*  b                          N  V
THE WORLD BANK
SECTOR POLICY AND RESEARCH STAFF
Environment Department
Assessment of Soil Degradation in the
Southern States of Nigeria
Rattan Lal and Bede Okigbo
September 1990
Environment Working Paper No. 39
This paper has been prepamd for internal use. The views and interpretations herein are those of
the author(s) and should not be attributed to the World Bank, to its affiliated organizations or to
any individual acting on their behalf.


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This paper has been prepared by Drs. Rattan Lal and Bede Okigbo,
both latterly of the International Institute for Tropical Agriculture
(IITA), Ibadan, Nigeria, acting as consultants to the Environmeni
Department of the Bank. Dr. Lal is currently Professor of Agronomy at Ohio
State University and Dr. Okigbo, Visiting Fellow at the Department of
Agronomy, Michigan State University. This study is part of a program of
work undertaken by the Environmental Policy and Research Division.
The authors would like to thank all those who assisted Dr. Okigbo
during his visit to Nigeria in early 1989, and are grateful for comments
received on earlier drafts from Steven Carr, Lewis Campbell, Bill Magrath,
Ridley Nelson and Ashok Seth of the World Bank and for assistance received
in preparing the report from John English.
Departmental Working Papers are not formal pablications of the
World Bank. They present preliminary and unpolished results of country
analysis or research that is circulated to encourage discussion and
comment; citation and the use of such a paper should take account of its
provisional character. The findings, interpretations, and conclusions
expressed in this paper are entirely those of the author and should not be
attributed in any manner to the World Bank, to its affiliated
organizations, or to members of its Board of Executive Directors or the
countries they represent.
Because of the informality and to present the results of research
with the latest possible delay, the typescript has not been prepared in
accordance with the procedures appropriate to formal printed texts, and the
World Bank accepts no responsibility for errors.


ABSTRACT
This study was carriad out as part of a program of work on soil
degradation in Nigeria undertaken in support of agricultural sector studies
on the country. A companion report on soil dagradation in the northern
states has also been published (Mortimore, 1989). The objective of the
study was to assess on the basis of secondary data and a quick diagnostic
field survey conducted in 1989 the extent and characteristics of soil
degradation in the southern states, the causes of this degradation and
measures available to counteract it.
Southern Nigeria has a generally humid climate with rainfall above
1,500 mn with a dry season. Physically, most upland soils in the region
are easily compacted, have low available water holding capacity, and are
susceptible to accelerated erosion. Structural deterioration sets in as
these soils are cultivated intensively. Chemically, the soils have low
nutrient reserves and although initial levels of organic matter are
generally high, they decline rapidly with deforestation intensive
cultivation and removal of crops. Nutrient losses are also caused by
increased leaching, volatalization, especially during bruning, and runoff
and erosion. These processes are most rapid on areas cultivated under
shifting cultivation or with extensive mechanical methods.
On farm studies have shown that yields in intensively cultivated
"gardens" (comnounds) close to homes were not related to inherent soil
fertility. These areas are heavily manured and the soils' fertility status
is generally maintained at a satisfactory level. However, in less
intensively cultivated, more distant fields, most commonly used for staple
crops, fertility and yields are significantly lower.
Possibilities exist to introduce some of the attributes of the
compounds into the utilization of the more distant crops. These include
active management of "fallows", and increased use of perennial species
either as crops, in contouring, provision of cover and bionass for mulch,
etc., and with legumes in nitrogen fixation. One variant of these has been
dubbed "alley-cr<pping" but others are possible. The attractiveness of
these possibilities to farmers is increasing but dependent on local
circumstances.
Because of increased population the pressure to intensify will
continue. Research is needed to assist in the development of viable
alternative technologies and systems. In particular greater, knowledge is
required of the characteristics and degradation processes of soils in the
region and work is also required on indigenous plants of the region to
determine those which may have characteristics and uses which will enable
them to be utilized as components of sustainable agricultureal systems.


ASSESSMENT OF SOIL DEGRADATION IN THE
SOUTHERN STATES OF NIGERIA
Table of Contents
.PaRe
I.       INTRODUCTION................................................  1
II.      GEOGRAPHICAL  BACKGROUND.....................................  2
III.     CHARACTERISTICS OF UNDEGRADED SOILS IN SOUTHERN NIGERIA..... 5
IV.      EVIDENCE  OF  SOIL  DEGRADATION................................  6
Acidificationf...........                                     7
Depletion of Soil 0.7ganic Matter and Principal Nutrients.... 7
Deforestation  of  Soil  Structure.............................  8
Soil  Fauna  and  Flora........................................  10
V.       EFFECTS OF SOIL DEGRADATION ON CROP YIELDS.................. 11
VI.      CAUSES OF SOIL DEGRADATION IN SOUTHERN NIGERIA.............. 15
Fire  and  Burning  of  Vegetation.............................. 15
Deforestation  and  Land  Clearing.............................  16
Increased Intensity of Farming and Shortening the
Period  of  Fallow...........................................  16
Low  Input  Agriculture.......................................  17
Accelerated  Erosion.........................................  18
Roads, Buildings and Industrial Construction........... .... 18
VII.     MEASURES USED TO PREVENT OR COMBAT SOIL DEGRADATION......... 19
Manuring  and  Mulching.......................................  19
Planted  Fallows  and  Cover  Crops............................. 19
Sustainable  Farming  Systems.................................  20
Chemical  Fertilizers........................................  22
VIII.    IS CONTINUOUS CROPPING ECOLOGICALLY AND
ECONOMICALLY  SUSTAINABLE?..................................  24
Interagency  Collaboration...................................  28
Research....................................................  29
Extension  and  Public  Awareness.............................. 31
IX.      RESEARCH AND DEVELOPMENT PRIORITIES......................... 29
BIBLIOGRAPHY................................................  36


I. INTRODUCTION
1.       Soil is the natural resource that provides the basis of the food
chain that ends with human consumption and waste disposal. There is no
other practical physical and chemical source of supply of human needs and,
in fact, needs of all living things, that can replace soil as the most
basic component of the life support system. Soil, under the normal
conditions of equilibrium, where the rate of soil loss equals that of soil
formation, is regarded as a renewable resource. But, it can be rendered
non-renewable by several human activities that characterize present
civilizations. Soil degradation is defined as the decline in soil quality
caused through misuse by humans. It results in deterioration of soil's
life support processes and decline in its capacity to produce food, feed,
fiber and fuel. Specifically, soil degradation is the diminution of the
soil's current and/or potential capability to produce quantitative or
qualitative goods or services as a result of one or more degradative
processes (UNEP, 1982; 1984).
2.       There is a need to distinguish between agents, processes and
factors of soil degradation. Agents of soil degradation are the frrces
involved, e.g-, the population pressure, development of infrastructure
including roads and buildings, socioeconomic conditions, etc. Processes of
soil degradation are the physical, chemical or biological mechanisms that
lead to decline in soil productivity. The most common processes of soil
degradation include: depletion in soil fertility, deterioration of soil
structure leading to compaction and accelerated erosion, nutrient imbalance
caused by salt accumulation or leaching, and buildup of organic or
inorganic pollutants. Factors of soil degradation are the natural or man-
made parameters that determine the magnitude and relative predominance of
the process of degradation, e.g., deforestation, burning, continuous
monocropping, cultivation of marginal lands, and indiscriminate use of
agrochemicals.
3.       The term degradation is, however, vaguely used leading to
ambiquity and confusion. It is important, therefore, that soil degradation
be expressed quantitatively by delineating critical limits of properties
beyond which soil's life support processes are drastically affected. These
limits may differ among soils, crops, landuse, and agroecological
environment. There is a paucity of basic research information on crop
growth in relation to degree of soil degradation caused by fertility
depletion and reduction in soil organic matter content, soil compaction,
reduction in rooting depth due to erosion and decrease in plant-available
water reserves, and buildup of toxic levels of Al and Mn in acid soils.
The quantity and quality of organic matter necessary to maintain an
adequate structural condition differ for different soils and environments
and are not known.
4.       Soil degradation is one of the major concerns of modern times,
because it has global implications even when it occurs in geographically
isolated conditions. Soil-related processes are a major cause of the so-
called "greenhouse effect". Deforestation leads to a sizeable emission of
C02, widespread fire and burning of biomass eject many radiatively active
gases into the atmosphere, continuous cropping leads to decomposition of
soil organic matter and release of 002 into the atmosphere, accelerated


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soil erosion caused by deforestation and cultivation of marginal lands is a
major cause of fertility depletion, transport of sediment-laden pollutants,
eutraphication of water and pollution of environment. Because of its
local, regional and global implications, we can no longer afford to loose
our basic resources to soil degradation.
5.       Obiective of this Report: The main objective of this study is to
determine in the basis of (a) available secondary data or already published
materials, (b) responses to a questionnaire sent to individuals or agencies
likely to be concerned with soil degradation in southern Nigeria, and; (c)
a quick diagnostic field survey conducted in late February 1989, the extent
of soil degradation in the southern states of Nigeria, the causes of this
degradation and measures being taken to combat it. An attempt is also made
to provide guidelines for preventive and restorative measures and resource
management strategies. Recommendations are made on some measures against
soil degradation.
6.       Procedure and Data Collection Methods: As much relevant
information on soil degradation in 10 southern states of Nigeria was
collected by (1) review of available literature published in Nigeria and
elsewhere, (') discussions with scientists in Nigerian universities and at
the Internat:Lonal Institute of Tropical Agriculture (IITA) and
Internationa:. Livestock Center for Africa (ILCA) at Ibadan, working on or
concerned with soil degradation, (3) discussions with scientists in the
agricultural research institutes in southern Nigeria, and with relevant
personnel in Federal Ministry of Science and Technology in Lagos, and (4)
discussions with staff of River Bisin Development Authorities and other
government and private agencies.
II. GEOGRAPHICAL BACKGROUND
7.       Southern Nigeria extends from latitude 4015. to 90 north of the
equator and from longitude 30251 to 9024D west of the %reenwich line. It
has an area of about 193,000km2 and is bordered by the Atlantic Ocean of
the Bight of Benin in the south and by the Middle Belt states of Gongola,
Benue and Kwara in the north. The 10 states of region consist of Akwa-
Ibom, Anambra, Bendel, Cross River, Imo, Ogun, Ondo, Oyo and Rivers.
Population densities in this region are among the highest in sub-Saharan
Africa. The 10 states have a total population of 42.7 million, average
density of 211/km2 and a range of from 138/km2 in Bendel to 618/km in Imo
and 1,239/km2 in Lagos state. While settlement in southeastern states is
dispersed, it is highly urbanized in the w,stern states with over 10 large
cities having populations of over 100,000 people each.
8.       Tbooraphy: The relative locations of the states and relief are
shown in Figure 1. Topographically, much of the area is lowland below 300
m but there are isolated hills or highland areas consisting of the Oban and
Obudu hills in the eastern border which are 1,000-1,500 m in elevation, and
the 800-920 m high Ishan and Idanre hills in the west. Geologically, much
of the area is dominated by sedimentary rocks, shales, sand, silt and clay
of Cretaceous and Tertiary ages, Pre-Cambrian Basement Camplex granitic
gneises, diorites and basic rocks in the eastern part and Pre-Cambrian
Basement Complex rocks consisting mainly of gneises; quarzites, schists and
granitic gneises in the western states.


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0 0o
w Å
Land above 310m (1000 ft)                Land above 610m
Land below 310m                          Escarpment
FIG. 1. Relief aud drainage of the southern states of Nigeria (Adapted
from Udoh , 1978).


