organic matter and soil fertility restoration in forest and savanna fallows in southwestern nigeria

GeoJoumal 6.2 183-192/1982 © Akademische Vedagsgesel lschaft • Wiesbaden

183

O r g a n i c M a t t e r and So i l F e r t i l i t y R e s t o r a t i o n in F o r e s t and

Savanna F a l l o w s in S o u t h w e s t e r n N i g e r i a

Areola, O., Aweto, A.O. and Gbadegesin, A.S., Department of Geography, University of Ibadan, Ibadan, Nigeria

Abstract: This paper compares the trend and relative importance in soil fertility restoration, of organic matter build-up in forest and derived savanna fallow soils in parts of SW Nigeria.

The analysis of the trend of organic matter build-up in each area was based on the comparison of the mean levels of soil organic matter in fallow soils of different ages as well as the comparison of those mean values with the equilibrium level of organic matter that is attainable in soils under the climax vegetation in each ecological zone. Intercorrelations between all the soil properties were computed in order to assess the relative importance of soil organic matter vis-avis soil intrinsic properties in soil fertil ity restoration during the fallow period. The significance of improvements in soil conditions with length of fallow was tested by computing correlation coefficients between age of fallow and soil properties.

With regard to the forest fallow soils the results show that there is a progressive build-up of organic matter over the years. The build-up is very rapid initially and by the tenth year of the fallow period the organic matter in the soils has attained 77% of the equilibrium level. Thereafter, the rate slows down as the equilibrium level is approached. Compared with soil textural composition, organic matter exercises a greater influence on other soil properties and, therefore, on soil fertil ity regeneration during the fallow period. In the case of the derived savanna fallows the trend of organic matter build-up and of improvements in soil conditions with length of fallow is rather erratic. There is not as strong a correlation between age of fallow and improvements in soil organic matter content and other soil properties as in the forest region. The build-up of organic matter in the savanna fallow soils is rather slow and fortuitous. This is probably due to the smaller vegetation biomass and the annual burning of the bush and the soil litter layer. The slower rate of organic matter build-up in savanna fallow soils can also be explained in terms of the lower equilibrium level. In contrast to the situation in the forest region the influence of soil textural properties appears to be greater than that of the organic matter in soil fertility regeneration.

I n t r o d u c t i o n

The restoration of the fer t i l i t y status o f the soil under

fa l low largely depends on the extent to which organic

matter builds up in the soil. Organic matter exercises a very

strong influence on the nutr ient status of fa l low soils be-

cause Kaol in i te, the dominant clay mineral in most tropical soils, has a very low cation-exchange capacity. Consequent-

ly, soil organic matter is the chief soil component that determines the cation-exchange capacity o f these tropical

soils (Nye and Greenland 1960, Ol lat and Combeau 1960,

Nye 1963, Jones and Wild 1975). The impl icat ion o f this

is that soil organic matter largely determines the capacity

of fa l low soils to retain plant nutrients. In addi t ion, soil

organic matter is a major source of p lant nutr ients such as phosphorus, nitrogen and sulphur and also nutr ient cations

such as calcium, potassium and magnesium which are re- leased s lowly into the soil when organic matter is mineralis- ed. In fact, the extent of nutr ient bui ld-up in fa l low soils

depends mainly on the amount of organic matter that accumulates in fa l low soil (Nye and Greenland 1960).

Soil organic matter also exercises marked effects on

184 GeoJoumal 6.2/1982

soil physical properties. Restoration of soil crumb structure and improvements in soil water holding capacity during the fallow period depend mainly on soil organic water. It has been observed that soil structure improved when land is under grass or bush fallow as a result of the build-up of soil organic matter (Pereira et al. 1954, Stephens 1960, Wilkin- son 1960).

The brief review above points to the importance of organic matter in restoring and maintaining the fertility status of soil under fallow. In spite of the overwhelming importance of organic matter in maintaining the produc- tivity of fallow soil, relatively few studies have been carried out to investigate the trend of organic matter build-up in the soil under natural fallow, that is, the natural bush regrowth that develops on "abandoned" farmlands which helps to restore soil fertility status. Even the comprehen- sive review of Nye and Greenland (1960) on the soil under fallow and the recent account of Jones and Wild (1975) on savanna soils did not include a systematic account of the trend of organic matter build-up in fallow soil. The work of JuG and Lal (1977) which marginally relate to the organic matter status of fallow soils was confined to the first few years following the cessation of cropping and as such did not give an adequate account of the trend of organic matter build-up in fallow soil.

This paper, using the results of three research investiga- tions conducted in parts of SW Nigeria attempts a com- parative analysis of the trend of organic matter build-up in fallow soils in two major ecological zones: the forest and the derived savanna. Furthermore, it examines the effects of organic matter build-up on soil properties in the two ecological zones.