9.       CliMate: The area has a tropical climate with mean annual
temperatures ranging from 21-350C in coastal areas to 21-400C further
inland. Minimum temperatures of slightly below 180C occur in December and
January when northeastern cold, dry and dusty harmattan winds blow from the
Sahara. Hilly areas have lower mean annual temperatures of 21-240C.
Relative humidity is usually high except during the harmattan in the
northernmost areas. The mean annual rainfall of a largely bimodal pattern
with an 'August break' ranges from above 4,000 mm in the extreme southeast
to less than 1,300 mm in the northwestern margin of Oyo state. The
duration of the rains range from 330 days in the southeast to less than 240
days in the northeastern parts of Oyo state during each year. Considerable
variation in rainfall occur from year to and from one location to another.
10.      Vegetation: This consists of parallel east to west zones of
varying width and from the coastal areas inland is made up of (1) coastal
vegetation of mangrove, aquatic grassland and herbaceous swamp, (2)
freshwater swamp forest, (3) lowland or moist tropical forest and (4)
derived savanna and forest/savanna mosaic in the northernmost areas.
However, most of the natural climatic climax vegetation is now of anthropic
origin having been greatly modified by increasing intensity of cultivation,
deforestation, annual fires, construction and human settlements under
increasing population pressure. As a result, with the exception of the
Oban Hills in Cross River State, much of the eastern states is now oil palm
bush north of the swamp vegetation.
11.      Farming Systems: The predominant farming system in the region is
slash and burn, bush or rotational bush fallow with fallow periods of 0-5
years in most of the southeastern states, and less than 10-12 years in the
western states, alternating with two or more years of cropping. Dominant
staples consist of cassava, yams, maize, cocoyams, rice, some sweet
potatoes, bananas and plantains. Major tree crops include oil palm, cocoa,
kola, rubber, coconut, mango, avocado, guava, citrus fruits and cashew.
The areas in utilization are small (less than 5 ha on over 70% of farms),
fragmented into 5 or more plots per farmer with staple food crops grown
mainly on mounds and ridges with predominantly manual labor, very little
mechanization and increasing but limited use of fertilizers. In the
southeastern states, there are elaborate home gardens with high crop
diversity including exotic tree crops, some indigerous tree crops (African
mango, breadfruit and pear), staple foods crops and vegetables forming a
multistoried structure with fertility maintained with animal manure and
household refuse. Fields adjacent to the Lome gardens are also intensively
cropped. There are no elaborate home gardens in highly organized western
states where small livestock are kept on free range unlike in the eastern
states where they are restricted. Cash crops in the southeastern states
consist of oil palms in addition to rubber (Cross River and Rivers), cocoa
in parts of Imo and Cross River but rice is increasing in importance. In
the western states, main cash crops are rubber, oil palms, yams and some
cocoa in Bendel state and cocoa, kola and some rice in Ogun, Ondo and Oyo
states.
12.      Overall Changes in the Cultural Landscape: During the last 20
years there has been significant change in the cultural landscape related
to changes in vegetation, land use, human settlements, roads and farming
systems which either influence land degradation or are manifestations of
it. Some of the changes in the cultural landscape include:


- 5 -
.    reduction of area and species complexity of forests secondary bush
and relic vegetation;
.    increasing sparsity of vegetation and marked increase in areas
colonized by the Siam weed (Chromolaena odorata) including
roadsides (Figures 3-5);
.   marked increase in the network of roads tarred or utarred with
hamlets, villages or markets springing up along them (Figure 3);
.    increased number and proportion of houses covered with corrugated
iron and aluminum sheets and large areas of compounds surfaced
with concrete;
.    many streams partly silted up or laden with sediment;
.    some indications of environmental degradation is evidenced by the
number of gullies along roads in outskirt of cities.
13.      Significant changes in farming systems and associated product
mixes have also occurred including:
.    increase in area of cassava and area under continuous cropping
with cassava;
.    decline in area under yams especially in southeastern Nigeria;
,    increasing area under more than one year continuous sole crop
cassava or maize, especially in western Nigeria;
.    increasing area of mechanized large scale farms of staple food
crops and plantains, especially in Rivers and Bendel States in
addition to oil palm plantations; and
.    increased production of vegetables and ornamental plants for
propagation or sale near to urban centers.
III. CHARACTERISTICS OF UNDEGRADED SOILS IN SOUTHERN NIGERIA
14.      When in equilibrium with its climax vegetation, soils have the
most favorable characteristics achieveable within the ecological limits
governed by the parent material and the climate. With the exception of
soils of the delta region, those derived from coastal sediments and recent
alluvium, most soils of south and southeastern Nigeria are very old.
Consequently weathering has occurred to greater depths in the bed rock, the
clay has eluviated to deeper horizons and is strongly aggregated.
Furthermore, strong leaching has caused depletion of bases and
acidification of the surface horizons.
15.      Soils in the region consist mainly of Ultisols, Oxisols, Alfisols,
Iceptisols and Entisols in hydromorphic and valley bottom areas. In


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general, with the exception of the Entisols and soils of younger volcanic
origin in Cross River State, most of these soils are highly weathered, acid
and of low inherent fertility.
16.      The soils of the Western states (Ogun, Odo, Oyo and small areas of
Bendel) are formed from parent materials consisting mainly of Pre-Cambrian
basement Complex rocks (gneises associated with quartzites, quartz, schists
and granitic gneises). They are mainly high base status soils consisting
of Alfisols, Mollisols and Inceptisols ui ler semi-deciduous vegetation with
pH values of above 6.0 and high base saturation of up to 90%. On the other
hand, most of the soils of southeastern Nigeria with the exception of those
of the younger volcanic origin are formed from sedimentary rocks, shales,
sand, salts and some clays of Cretaceous and Tertiary ages, underlain by
Pre-Cambrian Basement Complex granitic gneises, diorites and basic rocks.
They are mainly soils of low base status (Ultisols and Oxisols) with PH 4.5
to 5.0 and base saturation of less than 50% even when undegraded. There
are also Ultisols and Oxisols derived from beach sands, mangrove and
coastal alluvium in flood plains and other soils derived from shales and
basalt which are also highly acid and of low inherent fertility.
17.      Texturally, most of the highly leached Ultisols, Oxisols and
Alfisols vary from sandy to loamy sand, while the valley bottoms and
depressions with Inceptisols and Entisols are high in silt and clay. Many
of the soils have 60-90% sand, generally with gravel content ranging from
50-70% in the subsoil (B2t) horizon in Shepetiri sequence in Oyo and Ondo
states, very low level of silt (in most cases below 12%), and low in clay
content which is often below 20%. Soils east of the Niger and in Bendel
state are much coarser than those in the West and very highly weathered
with deteriorated lattice structure dominated by low activity clays
(kaolinite) and iron and aluminum oxides. Bulk densities of most soils are
low and may be less than 1.2 g/cm3 and increase with depth. These are low
undegraded soils where there is high activity of soil fauna (earthworms and
termites). As indicated above, soils in the southeastern states are lower
in exchangeable bases (Ca,+2 Mg+2 and K+) and available phosphorus, but are
higher in aluminum and total soil acidity as compared to those in the
western states. With the exception of soils derived from beach sands and
coastal sediments, soil organic carbon and total t.itrogen contents of
undergraded soils are usually high while phosphorus and effective cation
exchange capacity are generally low. Considerable variation in many of
these characteristics occur from location to location.
IV. EVIDENCE OF SOIL DEGRADATION
18.      With the exception of isolated islands of protected forest
reserves, the primary rainforest which prevailed several centuries ago has
long been degraded to secondary regrowth and thicket. Consequently, the
true shifting cultivation, characterized by short duration of cultivation
phase followed by long forest fallow, is nowhere to be observed. Due to
high population pressure, especially in eastern parts of Nigeria, shifting
cultivation has been replaced by rotational bush fallow in relatively poor
soils and continuous farming on fertile Inceptisols and flood plains.
There is a minimum of unused lands in the region and the soils are rapidly
being degraded. Severe soil degradation is evidenced in farms where the
fallow period is minimal or none at all. Most of the evidence of soil


-7-
degradation presented below is from analyses of soil sampled from
experiments conducted at research stations. There is little, if any,
research done on farmers' field to provide concrete and reliable
quantitative information on the rate, extent and distribution of soil
degradation.
19.      Symptoms of soil degradation include: crusting and formation of
surface seal, compaction of the soil surface horizon, reduction of pore
space and infiltration rate (Lal, 1986), low available-water holding
capacity, increased susceptibility to soil splash and rain drop impact,
accelerated runoff and erosion which are most evident on undulating ground,
decline in soil organic water content, decrease in pH, and nutrient
imbalance. Many of these symptoms are manifested in the cultivated soil of
southern Nigeria and especially in the southeastern states. In soils of
relatively flat terrain and gentler slops, the leaching out of the clay and
colloidal fraction of the soil and their deposition in the subsoil may be
the cause of textural and structural -changes. On the steep slopes and
undilating lands accelerated runoff and erosion are most pronounced. Gully
erosion is very severe in eastern states.
20.      Acidification: Continuous cultivation leads to depletion of bares
resulting in decrease in pH and increase in exchangeable aluminum and
hydrogen. The rate of decline in pH is more rapid in coarse-textured soils
of low cation exchange capaciv! (CEC) and low organic carbon content than
in heavy-textured soils of high buffer capacity. An experiment conducted
on an Entisol, of relatively high inherent fertility, showed that pH
declined with continuous cultivation. Soil pH declined from an initial
value of 6.1 to 5.65, 5.50, 5.45 and 5.42 with first, second third and
fourth cultivation, respectively. A similar evidence of decline in pH with
cropping is observed from an Ultisol derived from coastal sediment at Onne,
in eastern Nigeria (Maurya and Lal, 1979). A rapid decline in pH with
cropping is also observed for relatively fertile Alfisols of the subhumid
regions of southwestern Nigeria. Lal (1985) observed that soil pH declined
from an initial value of 6.1 to 4.4 within 6 years of continuous
cultivation. The rate of decline in pH was more in the case of plow-based
system than with no-till farming.
21.      Decline in soil pH is associated with depletion of bases. Once
again depletion of bases is expectedly more rapid in Ultisols and Oxisols
than in Alfisols and Inceptisols. The data in Table 1 indicate that
5-years of continuous cropping lead to a decrease from 4.8 to 4.0 meq/100 g
for calcium, 1.3 to 0.8 meq/100 g for magnesium, and 0.6 to 0.4 meq/100 g
for potassium. Similarly, the data for a plow-based system indicate even
more drastic decline in exchangeable bases. Six years of continuous
cropping led to a decrease from 7.9 to 1.9 meq/100 g of calcium, 2.0 to
0.13 meq/100 g of magnesium and 0.8 to 0.18 meq/100 g of potassium. In
contrast, there was an increase of 0.05 to 0.3 meq/100 g of manganese, and
from traces to 0.95 meq/100 g of total acidity. These apparently drastic
reductions in exchangeable bases and sharp increases in total acidity were
brought about by maize grain production of only 2 to 4 t/ha/yr.
Acidification is a severe problem especially with intensive use of chemical
fertilizers such as Ammonium Sulphate.
22.      Depletion of soil organic matter ad principal nutrients: Soil
degradation is also evidenced by reductions in soil organic matter, and


- 8 -
total and available fractions of nitrogen and phosphorus. Low levels of
soil organic matter content is an important indication of soil degradation.
Continuous cropping for production of food crop annuals leads to decline in
soil organic matter content due to a multitude of interacting factors.
High soil temperatures, accelerated erosion, and removal of stover and
other biomass are some reasons of decline in soil organic carbon during the
cultivation phase. Some of these factors will be discussed in the
forthcoming section. There are some examples of data from research
stations indicating decline in soil organic matter with cropping. Data
from an acid tropical soil of eastern Nigeria indicate that organic carbon
content declined from an initial value of 0.9% to 0.62% in the first year,
0.5% in the second year, 0.45% in the third year, and 0.42% in the fourth
year of continuous cultivation. Drastic reductions in organic carbon
content are also reported for Alfisols in Western Nigeria from an initial
value of 1.7% under forest to 1.2% with 5-years of continuous cropping.
The data in Table 1 also show decline in organic carbon content from an
initial value of 1.7% to 1.2% following 6-years of cropping with a plow-
based system.
23.      Reduction in soil organic carbon content also leads to decline in
total nitrogen and available phosphorus. The data in Table 3 show
reduction of total nitrogen from 0.16 to 0.12 percent with 6-years of
continuous cropping with a plow-based system. These reductions were
observed despite regular additions of 100 to 150 kg/ha/yr of nitrogen
applied as urea. These research plots also received 40 to 60 kg/ha/yr of P
as single or triple superphosphate. Consequently, soil levels of available
P increased with the cropping duration (Tables 1 and 2). Nutrient
depletions are expected to be severe in farmers' field where chemical
fertilizers are either not applied or are applied at extremely low rates.
24.      Deterioration of soil structure: In comparison with soils under
forest or bush fallow, cultivated soils in south and southeastern Nigeria
are characterized by deterioration of soil structure and impairment in
their ability to regulate water and air movement through the soil profile.
Soils with degraded structure exhibit symptoms of crusting and formation of
surface seal, compaction of the surface horizon, reduction in infiltration
capacity, increased susceptibility to soil splash by raindrop impact, and
accelerated runoff and erosion. The latter is evident only in sloping
lands. In soils of relatively flat terrain and gentler slopes, however,
the clay and colloidal fraction is leached or eluviated into the subsoil.
25.      Soil compaction is a widespread problem on all farms. Increase in
soil bulk density occurs due to cultivation regardless of the methods used.
However, the problem is comparatively more severe in mechanized than
nonmechanized farms. Experiments conducted on Alfisols at the research
farm of IITA have shown that the deterioration in soil structure, as
evidenced by increase in bulk density and decrease in infiltration
capacity, is less with traditional farming and slash and burn methods than
with intensive mechanized agriculture. (See Tables 3 and 4.) Similarly,
decline in equilibrium infiltration rate was also less with traditional
than with intensive mechanized farming (Figure 2). Furthermore, the
infiltration rate showed an improvement even during the first year of the
fallow phase. An universal evidence of compaction on farmers' fields is
the accelerated runoff and erosion leading to sheet wash and rill eresion.
Uncontrolled rills turn into guilies within a short period.


-9-
aoo
o.- oTRAOTIONAL FARMING
8... -x JNTENSIVE MECHANSEo FARMING
I (X) STANOARO OVIATN
tIoo
-             PHV4IoW  MASZ    I   rALLow  mSE
TIME (æm)
Fig. 2   Reduction in infiltration rate of an Alfisol in western Nigeria due to continuous cropping
(Lal, 1986).


- 10 -
26.      Soil fauna and flora: The clearing and burning of vegetation and
subsequent cropping usually results in changes in the number and
composition of soil fauna and flora (Nye and Greenland 1960). There is an
overall decline in soil fauna population following cultivation of forest
soils. Seasonal migrations from the top soil also occur in cultivated land
due to drying up of soil and high insolation. After an initial decrease,
the population of microflora usually redevelops to a level greater than
before thereby causing increases in nitrogen mineralization. After
clearing, burning and cropping there is a tendency for pests and diseases
to increase especially where rotation of crops is not practiced. Weed
growth also increases where cultivation is continued without fallowing.
27.      Lal (1987) reviewed the occurrence, activities and effects on soil
of a diversity of soil organisms and effects of deforestation clearing and
cropping on them. Deforestation results in significant reduction of the
invertebrate fauna consisting of crickets, spiders and millipedes.
Critchley (1979) reported from Western Nigeria that catches of microfaunna
in cultivated plots were 40-60 percent lower than in uncleared plots. With
the exception of Prostigmata, which later became the dominant
microarthropods in the cleared plots, there was a reduction in population
densities of all groups of animals. Caveness (1982) observed that
population densities of plant parasitic nematodes increased after land
clearing. Deforestation apparently has both direct adverse effects on soil
fauna through disturbance of the soil and scraping, which damages the
habitats and lowers faunal populations, and indirect effects through
changes in microclimate, decreased food diversity, and exposure to
predators and parasites.
28.      Various cultural practices including burning and the use of
herbicides and insecticides adversely affect soil fauna on cropped land,
but very scanty information about this phenomenon in the tropics is
available (Lal, 1977). Studies by Crichley et al. (1979, 1980) in Western
Nigeria indicated that DDT treatment reduced activities of adult and
immature Gryllids (crickets), lycosid spiders, subterranean mesofauna, ants
and some beatles and millipedes which ingested contaminated food. Perfect
et al. (1981) reported DDT to reduce populations of Collembolla and Acari
in treated plots to 50 percent of the untreated ones. Cultural practices
also significantly affect microbial activities in soil and, while initially
such treatments as burning may drastically reduce their abundance and
activity, subsequent cropping practices and crop species may produce a
range of differential effects.
29.      As regards earthworm activities in the soil, deforestation reduces
activity of Hyperiodrilus africanus but not as much of that of Eudrilus
eugeniae. No-till practices which retain plant residue on the surface tend
to encourage more earthworm activity than plowing. Mulching either with
dead plant residue or live mulch has beneficial effects on earthworms as
compared to bare plots which are detrimental to them. Fallowing with
grasses or leguminous covers enhances earthworm activity to the extent
dependent on crop species. Burning, and the subsequent compaction and
erosion which may result, also causes reduction in earthworm and other
faunal activities. Various other treatments such as fertilizers and
pesticides may adversely or beneficially affect earthworm activity
depending on the circumstances (Lal, 1987).


- 11 -
V. EFFECTS OF SOIL DEGRADATION ON CROP YIELDS
30.      Reduction in crop yield is a serious economic consequence of soil
degradation. Crop yield is an integrated response of many factors
including climate, pests and management. However, adverse effects of soil
degradation are accentuated due to suboptimal status of other environmental
factors. For example, crops grown on degraded soils are highly susceptible
to drought and infestation by pests.
31.      There is considerable evidence of decline in crop yield due to
soil degradation caused by continuous cropping. In the subhumid region of
western Nigeria, Couper et al. (1979) observed drastic reductions in maize
yield due to continuous cropping. Land was cleared from a secondary forest
in 1974, and yields of the first-year unfertilized crops were low. In
comparison with the best yields obtained for the plow-based system in the
second year (1976), maize grain yields declined by 2.5, 0.0, 27.5 and 75.0
percent in the third, fourth, fifth and sixth year respectively. Yield
decline also occurred in the no-till treatment but only during the fifth
and sixth years of cropping. Furthermore, the relative decline was less in
no-till compared with the plowed system. For example, in comparison with
maize yield in the second year, there was a reduction of 15.6 and 33.3
percent in the fifth and sixth year, respectively. Severity of yield
reduction in the plow-based system was related to rapid depletion of soil
fertility and degradation of soil structure. Another evidence of yield
decline of maize and cowpea due to soil degradation is shown by the data in
Fig. 4 (Lal, 1989). Despite intensive input (of fertilizers and
pesticides) and use of Leucaena and Gliricidia, there was a reduction in
yield of maize and cowpea. For example, yield declined at the rate of 0.34
t/ha/yr for maize and 96.3 kg/ha/yr for cowpea (Fig. 3). Similar to the
data in Table 5, yield decline in this experiment was also attributed to
degradation of soil physical and chemical properties due to continuous
cropping.
32.      Degradation-induced yield reductions are usually more severe in
acid infertile soils of eastern Nigeria than in relatively fertile Alfisols
of western Nigeria. A principal cause of rapid and severe yield decline is
the depletion of soil fertility and deficiency of principal plant
nutrients. The results of a long-term study conducted on a sandy soil in
eastern Nigeria relating the effects of a cover crop and green manuring on
maize grain yield show a drastic decline in maize yield with cropping
duration. For example, grain yield of 1800 kg/ha in 1934 declined to 1050
kg/ha in 1935, 1200 kg/ha in 1936, 250 kg/ha in 1937 and 200 kg/ha in 1938.
Grain yields in the fourth and fifth years of cultivation were about 10% of
the initial yields. Odurukwe and Orji (1981) evaluated yields of yam,
cassava and maize under continuous cropping on acid soils in eastern
Nigeria (Fig. 4). The data show that yield reduction was more severe in
maize compared with cassava or yam. In the 4th year of continuous
cropping, the relative decline in yield was 65% in maize, 38% in yam and
only 25% in cassava. Nitrophilic grain crops such as maize are more
susceptible to fertility depletion than long-duration root crops such as
cassava.
33.      Lagemann (1977) examined crop yields in relation to land use
systems of small holders in high population density of eastern Nigeria. He


- 12 -
5.0-
4 Cy 68 - 0.340x
e Ra =45.2%6
0
ac
1.0
(                          '
10001-
y=933 - 96.3x
-~~R 80 -74411'*
8 00-                                  R2749
400C -
! eoo
01
200-
1982     1983     1984     1985      1986     1987
Fig. 3.  Yield decline of maize and cowpea with continuous cultivaLion
using alley cropping (Lal, 1989).


- 13 -
CROP                4976 YIELD
i YAM                   23*6  T/HA
2 CASSAVA               16*4  1/WA
3 MAIZE                  4.6  T/14A
4, CALOIES             17.3 X 107 CALS/MA
60
0
1404
20
of
1976         1977         1978        1979
YEARS OF CONTINUOUS CROPPING
FIC 17.0tCLINE IN YIELD OF YAM, MAIZE AND CASSAVA
WITH 4 YEARS OF CONTINUOUS CROPPING
YIELDS AE AVERAGED OVER ALL PHASES CARRYING
THE RESPECTIVE CROPS.(SOURCE 00UAUXWE
AND ORJI 1981)