The Study Areas

This paper is based on research projects conducted in three parts of SW Nigeria namely: (1) the Ijebu-Ode/Shagamu area of Ogun State (Aweto 1978), (2) Ibarapa division (Areola 1979) and (3) Oyo Local goverment area (Gbadegesin 1979). The first study area is a part of the rain forest zone while the latter two occur in the derived savanna zone of Oyo State.

The Ijebu-Ode/Shagamu area is underlain by sedimentary rocks, mainly sandstones of Cretaceous age. The soils are ferrallitic tropical soils derived mainly from the sandstones. They are intensely weathered and consequently they contain low amounts of weatherable mineral reserves. Kaolinite is the dominant clay mineral and as a result the soils generally have a low cation-exchange capacity. The natural vegetation is rain forest. However, over much of the area, the original forest has been cleared and replaced by fallow vegetation in different stages of seconda'ry succession as a result of the prevalent practice of rotational

bush fallowing. Mature forest is now largely confined to sacred groves which have been deliberately preserved. The length of the fallow period usually ranges between three and seven years, but in some cases it may be as long as ten years. The mean annual rainfall is about 1,500 mm.

Ibarapa division and Oyo Local government area are both underlain by crystalline rocks of the basement complex formation. The soils are mainly ferruginous tropical soils. Like the soils of the Ijebu-Ode/Shagamu area, these soils are predominantly sandy and contain mainly kaolinite clay minerals. The natural vegetation in Ibarapa division and Oyo Local Government areas in the distant past was probably dry forest such as is found in the nearby Oloke- meji Forest Reserve. However, over the years, the natural forest vegetation has been degraded into derived savanna vegetation, largely because of intensive cultivation coupled with severe and recurrent annual burning. Fallow periods in both Ibarapa division and Oyo Local government area tend to be longer than in the Ijebu-Ode/Shagamu district and usually range between five and fifteen years. Ocassionally, the land may be left fallow for as long as twenty years or even longer. The mean annual rainfall for Oyo Local Government area is between 1,250 and 1,500 mm per annum while it is between 1,000 and 1,250 mm in Ibarapa. The most important arable crops grown in the two derived savanna areas are yam, tobacco, cassava, melon and vegetables.

Materials and Methods

Perhaps the best and most reliable approach to the study of organic matter build-up in fallow soil is to monitor on the same plot or plots, the changing levels of soil organic matter for a very long period. Organic matter accumulates in the soil very slowly, and this is particularly so in savanna fallows, and as such it would take several years or even decades to obtain adequate data for a meaningful analysis of the trend of organic matter build-up if this approach is adopted.

Consequently, an inferential approach which involved the side-by-side comparison of soils under fallows of different ages in separate geographic locations was adopted in these studies. For such an approach to be valid, the factors that affect soil organic matter content in each of the two ecological zones studied namely texture, ground drainage and land use practices should be kept constant or at least, their effect substantially reduced. The effects of ground drainage were eliminated by selecting only fallowlands on well drained sites. The effects of soil textural composition were reduced by selecting areas that are lithologically homogeneous for investigation.

Although the forest study area consists of sedimentary rocks and the savanna areas of basement complex rocks, this lithological difference between the two ecological zones is not likely to significantly affect the trend or extent

GeoJoumal 6.2/1982 185

of organic matter build-up since all the study areas have soils of similar textural composition. The soils in all the study areas are predominantly sandy. Within each study area, fallowlands were selected for study from a zone that is homogeneous with respect to farming conditions and land use practices. This was to ensure that comparison between the different age-grades of fallows would be valid and the trend of organic matter build-up in fallow soils mean ingful.

The fallow categories studied in the three study areas differed from one another. This was largely because the length of time the land is left fallow varies from one area to another depending, among other things, on population pressure on available cultivable land, inherent soil fertility and the length of time it takes before the soil restores its fertility status after cropping.

The choice of fallow categories studied in each of the study areas was largely determined by the traditional practice of shifting cultivation in that area particularly the maximum length of time the land is left fallow before it is put under cultivation. In the Ijebu-Ode/Shagamu area, that is the forest zone, fallows that belonged to the 1-year, 3-year, 7-year and 10-year categories were selected for study. In addition, soils under mature secondary forest communities of about 80 years old were studied as a control for assessing the extent of organic matter build-up in the different age-grades of fallow not exceeding 10 years old.

Ten sample plots, were studied for each fallow category and for mature forest community, making a total of 50 sample plots on the whole. In Ibarapa division eleven fallowlands that belonged to the 5-year, 10-year, 15-year, 25-year and 30-year age categories were studied. The number of sample plots for each category is 3, 2, 1, 3, and 2 respectively. Twenty five fallow lands that belonged to the 1-year, 5-year, 7-year, 9-year and 15-year age categories were studied in Oyo area. The number of fallowlands selected for each age-grade of fallow is respectively 5, 3, 5, 4, 4 and 2.