- 14 -
related crop yields on farmers' fields to soil chemical and nutritional
properties. The results of regression analyses relating yield to soil
properties are shown in Table 6. Soil organic carbon, and the level of
phosphorus in soil were positively correlated with yields. The correlation
coefficient, however, was more; significant for organic carbon than
phosphorus. Lagemann (1977) also related grain yield of sole-cropped maize
from 69 farms to organic carbon and Bray-1 P levels of soil. He found that
yield of maize was very sensitive to level of organic carbon in the surface
soil. Grain yield of maize increased by 1.23 percent with every 1 percent
increase in level of soil organic carbon. However, the range of soil
organic carbon content for 69 farms studied was rather narrow, 2.0 to 2.9
percent. In comparison, there were wide ranges for soil phosphorus (1.36
to 20.73 ppm) and base saturation percentage (18.1 to 66.0 percent). Maize
grain yield increased by 0.18 percent with every one percent increase in
level of soil test phosphorus. Maize grain yield also increased by 0.83
percent for every one percent increase in base saturation. An important
conclusion of Lagemann study from 69 farms in 3 villages in eastern Nigeria
was that as much as 73 percent of the observed differences in maize yield
among farms could be explained in terms of differences in soil fertility
and management.
34.      A similar study based on evaluation of soil properties and
management characteristics for several hundred farms in eastern Nigeria was
conducted by Armon (1984). He conducted a survey of soil degradation on 6
locations in eastern Nigeria. Soil characteristics and crop yields were
evaluated on farms in the vicinity of Calabar, Onitsha, Owerri, Ogoja,
Umudike and Enugu for both compound and distant farms. Yields of yam,
cassava and cocoyam were related to the length of the fallow period and
soil properties. On distant fields yields were significantly higher on
fields with fallow period exceeding 4 years than on fields with a fallow
period of less than 2 years. Furthermore, yields on distant fields were
significantly correlated with total nitrogen content of the soil (Eqs. 1
and 2) and with length of the fallow period (Eqs. 3 and 4).
Y - 147.2 + 334.6 N,                r - 0.41 **          Eq. 1
Y - 253.6 - 761.7 N + 1050.3 N,2    R = 0.60**           Eq. 2
Y - 131.9 + 21.8 F,                 r - 0.23*            Eq. 3
Y - 129.6 + 23.6 F - 0.26 F,2       R - 0.34*            Eq. 4
where Y is yield expressed as MJ/ha, N is soil nitrogen content (%), and F
is length of the fallow period in years.
35.      The analyses of yield in relation to soil properties for distant
fields indicated that about 40% variability in crop yields among fields was
attributed to differences in soil test value for total nitrogen, and about
12% to differences in the length of fallow period. Apparently, nitrogen
content of the soil was an important determinant of yields of root crops on
farmers fields in eastern Nigeria.
36.      Armon's study led to two important conclusions. Firstly, crop
yields in compounds were not related to inherent soil fertility. These
areas are heavily manured by the household waste and animal dung, and the
soil's fertility status for nitrogen and other nutrients is generally
favorable. Secondly, crop yields on distant fields were significantly
affected by the soil test for total nitrogen.


- 15 -
37.      Studies by Lagemann and Armon have important management and policy
implications. Their data clearly indicated that in areas of high
population densities in eastern Nigeria, soil degradation sets in due to
reduction in length of the fallow period. Furthermore, degradation-caused
yield decline is related to reductions in soil tests for organic carbon,
total nitrogen, available phosphorus, and base saturation percentage.
These conclusions are further supported by the high fertility status of
compound vis-a-vis distant fields. The data of soil chemical analyses for
three sites studied by Lagemann is shown in Table 7. The data indicate
significant differences among compounds and distant fields in
acidification, and soil test for phosphorus and exchangeable cations. Soil
pH in compounds had not deteriorated when compared with near and distant
fields. Exchangeable cations and phosphorus levels were also significantly
higher in the compound than in the outer fields.
38.      The survey conducted by Armon showed that fertility status of
.ompounds, heavily manured by traditional methods of organic farming, is
significantly superior to those of unmanured distant fields (Table 8). The
high fertility status of compounds is despite the intensive cultivation
without fallowing. Taking the mean soil properties for all sites in
compound versus distant farms, respectively, showed that soil test values
were: 5.84 vs 5.23 for pH, 1.62% vs 1.20% for organic carbon, 0.15% vs
0.17% for total N, 96.6 ppm vs. 44.8 ppm for P, 3.71 meq/100 g vs 1.50
meq/100 g for Ca, 0.63 meq/100 for g vs. 0.31 meq/100 g for Mg, and 0.24
meq/100 g vs 0.09 meq/100 g for K. Statistical analyses of the data
indicated that soil test values in compounds were significantly higher than
in distant fields for pH, organic carbon, available phosphorus and
exchangeable calcium, magnesium and potassium. Addition of plant nutrients
through mulch, household and animal wastes, and by recycling due to deep-
rooted perennials maintain high level of soil fertility in compounds
despite intensive land use and continuous cropping. This type of intensive
management, however, is impossible to be followed on large areas of
regularly cultivated distant fields.
VI. CAUSES OF SOIL DEGRADATION IN SOUTHERN NIGERIA
39.      Causes of soil degradation encountered either in the literature or
during the field survey and discussions in Nigeria consist of (1) fire and
burning of vegetatinn, (2) deforestation, (3) increasing intensity of
farming and cultivation, and tillage and related practices, (4) low input
agriculture, (5) accelerated erosion, (6) road building and other
construction.
Fire and Burning of Vegetation
40.      There has recently been increasing frequency and intensity of
fires and burning of vegetation, especially in the dry season. Fires may
be (1) purposely used for clearing of vegetation in traditional farming
systems, (2) accidaitial, (3) used to drive out game and wild animals and
(4) used to eliminate undesirable refuse or some inflammable waste. During
the field survey in February and March 1989 many fires were observed
burning uncontrolled along roadsides in all of the 8 states visited to the
extent that 5 to above 30% of roadsides were burnt during the dry season
(see Table 9). Despite laws against burning of vegetation many economic


- 16 -
trees (oil palm, rubber, cocoa, bananas and plantains), food crops
(cassava, cocoyam, pineapples, pepper, vegetables), forest plantations,
ornamentals and vegetation were burnt. Many fires start from dry Siam weed
growing along roadsides and penetrate far into nearby bushes except where
thick vegetation and planted fallows of Acioa and Anthonotha blocked their
path.
41.      Uncontrolled fire causes soil degradation through short-term and
long-term effects. A short-term effect is that related to high
temperatures generated during the fire. Temperature at the fire base, at
the soil-air interphase, may exceed 2500C (Lal, 1987). Although, the
heatwave may not penetrate deep, the top 5-cm of soil may be completely
sterilized. In addition to alteration of soil chemical properties,
excessive heat changes soil structure and its wettability. Fire-burned
soil is hydrophobic and becomes prone to excessive runoff and accelerated
erosion. Ecological instability caused by repeated cycles of fire have
resulted in denudation of vast areas of its protective vegetation cover,
soil compaction, and unprecedented runoff and accelerated rates of erosion.
Furthermore, economic crops damaged by fire are prone to infestation by
pathogens or insects.
Deforestation and land clearing
42.      Deforestation, which increases with intensity of cultivation, and
harvesting of timber and removal of vegetation along roads and on
construction and mining sites, expose the soil to the elements with
increased runoff, erosion and soil degradation. Mechanized clearing which
is replacing manual clearing and logging operations cause scraping away of
rich top soil and compaction of subsoil which restricts water infiltration
and root penetration and increases runoff and erosion. Experiments
conducted in the subhumid regions of western Nigeria showed that runoff and
erosion from mechanically cleared lands were several orders of magnitude
greater than those from traditional or manually cleared land (Table 10).
The data in Table 11 from western Nigeria indicate drastic reductions in
fertility status of the surface soil horizon of mechanically cleared land
in comparison with manual clearing or slash-and-burn traditional farming.
Increased Intensity of Farming and Shortening the Period of Fallow
43.      Increasing population pressure has resulted in decreased periods
of fallow and intensification of cultivation. A survey by Goldman (1988)
in Imo state of Nigeria has indicated that during the last 20 years fallow
periods have declined from 1-9 years to 0-6 years--a period much shorter
than the 5-7 years required to restore soil fertility. The decline of
fallow periods had rendered fallows incapable of supplying stakes,
fuelwood, fodder or browse, edible mushrooms, drugs, fiber, fruits and
vegetables to rural communities. This either causes the farmer to spend
money to purchase these things or deforest other areas to obtain them.
Some highly degraded areas are colonized by spear grass.
44.      Similar to the data on degradation of soil physical properties,
there is little additional information available from farmers' fields on
depletion of soil fertility with continuous cropping. Although it is
highly suggestive of its impact, the data by Lagemann and Armon do not
provide sufficient information for establishment of the direct cause-effect


- 17 -
relationships. Several studies conducted on research farms, however,
indicate drastic decline in soil fertility due to continuous cropping.
Experiments conducted at the research farm of IITA showed rapid
acidification, ano reduction in organic carbon, total nitrogen, effective
CEC and exchangeable cations due to continuous cropping (Table 2).
Fertility depletion occurred despite addition of the recommended rates of
fertilizers and return of crop residue as mulch. In another experiment
also conducted at IITA, Lal (1985) showed drastic changes in soil chemical
properties over a short period of 6-years of continuous cropping. In a
plow-based system soil pH decreased by almost 2 units, organic carbon by
0.2 to 0.5%, total N by 0.1 to 0.2%, and ECEC by 5 to 6 meq/100 g. The
rate of fertility depletion, however, can be somewhat less with no-till,
mulch farming and alley cropping systems than with conventional plow-based
systems. The data in Table 12 is indicative of beneficial effects of the
no-till system on soil fertility. Nonetheless, fertility decline is
inevitable with intensive continuous cropping regardless of the management
system. At the research farm of the University of Ife, Aina (1979)
observed severe decline in soil chemical and physical, properties. Decline
in soil pH, exchangeable cations and CEC were severe with continuous
cropping both with and without fertilizer application. Expectedly,
however, fertility decline was more drastic in relatively less fertile Oba
than fertile Iwo soil series. In addition to decline in soil chemical
properties, continuous cropping also results in degradation of soil
physical properties. A principal victim of continuous cropping especially
by mechanized means, is soil structure. Structural degradation is
evidenced by rapid increase in bulk density, decrease in total and macro-
porosity, reduction in infiltration capacity, and decrease in percent
aggregation. Some of these effects were discussed in several reviews by
Lal (1986; 1987).
45.      If fertility depletion is rapid and drastic on research farms even
with intensive input of chemical amendments, the problem is likely to be at
least as severe on farmers' fields. Major differences, however, are in
intensity of cropping and in cropping systems used. Farmers rarely use
monocropping commonly used by research farms. Furthermore, farmers
deliberately use low plant stand and grow crops in association with trees
and woody perennials. The rate of fertility decline under farmers'
conditions may, therefore, be less thar that observed on research farms.
Low Input Agriculture
46.      Intensive farming without restorative fallowing or exogenous
inputs is wasteful and degradative. It is a system of mining the inherent
fertility that leads to degradation of the entire ecosystem. Fertilizer
use on traditional farms is still negligible. While the excessive and
indiscriminate use of fertilizers and other chemicals must be avoided,
exploitation of native fertility through continuous mining is an
ecologically incompatible system. Increasing cropping intensity without
external input is to aggravate the degradative trend. In the studies cited
above Armon (1984) and Lagemann (1977) reported severe degradation on
distant farms because these fields do not receive mulch, manure or
fertilizers. Poor crop stand and low plant vigor results in scanty canopy
cover, especially at the onset of heavy rains. Native trees are severely
damaged by repeated cycles of uncontrolled fire. All biomass is purposely
removed away or burnt. Soil degradation, especially on distant farms, is


. 18 -
caused by low-input, exploitive agriculture driven by high population
density and shortage of good arable land.
Accelerated Erosion
47.      Erosion, which is the wearing away and removal of weathered rock
material or soil from the surface of the land, varies in intensity from one
state to another in southern Nigeria. It is a selective process which
removes the topmost fertile 0-5 cm of soil and its ability to grow crops.
While many human activities may cause erosion, it has been observed that
the extent of erosion often depends more on relief, surface materials,
climate and vegetation interacting with each other and human activities.
The coarser soil texture in southeastern Nigeria and presence of cuestas
running north to south has made the area more subject to erosion than in
the western states of Nigeria. Erosion is more of an urban phenomenon in
the West as compared to the East where it is rural. Erosion may be
manifested as (1) sheet erosion which is the removal of a very thin layer
uniformly over the surface which is most serious in Ogun, Oyo, Imo and
Anambra states; (2) XJl_ erosion which removes materials from relatively
shallow and narrow channels, especially in Anambra, Imo and Lagos states;
and (3) gully erosion which removes materials in deep channels and is most
pronounced in Imo, Anambra, Cross River, Akwa-Ibom and Rivers States; (4)
coastal erosion causing wearing out of ocean shoreline in Rivers, Cross
River, Akwa Ibom, Lagos and Bendel states. About 80,000 km2 or 45% of
southeastern Nigeria is affected by rill and sheet erosion and about 2-3%
by gully erosion (Figure 19).
Roads. Buldings and Industrial Construction
48.      The oil boom in Nigeria resulted in building of a network of
express roads and ordinary tarred roads in many states of Southern Nigeria.
Coupled with these are untarred roads and footpaths to all nooks and
corners. Recently the Directorate of Food, Roads and Rural Infrastructure
(DFRRI) has increased the network of untarred rural roads, that are not all
properly provided with drainage. The Enugu-Onitsha, Enugu-Nsukka and the
Port Harcourt-Aba-Enugu roads have been reported by Fubara (1988) to be
treated or cut by gulley erosion. Many untarred roads are damaged or are
turning into gullies because of regular scraping to level them after damage
by torrential rainstorms. Roadsides covered by Siam weed, which is not
regularly mowed constitute fire hazards in the dry season.
49.      Rain water rushing down corrugated iron roofed buildings in rural
and urban areas especially when surrounded by concreted compounds without
adequate drainage cause channels of runoff and erosion. Various industrial
buildings, airports, playing fields, car parks, and markets without
adequate drainage are associated with a lot of soil degradation. Grazing
along roadsides also contributes to exposure to soil to erosion.
VII. MEASURES USED TO PREVENT OR COMBAT SOIL DEGRADATION
50.      Measures used to combat soil degradation may be preventive or
remedial. Preventive measures are practices used to minimize the chances
of soil degradation occurring or the magnitude or severity of the damage
thereof when they do occur. The preventive measures consisting of soil
conservation practices or aspects of agricultural production are reviewed


- 19 -
briefly under different subheads below followed by a brief survey of
sustainable agricultural production systems which have the potential foi
minimizing soil degradation while achieving economically viable yield
levels. Remedial measures taken by government are considered later in -:he
next section.
Manuring and Mulching
51.      Manuring involves the application of organic residues consisting
of rotted or decomposed organic materials, household refuse, compost,
kitchen refuse including ashes and animal manures to the land on which
crops are grown. These manures are used to restore soil fertility and
productivity. Very often pits are dug in which manure is accumulated.
Animals are kept in pens where their droppings or dung are made into
farmyard manure (FYM). This kind of manure is used to maintain fertility
in home gardens, adjacent fields and sometimes market gardens. A lot of
soil waste in urban areas can be compacted but is not a widespread
practice.
52.      Mulches usually consist of plant residues; industrial wastes such
as sawdust; crop residues such as straw and rice husks and even polythene
films which are used to cover the soil in mounds, beds, ridges and flat,
where crops are growing. Mulching protects the soil from the beating
action of the rain, increases water infiltration rate and prevents
compaction of the soil thereby reducing runoff and erosion, supplies
organic matter and nutrients to the soil, reduces the amplitude of
temperature fluctuations and increases yields. Both manures and mulches
are only economical to use on very small farms and high labor costs
minimize extent of their use. Living mulches such as Pueraria and
Flemingia have been found useful in tree crop plantations and in the
growing of plantains, respectively.
Planted Fallows and Cover Crops
53.      Properly managed planted fallows and cover crops are more
efficient and require less time to restore soil properties than natural
fallows. Because of high rate of biomass production, soil organic matter
can be rapidly built up and th, soil structure improved even on severely
eroded and degraded lands by growing appropriate planted fallows for 3 to 5
years (Jaiyebo and Moore, 1964; Juo and Lal, 1977; Wilson et al., 1982).
There is a considerable amount of research information available from
western Nigeria regarding the beneficial effects of cover crops in
restoring soil properties. Studies by Jaiyebo and Moore (1964) indicate
drastic increases by Pueraria in soil organic matter, total and N03-N,
total P, CEC and exchangeable bases. Short-term fallowing by Pueraria was
as effective as long-term bush fallowing. Lal et. al., (1979) reported
significant improvement in soil organic matter by 3-years of fallowing by
several grasses and leguminous cover crops. Stylosanthes and Cynodon were
more effective than Paspalum and weed fallow control. Similar improvements
in soil fertility were reported by Wilson et al. (1982) Improvements in
soil organic matter content and fertility status are also accompanied by
restoration of soil structure. In western Nigeria, Lal et al. (1978)
observed that planted fallows enhanced biotic activity of earthworms.
Stable and continuous macropores and biochannels thus created decreased
bulk density and increased infiltration rate. Some important cover crops


- 20 -
adaptable for southern Nigeria are Calopogoni  muconoides, Centrosema
pubescens, indigfera spictata, Pueraria phaseoloides Stylosanthes
guianensis, Dolicos hosel, Glycine Javanica, Muouna utilis, Brachiaria
ruziziensis, and Panicum maximum.
54.      Improvements in soil fertility are reflected in higher yields of
crops grown after the cover crops (Lal, 1986). However, growing cover
crops for a short period without chemical fertilizers and manure is not
adequate to enable high yields on a continuous basis. Data from eastern
Nigeria show rapid yieli decline of maize grown after Mucuna and Dolichos
after the second consecutive crop. Grain yield for the sixth consecutive
maize crop was 21% of the first crop in case of Mucuna and only 14% in case
of Dolichos. Nutrient removed by nitrophilic crops such as maize cannot be
replenished by short fallowing alone without supplemental applications of
chemical fertilizers or organic amendments.
Sustainable Farming Systems
55.      Soil degradation could be prevented or minimized where various
crops are grown on soils in parts of the landscape where they are most
adapted and have both economic and ecological advantages. The production
systems which are most adapted to the humid and subhumid areas of Nigeria
include: (1) tree crop plantations, (2) home gardens or compound farms,
(3) alley cropping and related agroforestry systems, and (4) wetland rice.
56.      Tree crops such as rubber, cocoa, oil palm and coffee when grown
in plantations, and especially with leguminous cover crops, effectively
protect the soil. Such tree crops give good yields 'or 20-25 years or
more. Tree crops enterprises vary in size from small holder plantations of
under 5 ha to large scale farms of more than 100 ha. Small holder tree
crops plantations have naot been economically viable due to size
limitations, and the tendency of farmers not to replant uneconomic
plantations with improved cultivars when yields start to decline after 20
years. The result is reduced yields and ineffective soil cover.
57.      Home Gardens or Compound Farms: This is one of the most
widespread sustainable agricultural production systems in the humid and
subhumid tropics and, for Southeastern Nigeria, has been described in some
detail by Okafor and Fernandes (1987). Compound farms are a type of
agroforestry system involving the deliberate management of multipurpose
trees and shrubs in a multi-storied association with agricultural crops and
small livestock within the compounds of individual houses. These
intensively cultivated areas are usually operated in conjunction with a
more distant, less intensively cultivated area, used for staple crop
production (e.g., of yams, cassava, rice, maize, etc.). The stability of
the system is, no doubt, related to the species diversity, production of a
wide range of food and useful products, effective integration of crop and
animal production with fertility maintained under continuous cultivation by
the use of household refuse, animal manure, farmyard manure and nutrient
cycling by trees and shrubs, with some of the leguminous crops also fixing
nitrogen. The animals yield meat, manure and cash. Ornamentals in home
gardens also perform an aesthetic function. Increasing population pressure
in rural areas, soil degradation and the building of houses on ancestral
lands are causing some breakdown of the system. There is a need for a


- 21 -
special research program on home garden improvement if the system is to
survive.
58.      Alley Cropping and Related Agroforestry Systems: Alley cropping
is a cropping system in which arable crops are grown in between two
hedgerows that are regularly pruned in order to obtain cuttings or twigs
for mulching and for soil fertility maintenance. With the development of
alley cropping at IITA there has also been developed alley farming which is
a silvopastoral production system in which fallow species grown in the
hedgerows are used as browse or sources of fodder for small ruminants.
Advantages of alley cropping include:
.    supply of mulch for covering and protecting the soil thereby
reducing runoff and erosion
.    increased supply of organic matter and nutrients for maintaining
soil fertility and increasing yields
.    increased infiltration
.    supply of stakes and miscellaneous useful products
.    supply of fuelwood, and
.    supply of fodder or browse
59.      Despite all these advantages, there has not been widespread
adoption of this practice due to high labor requirements for pruning, very
narrow species of crops that can be grown in small plots under alley
cropping and hence high risk of crop failure. Alley cropping without
herbicides was not found to be as attractive as traditional practices.
Alley cropping with Gliricidia and Leucaena was found to be promising in
the keeping of sheep and goats, but its acceptance remains limited even in
southeastern Nigeria where it has been demonstrated to be viable. This is
mainly because of high soil acidity and low fertility, incompatibility of
woody species, division of labor and decision-making process within
households and tree tenure rules, need for more sustained extension effort
and longer period of orientation of the farmer to ensure better
understanding and appreciation of interaction of various factors (Francis
and Atta-Kra (1988), Ehui (1988), Kang et al., (1989) and Osembo (1987).
Alley farming was more attractive in Western Nigeria where animals are on
free range than in Eastern Nigeria where animals are usually confined.
60.      Alley cropping is a low-input system designed to bring about
improvements in yield when the fallow period is reduced. Similar to the
use of cover crops, this system has also inherent limitations. There is a
limit to the amount of bases that can be recycled from the deeper layers of
impoverished acid sands. With continuous cropping, therefore, decline in
soil fertility and reduction in crop yields are inevitable unless the
nutrient capital is augmented by external input or supplemented by
fallowing. Data from a relatively fertile Alfisol show that continuous
cultivation of maize-cowpea with alley cropping led to a serious decline in
total nitrogen (from a range of 0.214 to 0.397 in 1982 to 0.038 to 0.105%
in 1986). This drastic reduction was observed despite the addition of at
least 120 kg/ha of N (Lal, 1989). Although N contents decreased, the soil


- 22 -
test value for exchangeable K showed some improvements. Such a level of
improvement , however, was not observed for calcium and magnesium. The
overall fertility decline in these plots was also reflected in yield
decline.
61.      It is important to realize that restorative effects of forests and
agroforestry systems are usually over-exaggerated. With improper
management and intensive landuse without adequate input, severe land
degradation and fertility depletion can occur even with tree-based systems.
Undoubtedly with good management, however, the chances of attaining
economical success and ecologically compatible systems are better with
tree-based systems than with monoculture of seasonal crops. However,
without special conservation measures and supplemental applications of
nutrients, intensive landuse even with tree-based systems will inevitably
result in soil degradation in the form of decreased soil organic matter
content and nutrient levels and loss of top soil structure and porosity
(Lundgren, 1978; Lal, 1989).
62.      Wetland Rice: Of most of the arable crops production systems in
the tropics, the Asian wetland rice appears to be the most sustainable
since it is adapted to flooded areas where most other crops are not
adapted. Moreover, although lowland rice gives high yields when alternated
with upland crops, rice can be grown year in year out without significant
reductions in yields. While rice is becoming a very important cash crop.
especially with the fanning of rice imports in Nigeria, it has not become
as widely grown as expected in many valley bottoms and hydromorphic soils
where it has the highest potential. This is partly due to cultural reasons
and partly because of health hazards in hydromorphic areas.
Chemical Fertilizers
63.      It is apparent from the foregoing analyses that economic levels of
intensive cultivation cannot be maintained with traditional methods of
restoring soil fertility. Furthermore, excessive soil degradation now
prevalent in the region is driven by low-input, subsistence, and fertility-
mining farming practices. Although low-input systems are to be preferred,
it does not imply that use of synthetic fertilizers and soil amendments is
prohibited.
64.      Many experiments have been done since the early 1920's to evaluate
the agronomic and economic benefits of chemical fertilizers. Experiments
conducted during 1930s in acid soils of eastern Nigeria showed increase in
yields of maize by application of lime. Research data from the
Agricultural Research Station at Umudike showed significant increase in
yield of cassava tubers by the application of NPK fertilizers. Yield of
cassava tubers increased between the application rate of 0 to 630 kg/ha.
Another set of experiments conducted in 1960s demonstrated drastic
increases in yields of yam, maize, cowpea and cassava due to application of
fertilizers. Fertilizer application increased yields by 7 to 41% for yam,
90 to 150% for maize, 4 to 354% for cowpea, and 71 to 150% for cassava.
These improvements were obtained despite the use of Stylosanthes in
rotation.
65.      Similar beneficial effects of fertilizers on crop yields have been
reported from relatively fertile Alfisols of the subhumid regions of