In each of the three study areas, soil samples were collected from a uniform predetermined depth of 0 - 10 cm, that is the top 10 cm of the soil profiles. The soil samples collected were air-dried, passed through a 2 mm sieve and analysed for the following parameters: (1) organic matter, (2) particle size composition (3) water-holding capacity (or moisture content) (4) bulk density (5) total porosity (6) total nitrogen (7) available phosphorus (8) exchangeable cations (calcium, potassium, magnesium and sodium) (9) cation exchange capacity and (10) pH.

Soil organic matter was determined by the Walkley- Black method and soil particle size composition by the pipette method. Bulk density was determined by the core technique (Blake 1965) while total porosity values were computed from those of soil bulk density using the assum- ed particle density value of 2.65 g. cm - 3 (Vomoci11965).

Total nitrogen was determined by the Kjeldahl method; available phosphorus by the method of Bray and Kutz (1945) and soil cation exchange capacity by the summation method (Chapman 1965). Exchangeable calcium, potassium and sodium were determined by flame photometry while magnesium was determined by atomic absorption spectro- photometry. Soil pH was determined potentiometrically in 0.01 m calcium chloride solution using a soil to solution ratio of 1:2.

In order to examine the effects of organic matter build- up on soil properties, bivariate correlations between soil organic matter and soil physical and chemical properties were computed. Intercorrelations between soil properties were also computed so that the relative effects of soil organic matter and other soil properties, particularly the intrinsic properties such as textural composition could be compared. Correlation coefficients between age of fallow and soil organic matter content were also computed so as to find out the nature and strength of the relationship between the length of the fallow period and soil organic matter status. The correlations discussed at length are those that are statistically significant at least at the 5 % level.

Since this paper is based on the results of three different research projects, a few soil parameters such as bulk density, total porosity and available phosphorus were not analysed for all the three study areas. In all cases, however, the same analytical procedures were used in analysing the soil samples collected from the three study areas.

Results and Discussion

1. Trend of organic matter build-up

Tab 1 shows the organic matter content of the soils under fallows of different ages in the three study areas. With respect to forest fallows, in the Ijebu-Ode/Shagamu area, it can be observed that there is a progressive build-up of organic matter in the top 10 cm of the soil profiles with an increase in the length of the fallow period. The mean organic matter contents of the 1-year, 3-year, 7-year and 10-year fallows and for mature forest are 2.49 %, 2.18 %, 3.10 %, 4.18 % and 5.57 % respectively. The mean level of soil organic matter under the 3-year fallows is slightly lower than that of the 1-year fallow and this suggests that the first three years or so of the fallow period are characterised by a slight decline in the amount of soil organic matter. Slight declines in the level of soil organic matter during the first three years of the fallow period have similarly been recorded by Juo and Lal (1977) in Ibadan area, also within the rain forest zone.

The reasons for this phenomenon are not well under- stood. In the Ijebu-Ode/Shagamu area, however, the slight decline in the level of soil organic matter during the first three years of the fallow period, is presumably due to the


Tab 1 The mean organic matter contents of fallows in the three

study areas

fact that fallowlands are dominated by forb vegetation during this period. The rate of litter production by the forb vegetation is much less than that of woody perennials, mainly trees, which latter replace it at about the fifth year of the fallow period. It seems that the rate of litter production by the forb vegetation barely matches the rate of soil organic matter decomposition: hence the slight decline in the amount of soil organic matter,

Between the fourth and fifth year of the fallow period in the ljebu-Ode/Shagamu area, a woody vegetation that is dominated by trees has become established in fallowlands. Such woody vegetation affords the soil greater protection against direct solar radiation (and hence accelerated soil organic matter decomposition) and generates more litter than the forb vegetation that precedes it. The progressive build-up of organic matter in fallow soil after the third year of the fallow period in the ljebu-Ode/Shagamu area may therefore be attributed to the establishment of woody fallow vegetation in fallowlands.

The trend of organic matter build-up in savanna fallows is generally similar to that of forest fallows. As in the case of forest fallows, there is an improvement in the organic matter status of soils under savanna fallows with an increase in the length of the fallow period. However, the rate of organic matter build-up in savanna fallows tends to be slower than that in forest fallows. For instance, the mean soil organic matter content of 30-year fallow in Ibarapa derived savanna area is 2.85 %. Land which had been left fallow for 15 years in the savanna area around Oyo had a mean soil organic matter content of 3.20%. These values are lower than the mean organic matter content of 10-year forest follows in the ljebu-Ode/Shagamu area.