- 23 -
western Nigeria. Research conducted at the University of Ibadan by Agboola
(1978) showed significant interaction between the level of soil organic
matter content and the rate of N application on grain yield of cowpea.
Expectedly, fertilizer response was evident only on degraded soils of low
organic matter content. That is why application of fertilizers is not
recommended for newly cleared lands. The data in Table 13 show that the
effect of 1% to 2% of soil organic matter contents were compensated by
addition of 50 to 100 kg/ha of nitrogen in soil with 0.5% of organic
matter. For coarse-textured sandy soils in the subhumid region, Lal and
Kang (1981) observed that use of crop residue mulch can partially
compensate the need for additional fertilizer. Retaining crop residue as
mulch increased yield of maize by 1700 kg/kg.
66.      It is apparent, therefore, that the use of fertilizers is
inevitable for increasing yields from impoverished and degraded soils in
southern states of Nigeria. However, more appropriate soil and crop
management systems need to be developed so as to reduce the amount and
frequency of their application while also increasing efficiency of their
utilization by crops. The nutrient requirements for high yields can be met
partly be decreasing losses and increasing efficiency, partly by symbiotic
nitrogen fixation through cover crops and alley cropping, partly by re-
cycling organic wastes and crop residue mulches. Nonetheless, the vital
difference in nutrients needed for high yields on continuous basis will
have to be met by adding synthetic fertilizers. Judicious management can
decrease the need for chemical fertilizers but cannot eliminate their use.
VIII. IS CONTINUOUS CROPPING ECOLOGICALLY AND ECONOMICALLY SUSTAINABLE?
67.      Presently, yields are limited by soil, environmental, and socio-
political constraints. Physically, most upland soils are easily compacted,
have low available water holding capacity, and are susceptible to
accelerated erosion. Structural deterioration sets in as soon as these
soils are cultivated intensively. Chemically, soils of southern Nigeria
have low nutrient reserves, especially of nitrogen, phosphorus and some
trace elements. Although the initial level of soil organic matter is
generally high, it declines rapidly with deforestation and intensive
cultivation. In addition to harvest, nutrient losses are also caused by
leaching, volatilization, especially during burning, and runoff and
erosion.
68.      Compound farming is an example of an intensive and economically
viable farming system at a small scale of farming. This technology,
however, cannot easily be extended to more distant plots. Subsistence and
degradative traditional farming systems, as practiced on most more remote
farm plots,can be replaced by conservative and economically productive
system provided that
the climate/vegetation/soil balance is kept at an ecologically
favorable level;
an adequate and regular supply of organic matter to the soil
surface is maintained;
the activity and diversity of soil fauna is enhanced to enable
favorable soil turnover;


- 24 -
.    structural conditions of the soil are improved to minimize
compaction and crusting;
.    a nutrient recycling mechanism is built into the system to
minimize nutrient losses due to leaching;
.    a desirable nutrient balance is maintained and risks of
acidification are minimized; and
.    nutrients harvested by plants and animals are replenished by
judicious use of chemical fertilizers.
69.      The success of compound farming is attributed to all the above
mentioned factors. However, nutrient replenishment through organic manure
at the level needed to restore a high level of productivity is not feasible
for distant plots. Improved systems must, therefore, be soil restorative,
ecologically compatible, and commercially or economically viable.
70.      A self-sustaining closed system, as the traditional true shifting
cultivation was, is outdated and a non-solution in the context of present
day realities. The system, at its best, was ecologically stable but at a
subsistence level of farming. While the desirable aspects of the system
can be preserved, the new system must enable intensive farming at a
commercial or semi-commercial level of production. For a soil low in
inherent fertility and depleted of its nutrient reserves due to natural
soil forming factors involving intense weathering, commercial farming
cannot be achieved without boosting its nutrient dowry. The nutrient
status can be improved by adoption of conservation-effective farming
systems, regular returns of some organic matter and biomass to the soil,
and supplemental applications of judicious levels and appropriate types of
inorganic fertilizers.
71.      Adoption of Improved Technologies: Based on the field surveys
undertaken in 1989, and a review of various technologies and associated
production systems that reduce soil degradation, an attempt has been made
to summarize the extent to which the different more sustainable production
systems and associated technologies are being adopted or used in Southern
Nigeria. (See Figure 5.) Reference to this table shows that most of the
sustainable production systems such as tree crop plantations are being
adopted widely. Tree crops are the most adapted and sustainable production
systems in the humid tropics. In most of southeastern Nigeria population
densities are very high and large scale plantations are out of the
question. But at the same time not much progress is being made in
rehabilitation of small-holder production systems by growing them with
arable crops in more permanent agroforestry systems. In areas of low
population density, where large scale plantations are appropriate, there is
an increasing tendency for widespread adoption of inappropriate mechanical
clearing techniques which usually result in irreversible soil degradation,
increased runoff, and low productivity. Unless tree crops or adapted
arable crops such as cassava and yams are promoted in humid areas, soil
degradation problems will continue to mount.
72.      Wetland rice, especially with the banning of rice importation and
high returns from lowland rice, is becoming widely adopted in low lying


- 25 -
Figure 5.   Extent of used or adoption of certain technologies in various sustainable
production systems that can potentially reduce soil degradation in the
southern states of Nigeria
Systems and Component
Technologies                 Extent of Use                   Remarks
A.FARMING OR PRODUCTION SYSTEMS
1. Tropical Tree Crops
a. Small holder                    +++            Declining yield, not being improved
b. Large scale                       ++           Increasing and destructive
2. Wetland Rice                        ++r           Only in suitable niches
3. Traditional Agroforestry
a. Home Gardens/Compound
Farms                          +++-           Mainly in rural southeastern Nigeria
b. Arable Crop/Tree Crop
Farms                                         In rural areas
c. Rural Hedgerow Cropping          ++            Rural areas especially southeastern
Nigeria
4. 'Modern' Agroforestry
a. Taungya system                                 Causes genetic erosion, declining
b. Alley cropping                    NT           Promising still in on-farm trials
c. Alley farming                     NT           Promising still in on-farm trials
d. Permanent Tree Crop/
Arable Crop Farms                +            Not yet well developed and of
reasonably low input
S. Intensive Livestock
Production
a. Poultry                          ++            Especially in urban areas
b. Mixed farming                     +            Restricted to a few poultry farms
6. Commercial Crop Production
a. Market gardens                  +++            Especially in urban areas
b. Large scale farms                 ++           Increasing but destructive
B. SUB-SYSTEMS OF PRODUCTION
a. Mixed cropping                    +++-           Mainly traditional, no improvement
b. Relay cropping                     ++-           Not being involved in rotations
c. Crop rotations                      +            More of continous cropping
developing
C. COMPONENT TECHNOLOGIES/PRACTICES
a. Traditional Land
Clearing                         +++-            Increasingly being abandoned
because of high cost


- 26 -
Figure 5. (Continued . . .
Systems and Component
Technologies                Extent of Use
b. Selective Mechaniza-
tion                         N                Techniques and benefits not yet
widely known
c. Reduced Tillage               +                Not yet well studied for various
crops
d. Manuring                     ++                Maintain home gardens and by
fallowing
e. Mulching and Good Plant
Residue Management            +               More common to traditional systems
than in modern ones
f. Fertilizer application         ++              Move to cash crops, not much in
food crops
g. Fertilizer/Manuring            ++              In home gardens and market gardens
h. Integrated Pest or
Disease Management           N                Limited research to make this
possible
i. Use of Resistant
High-yielding*Vrieties         +              Not yet possible for many crops
j. Fallowing
- long term tree/
shrub                     ++"              Length of fallow declining
- short term cover
crop                      N                Not being actively researched
Key
4++    Widespread or widely practiced
++    Common
+    Limited Use
NT    Negligible and under trial or testing
!+    Increasing in use
Declining in use
Restricted to specific situations


- 27 -
areas and inland swamps where cost of land development and water control is
not too expensive. However, the priority being given to this in
agricultural development is not commensurate with wetland rice
potentialities.
73.      Home gardens are fairly sustainable, but under increasing
population density and urbanization which is extending to rural areas, they
may be declining and with increased growing of arable crops, soil
degradation is becoming a serious problem. There is still a lot of
opportunity for development of arable crop/tree crop farms, but this has
not received the attention it deserves in research and development. The
hedgerow cropping system could be adopted in all rural areas along
boundaries between farms, but there may be need for some land consolidation
to ensure that areas occupied by hedgerows are not proportionately too
large.
74.      The taungya (forestry) system is of limited use since it is not
only destructive but requires using the force of the law to extend the
length of fallow in addition to strictly controlling farmer's execution of
operations. Alley cropping and alley farming need to be given higher
priority in research and more innovatively adapted to the farmer's needs
and circumstances to ensure economic viability and cultural acceptability.
75.      Poultry farming which is not operated as a mixed farming
enterprise often constitutes a serious source of environmental pollution if
the waste is not properly disposed of, or used in growing crops. Use of
poultry waste in market gardens or use of compost in market gardening has
already been found to be a reliable practice that ensures sustainability
provided labor is available. The returns from the market gardening also
facilitate purchase of some limited amounts of fertilizers. Large scale
arable crop farms will not be sustainable until they are executed with
careful mechanized use of reduced tillage systems, adequate plant residue
management in addition to fertilizers, and suitable rotations.
76.      Monocropping, of annual or short-duration crops is highly
degradative and cannot be sustained even with high levels of fertilizer
application, which are not cost effective, but also have low residual
effect, without use of adequate levels of organic matter in a well proven
rotational sequence of cropping. The use of well designed and well tested
rotations of simple crop mixtures is imperative in the development of
sustainable arable cropping systems in the humid tropics. This needs to
receive higher priority, but because of the current emphasis on shorter
term evaluation of cropping patterns, it may take some time before this can
be done.
77.      Fallowing with trees and shrubs while ecologically sound, is
increasingly no longer economically viable and has to be replaced with more
intensive systems such as alley cropping or actively managed fallows, e.g.,
shorter duration (possibly one year), fallows with herbaceous legumes and
judicious application of fertilizers. Fertilizer use, adequate organic
residue management, reduced tillage systems and integrated pest and disease
management systems are ingredients that must be components of sustainable
agricultural production systems.


- 28 -
IX. RESEARCH AND DEVELOPMENT PRIORITIES
78.      This paper has indicated the complex nature both of the processes
of land degradation itself and of the social and economic conditions which
appear to facilitate its occurrence. No individual deliberately set out to
destroy these resources, but this can be the cumulative effect of actions
undertaken for other purposes. Given the rapid increase in population and
consequent pressure on land resources, their conservation is a justifiable
objective of national policy.
79.      While individual actions are important in causing or alleviating
land degradation, a sustainable pattern of land use, particularly in more
fragile areas, cannot be achieved by the independent and random actions of
individual land holders and users. It will require collaborative action at
the local level. In order to achieve this a national effort will be
required with actions in a number of areas:
(a) increasing public awareness of the problem; this will include
steps to raise the profile of related activities in the
public sector at the national and state levelsand ensure
adequate interagency coordination;
(b) research to increase understanding of the nature of the
problem and of the type of technologies and farming systems
discussed above;
(c) demonstration and extension efforts. In addition to
individual actions, soil and water conservation efforts
require collaboration among resource users at the local
level. Measures to foster and support such collaboration
will be required.
A. Interagency Collaboration
80.      A considerable number of institutions have been created over the
past two decades to address different aspects of soil and water management
and this structure is still evolving (FAO/CP, 1990). At present there is
excessive overlap of theoretical responsibilities, which leads to
inadequate and fragmented funding and, in consequence, poor liaison and
very limited action. There is need or one top level body and there are
indications that the Natural Resources Conservation Council (NRCC),
announced in 1989, may play this role, especially if the President assumes
its chairmanship. NRCC will be empowered to insist that State level
agencies carry out conservation programs. It will be advised by the
National Committee on the Conservation of Renewable Resources (NCCRR) which
was created at the same time and includes Directors of the relevant federal
departments, and representatives of other national agencies and the
President's Office of Budget and Planning.
81.      Several states have established Interministerial Committees or
Task Forces to address resource conservation issues. If local initiatives
are to be fostered it will be essential that the state bodies be given a
high profile and the full backing of state leadership.


- 29 -
82.      As indicated below a broad program of investigation and research
will be necessary to support action programs. In order to focus these
efforts it would be desirable to establish a National Soil Degradation
Study Group charged with the responsibility for (1) conducting adaptive
research in selected areas aimed at utilizing results of research to design
more practical methods of combating soil degradation in agriculture, and
determining constraints to farmers adoption of the practices and what
should be done to enhance adoption; and (2) monitoring of soil degradation
processes in different situations based on standardized procedures and
quantifying the extent and contributions from various processes. This body
could function under the umbrella of the NCCRR and the membership should
consist of university scientists in relevant disciplines in addition to
staff of some Federal research institutes. The adaptive research could be
conducted in the different sectoral activities in agriculture, forestry,
fisheries, construction, mining, etc. There is need for careful detailed
discussion and consultations to ensure that this body functions effectively
with the Federal Department of Agricultural Land Resources without being
stifled by bureaucratic civil service procedures and might have a semi-
autonomous status under a director possibly from the university system.
B. Research
83.      A principal constraint in improving agriculture in southern
Nigeria is the lack of reliable data on present status of the resource
base. No one knows for sure the type, degree, and extent cf soil
degradation. Most data, as reviewed in this report, are clincerned with
principles and processes of soil d6gradation observed oi researcher managed
on-station experimentation. There are few, if any, studies on evaluation
of soil resources on farmers' fields.
84.      Applied Research: It is difficult to improve soil characteristics
through better management without better knowledge of the current status of
soil properties and the rate of their change under different farming/land
use systems. An organized effort must, therefore, be made to conduct farm
surveys regarding the current status and future trends in the prevalent
farming systems, crop rotations and combinations, cultural practices, the
amount and type of inputs, crop yields and economic returns. In addition,
detailed and reconnaissance surveys should be undertaken to characterize
vegetation, water and soil resources. Vegetation surveys should establish
the successional changes in predominant species following deforestation,
uncontrolled burning, traditional farming and intensive land use. Water
resources should be evaluated in terms of stream flow, ground water level,
and their dissolved and suspended loads. Current status of the soil
(Physical, chemical and biological properties) must be related to the past
and prevalent farming systems, and compared with the initial level of soil
characteristics in an undisturbed state. The latter may be hard to find
except in forest reserves and communal protected lands. Traditionally, a
wide range of plants have been used for food, agricultural or other
purposes. However, in most cases little is known about their cultural
characteristics, which may include useful attributes which can be utilized
in a farming system. For example acia which has traditionally been used
in the south-east grows more vigorously in the sandy, infertiles acid soils
of the region than Leucaena or GliricidiA, which are generally recommended
for alley type systems or contour hedging. A systematic review of useful
species (see Okafor, 1987) for this purpose, should be a high priority.


- 30 -
85.      Standardization of methodologies used for field and laboratory
evaluation is an important consideration. While traditional methods of
soil survey can be upgraded, efforts must be made to use modern
technologies to assess land and water resources. Remote sensing provides a
unique opportunity to evaluate soil, water, and vegetation. Methodologies
for using remote sensing are available to assess change in land use,
vegetation cover, extent of the damage by fire, extent and rate of gully
erosion, level of water in the reservoir and stream (Paul et al., 1989;
Estes and Cosentino, 1989). Some soil properties and sediment load in
reservoirs and rivers can also be assessed by methods based on remote
sensing.
86.      Establishing farm demonstrations is vital in showing farmers the
use and benefits of new technologies. Farm demonstrations should be
established in indicate the benefits of different components or sub-systems
of improved agricultural practices. An example of the method of step-wise
evaluation of sub-systems is shown below:
A.   Traditional system.
B.   A+ improved cultivar, cropping systems and agro-forestry.
C.   B+ improved tools and labor-saving devices.
D.   C+ chemical fertilizers and amendments.
87.      Systems A through D involve incremental improvement of a sub-
system at a time. The farmer can judge by him/herself, the return from
each step. Local specific components for each of the steps listed above
are already available for major soils and ecological regions of Nigeria,
and may need only slight validation and adaptation. Benefits of chemical
fertilizers may be more substantial with improved cultivars, better
cropping system, and efficient tools and implement than with traditional
farming.
88.      Basic Research: Over and above the need for applied research
discussed above, there is also a need to conduct some basic research. The
term soil degradation is an emotional rhetoric and is used rather vaguely.
To avoid ambiquity and confusion, it is important that predominant soil
degradation processes in southern Nigeria be defined on a more quantitative
basis. To do so will require basic research in delineating critical limits
of soil properties beyond which crop growth and economic yields are
seriously jeopardized. These critical limits, of the life-governing soil
processes and properties that support them, are not known. Some of these
processes are affected by interactions, such as those between organic
matter and soil structure, soil structure and water availability, texture
and nutrient status, etc. These interactions are specific among soils and
are not known.
89.      Furthermore, critical limits of soil properties must be defined to
delineate different levels of soil degradation. These limits vary among
soils, antecedent soil moisture regime, climatic conditions, land use,
crops and agro-ecological regions. It is impossible to assess the type,
extent, and rate of degradation without knowing these limits for
predominant soils of south and southeastern Nigeria. For example, if the
critical limits of organic matter content, water and nutrient status,
porosity, compaction and effective rooting depth are not known for major


31 -
soils and crops of the region, it is difficult to judge whether a soil is
degraded and if so to what degree and by which process(es). Such
information is currently not available. It is apparent from the literature
presented that we do now know yields of major crops (yam, cassava, corn,
etc.) in relation to soil organic matter content, top soil depth, porosity,
plant-available water reserves, and soil test values for lime, N, P, and
basic cation. Without the benefits of this basic information, we cannot
judge the restorative effects of conservation-effective measures. Land
evaluation criteria, developed through the basic research proposed above,
should indicate: when a soil should be taken out of production and put
under a restorative and ameliorative phase, or when a soil is sufficiently
restored and should be put under cultivation.
C. Extension and Eblic Awareness
90.      It is recommended that initial efforts focus on two activities;
preparation of materials for general dissemination to increase public
awareness; and targeted demonstrations of improved technology. As an
initial step a small public enlightenment and education bulletin could be
prepared aimed at:
.    Indicating the nature and magnitude of soil degradation
occurring in Nigeria.
.    The causes of soil degradation and processes involved.
Actions being taken to understand and combat soil erosion and
limitations of such actions.
.    What really needs to be done that is not being done.
.    Roles of governments, private sector, non-government
organizations and private individuals in combating soil
degradation.
The bulletin should be non-technical, simple and well illustrated
and aimed at the man in the street, farmers and policy-makers.
91.      Demonstrations of various soil conservation and management
practices that have been proved to work in different situations either to
prevent soil degradation occurring or to check incipient manifestations of
soil degradation from becoming more serious and expensive to remedy. These
demonstrations should encompass agricultural soil conservation measures,
forestry, road and building construction mining and other sectoral
activities wherever they can be located and effectively demonstrated. The
agricultural demonstrations should be looked after by state agricultural
extension officers. Those dealing with road and building construction
should be under the Ministry of Works, Housing and Environment. There
should be arrangements for special field days to be held each year at
certain times for the public to see them and for students to visit such
sites as part of their educational training in agriculture, geography,
science and civics.
92.      In order to foster adoption of modified technologies to ensure
conservation of soil resources, awareness campaigns and demonstrations will


- 32 -
have to be backed up by action programs which provide some form of
incentives, and/or requirement, for group action by villagers. This might
include the preparation of village conservation plans as a basis for
provision of required inputs on a concessional basis or, perhaps, as a
condition for participation in other public programs. Effective
implementation of such a program would itself require a major effort to
increase the numbers of appropriately trained extension personnel and
possibly changes in the organization of extension. These issues, however,
are outside the scope of this report.


- 33 -
Table 1. Changes in chemical properties of the surface 0-15 cm layer of an Alfisol near lba6an, western Nigeria due to
cultivation (Wilson et al., 1982).
Forest fallow  Continuous cultivationl/   Planted fallow for 2 yrs
Soil property              (20-25 yrs)         (5 yrs)           Grasses            Legumes
pH (1:1, H20)                  65                  55                52                 5.6
Organic carbon (%)             1.7                 12                1.5                1.6
Total N (%)                    0.16                0.12              0.18               020
Bray P (ppm)                  15                  37                60                 33
Cat2 (meqI100g)                4.8                 4.0               62                 6.7
Mg+2 (meql100g)                1.3                 08                1.3                1.6
K+ (meqll 00g)                 0.6                 0.4               0.9                0.9
M/With application of fertilizer at the rate of 100 kg N, 60 kg K and 50 kg P per ha per year for maize-cowpea rotation.


.                              34 �
iаЫе 2. Decline in feпility of an Alfisol пеаг lbadan in Оуо state ипдег con�nuous cropping, (la1 t 985).
Pгe-cultivation, yrs   �      2 угs      4 угв                 6 yrs
Propetty           NT         Р             NT        Р   NT     Р           NT
рИ (з ;1 in H2f3)   в.з t о.з   s.з � о.а   s,a t о.в   5.в t о.з   s.в t о.за   5.в t о.зз   s.a t о.э   а.а t oz
Organic сагbоп (�,)   з.а � а.а   з.т t о.э   з.sг t o.s2   з.эг t о.зз   з.ss t о.за   з.зт t о.зз   з.sо t о.зт   з.гэ t о.оа
Totat N(�о}           а.за t о.зз  о.ао � о.оа   о.зв t о.оа   о.г2 � о.оа   о.г4 t о.зз   оzз t о.з2   o.зss � а.оа   o2os � о.оа
егау•t Р(ррт�          аtа          аtз          гs.зts.o   зз.5ts.e          э2tгв         ззtв         зstв        sstзв
E�cchangeablе cations (meq1100 g)                     �
�а+г      в.о ± з.т     тs � з.в          s.o t z.з       т.з t з.а         s.з � os     sz t з.з      з.о t з.а   � s� о.а
Mg+2      з.а t о.в     г.о � о.т         ts t а.т        з.9 t os         о.5а t о.от   oss t о.за   о.аз t оzз   о.зз t о.оз
�
к¢        а.т ± о.г     о.а t о.2              о.вт ± о.з    o.9s t од     о.аа � о.оа  о.за t о.от   о.зs ± o.os   о.за ± o.os
N8+         -             -                    ОА9 * о.03    0.09 t 0.05   А.з з t о.о2   а.1 з t 0.02   а.г4 t аАг   0.21 t о.02
мп+г      o.os ± о.о2 o.os t о.ог              o.os ± о.оз   о.оз t о.о2     --             -            о.за ± o.os   о.з±о.зз
т'otal aad;ty   -        -                       о.ов t о.аа   o.os ± о.оз     -              -            о.зs ± о.2з   o.ss ± o.as


- 35 -
Table 3. Changes in properties of the surface layer (0-15 cm) of an Alfisol near lbadan, western Nigeria, due to
cultivation (Wilson et al., 1982).
Forest fallow   Continuous cultivation      Planted fallow for 2 yrs
Soil property             (20-25 yrs)         (5 yrs)            Grasses             Legumes
Clay                          184                212                 -                   -
Silt (%)                      10.3                9.5                -                   -
Sand                          71.3               69.3                -                   -
Gravel                         5.0               20.0                -                   -
Bulk density (g/cm3)           1.00                                  1.33                129
Infiltration rate (cm/h)     100                  9                 17                  23


- 36 -
Table 4.   Effects of methods of deforestation aad cropping on bulk density and penetrometer resistance of 0.5 cm
layer of an Alfisol in Oyo stato (Lal, 1984).
Bulk density (glcm3)                     Penetrometer resistance (k/cn2j
Pre-                                      Pre-
clearing                                  clearing
Treatment        1978     1979       1980       1981       1978      1979        1980     1981
Traditional farming 0.64  1.06       1.07       1.27       0.21       0.96       0.52      1.32
Manual clearing  0.68     1.17       1.17       1.39       0.20        1.4       0.75     1.19
Shear blade      0.70     1.19       1.37       1.38       0.26        1.0       1.84     2.19
Tree pusher/root  0.60    1.24       1.32       1.42       0.20        1.3       0.73      1.23
rake
Each figure is a mean of 25 separate analyses.


-37 -
Table 5. Decline in grain yield of maize on newly cleared land with continuous cultivation for an
Alfisol in Oyo State (Couper et al., 1979).
GQrain.vield (t/havr)
Year                              Plowed                             No-till
1975                                 2.7                                2.8
1976                                 4.0                                4.5
1977                                 3.9                                4.8
1978                                 4.0                                5.0
1979                                 2.9                                3.8
1980                                  1.0                               3.0
Yield is total of two crops per year.


- 38 -
Table 6i Soil and management effects on crop yields from distant farms in three villages in eastern
Nigeria (Lagemann, 1977).
R2       F
(a) Linear function
Y = 231.9 + 64.5 C + 13.2 P + 616.7 F - 52.7 M - 31.2 W + 0.19 T  0.49  41"
(b) Cobb Douglas function
Y = 1366.5 + 0.07 C + 0.14 P + 0.11 F - 0.06 M + 0.09 W - 0.05 T  0.41  30*
(c) Quadradic function
Y = 579.2 + 1980.08 C - 502.2 C2 + 3.6 P - 0.06 P2 - 477.0 F + 162.8 F2  0.64  38*
- 194.6 M + 2.34 M2 + 76.02 W - 22.8 W2 - 0.35 T + 0.001 T2
Y = total yield (dry matter, in kg/ha) of all arable crops grown as mixture
C 7 soil organic carbon (%)
P= Bray-1 P (ppm)
F= fallow period (years)
M = planting month
W= month to first weeding
T= tree density


- 39 -
Table 7.  Effects of farm types on soil fertility status of 3 sites in eastern Nigeria
(Lagemann, 1977).
Exchangeable cations
(mea/100 a
Organic                                Bray-P     Total
Site/Farm              Carbon    pH       Ca*Mg       K        ppm      Nitrogen
(%)
A. CoTound -farms
Umuokile (25)         2.06    5.06     3.50      0.14       36.2      0.168
Owerre-Ebeiri (63)    1.06    5.06     2.77      0.17       19.2      0.085
B. Near farMs
Okwe (35)             2.35    4.59     1.64      0.09       25.01     0.178
Umuokile (32)         2.30    4.71      1.90     0.06       10.58     0.170
Owerre-Ebeiri (63)    1.20    4.45      1.28     0.11        9.77     0.092
C. Distant farms
Okwe (33)             2.37    4.47     0.89      0.08        9.78     0.169
Umuokile (44)         1.94    4.71      1.61     0.05"       8.03     0.151
Owerre-Ebeiri (63)    1.00    4.26     0.69      0.08        9.98     0.067
Figure in parenthesis is the number of soil samples analyzed.


- 40 -
Table 8. Fertility status (0-10 cm  depth) of compound and distant farms for 25
farms at each of five locations in eastern Nigeria (Armon, 1984).
Farm type                  Agula/    Calabar   Isiekenesi    Ohafia/   Uyo
Nanka                             Abriba
A. Compound farms
pH                       5.6         6.1       5.72        6.04       5.81
Organic carbon (%)       0.94        2.50      1.10         1.80      1.90
Total nitrogen (%)        0.1        0.15      0.14        0.16       0.18
Bray-P (ppm)             30.4      116.2      37.7        92.0      184.4
Exchangeable cations:
(meq/100 g)
Calcium              3.00       4.33      3.16         3.20      5.44
Magnesium            0.63       0.46      0.52        0.66       0.82
Potassium            0.24       0.21      0.23        0.13       0.25
B. Distant farms
pH                       5.11        5.28      5.28        5.50       5.03
Organic carbon (%)        0.75       0.83      1.67         1.22      1.30
Total nitrogen (%)        0.08       0.13      0.19        0.23       0.12
Bray-P (ppm)             10.3       13.40     22.4        52.1       37.3
Exchangeable cations:
(meq/100 g)
Calcium              1.24       1.40      1.54        2.0        1.30
Magnesium           0.31        0.20      0.34        0.40       0.25
Potassium            0.10       0.08      0.13        0.08       0.06


- 41 -
TABLE 9 . Extent of fire damage and soil degradation manifested by cassava,
other crops, plants and soils
State           Extent of fire damage              Condition of crops and soil as
observed along roads passed        observed on roadsides
Akwa Ibom       Not visited                        Not visited
Anambra         5-15% burn on roadside in          Poor cassava crop in most areas
between villages. Crop damage      exhibiting symptoms of mealy bug
slight. No serious damage in       and spider mite damage. Soils
planted fallow areas and home      degraded colored light gray, low
gardens.                           in organic matter. Spear grass,
common on some degraded soils.
Bendel          5-20% but along Benin Auchi        No serious stunted growth observ-
road up to 10-20% in some          ed in cassava crop except on the
places. Fire damage to             Benin-Asaba roadsides where con-*
economic trees and crops           tinuous cassava production in
(rubber, oil palms, cassava        some areas has resulted in poor
and cocoa) serious in some         growth.
areas.
Cross River     Not visited                        Not visited
Imo             5-15% crop damage slight.          Some areas with sandy soils de-
No serious fire damage             graded, soils low in organic
observed in areas with             matter, cassava in such areas
Acioa/Anthonotha fallow on         showing stunted growth as one
roadside.                          goes northwards.
Ogun State      10-25% and damage to economic      No serious stunting of cassava
trees (cocoa cola, oil palms)      crop but there are areas with
suffered slight to medium fire     invasion by spear grass.
damage.
Ondo State      10-25% in many stretches along     Cassava with stunted growth to-
Akure-Benin roads many economic    wards northern parts of the state.
plants (cocoa, oil palms,          Moisture stress serious in
cassava and cola) damaged by       bananas/plantains.
fire. Many teak plantations
were affected.


- 42 -
TABLE 9. (Continued . .
State          Extent of fire damage              Condition of corps and soil as
observed along roads passed       observed on roadsides
Oyo            15-30%, a lot of damage to         Some areas toward northern
economic plants (cocoa, oil       areas of the state with
palms, cola and cassava,          cassava exhibiting stunted
especially along Ibadan, Ife      growth, some areas with spear
and Ibadan-Oyo road. .            grass invasion.
Rivers          5-10% limited area visited.       Not much stunting noticed in
Only areas between Owerri         cassava crop except in some
- Port Harcourt and Port          areas near boundary with
Harcourt Aba roads observed.      Imo state. Some moisture
Slight fire damage to             stress noticed in plantains.
plantains, oil palms, cassava
and teak plantations.


- 43 -
Table 10. Effects of methods of deforestation on runoff and erosion (Lal, 1981).
Treatment                     Runoff (mmlyr)            Soil erosion (t/ha/yr)
Traditional farming                         3                         0.01
Manual clearing                            35                         2.5
Shear blade                                86                         3.8
Tree pusher/root rake                     202                        17.5


- 44
Table l1Effects of land clearing methods on degradation of soil fertility and chemical properties
for 0-10 cm layer of an Alfisol in Oyo State (Lal, 1981).
Land clearing
method                    ECEC         CA+2         Mg+2           K+
(meq/100 g)
Mechanical                    8.2          7.0          1.7           0.08
Slash and burn                14.0        11.3          2.4           0.10
Slash                         9.5          7.6          1.6           0.08
LSD (0.05)                     1.4         1.1          0.4           0.007


-45-
Table 12.7Illage effects on soil fertility and chemical properties of the 0-S cm soil layer of an
Alfisol at IITA, Ibadan, Nigeria (Lal, 1986).
Property                          No-till                    Plowed
PH (1: 1 in H20)                         5.9 :t0. 1                  5.3±*0.2
Organic carbon M%                      1.69 ± 0.27                 1.44 ± 0.07
Total nitrogen M%                     0.179 ± 0.04               0.093 ± 0.007
CEC (meq/100g)                          7.84 ± 1.4                 3.99 ± 0.91
Exchangeable C12+ (meq/100g)           5.31 ± 0.84                 2.87 ± 0.62
Exchangeable Me+~ (meq/100g)           0.97 ± 0.21                 0.31 ± 0.08
Exchangeable K+ (meqtlOOg)             1.06 ± 0.46                 0.32 ± 0.16


- 46 -
Table 13.Average effect of N and soil organic matter on the yield of
Ife - Brown Cowpea, 1973 - 1975.
Teanent          0.5%       1%       2%          3%.          5%
NO N                00 c    1,200 b   1,700a   1.750 a        1,800 a
10 kg N/ha       1,3006    1,650 a   1,800 a   1.700 a        1,750 a
20 kg N/ha        1,850 a   1,600 a  1,750 a   18008a        1,700A
50 kg N/ha        1,870 a   1,700a    1,800 a  1,850 a        1,800 a
100kg N/ha       1,750a    1,800 a   1,800 a   1,750 a        1,800 a
Note: Numbers within columns followed by the same letter are not
different (P = 0.05) according to Duncan's multiple Range
Tes t
+ All treatments had basal dressing ofP and K based on
soil test.
*Source: Agboola (1978).


- 47 -
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