Two reasons may be adduced to explain the relatively slow rate of organic matter build-up in savanna fallows as

compared to forest fallows. Firstly, the biomass of the standing crop of fallow vegetation in savanna fallows is usually much less than that of forest fallows. The implication of this is that, less litter is generated by savanna fallows for incorporation into soil organic matter complex. Second- ly, savanna fallows are burnt annually mainly in order to stimulate the growth of grasses for grazing animals or to drive out wild game from their hiding places to facilitate their capture. Forest fallows, in contrast, are not usually burnt. The burning of savanna fallows results in the destruc- tion of litter which could have been converted into soil organic matter. Furthermore, savanna burning results in the exposure of fallowlands to intense insolation and the direct impact of rain drops. This results in the accelerated oxidation and decomposition of organic matter as well as in considerable losses of organic matter from the topsoil through leaching and erosion when the rains set in.

The mean organic matter content of 1 S-year fallow in Oyo area is higher than that of 30-year fallow in Ibarapa division. This suggests that the rate of organic matter build- up is slower in savanna fallows in Ibarapa division and this is presumably due to the fact that the soils in Ibarapa division have been subjected to more intensive cultivation and the fallows to more severe burning. In fact, the cattle Fulanis have long settled in Ibarapa division and they burn the fallowlands towards the end of the dry season in order to stimulate the sprouting of grasses for their cattle to graze on. This would result in a slower rate of fallow vegetation regeneration on which largely depends the capacity of fallow soil to restore its soil organic matter status after the period of cultivation. Areola (1979) has shown that the old fallow plots in Ibarapa area have been largely degraded by sheet erosion, mainly as a result of intensive burning which had retarded the rate of fallow vegetation regeneration.

Nye and Greenland (1960) have observed that the extent of organic matter build-up in fallow soil should be evaluated in relation to the equilibrium level, that is, the maximum level of organic matter build-up that is attainable under the climax vegetation. They further pointed out that organic matter would only build-up in fallow soil if the amount of soil organic matter is below the equilibrium level at the beginning of the fallow period. In order to assess the extent to which organic matter in fallows of different ages have approached the equilibrium level, the mean organic matter contents of fallows in the three study areas were expressed as percentages of the mean level of organic matter under the 80-year forest fallows, that is, the mature secondary plots. The mean level of soil organic matter level under the 80-year forest fallows is 5.37 %. This value is reasonably close to that of 6.0 % obtained by Reed (1951) for ferralitic soils under virgin forest in Liberia and may therefore be regarded, with justification, as the mean equilibrium level of soil organic matter under climax rain forest communities with sandy soils.


Tab 2 The organic matter contents of fallows in the three study areas expressed as percentages of forest and savanna equilibrium levels

FE = Forest equilibrium level DSE = Derived savanna equilibrium

level

I

:? :i;i;;i i:":ii:!

"i!!1:5:1:,ii::~0 i:i::,~i(::?'i

. ' , E E " ~: : . . . . :FE : : " DSE : : : ..... FE DSE

: :~: i ::: ::: :: '.: Z34!82 :: 52:16~ : ... . 47130:: :71.55 ':i :~L7:3::: Z : ::::i:::, :~ : 1 47:.11 i : 71.27 : :i:::~ : :: ;i~ 49.72: : 75.2! '

", ; : i :;: '--:; i: : ii: ::: :32~40 49.0i 59.59: 90.14 ::":: 42.s:3 - :: -

:;::i ::: ' : ':: : : ii '::S3:07 s:oi2s c

The mean levels of soil organic matter in the two derived savanna areas were related to the equilibrium level of forest soils because the derived savanna zone was a part of the rain forest zone (Keay 1959) and the savanna vegetation would revert back to forest vegetation if it is not burnt periodically (Charter and Keay 1960). Thus relating the mean organic matter contents of savanna fallows to forest equilibrium level gives one an insight into the extent to which organic matter in derived savanna soil would eventual- ly build-up if burning were completely excluded and the land left to fallow for a very long period.

However, since the derived savanna vegetation is more or less stable and is prevented from reverting to forest vegetation as a result of man-induced burning, that is, it is an anthropogenic climax, it is also desirable to relate the organic matter contents of the savanna fallows to the soil organic matter equilibrium level that obtains under derived savanna vegetation. The mean organic matter content of soils under a savanna woodland at the Olokemeji experimental plot which was initially cleared, coppiced and then subjected to early burning for thirty years while it was left uncultivated, was regarded as having attained the equilibrium level of the derived savanna zone. Since this savanna woodland plot has started to revert to forest vegetation as evidenced by the invasion of fire-tender forest species (Charter and Keay 1960), soils under it can be regarded as having attained the organic matter equilibrium level of derived savanna soils.

The mean organic matter content of the top 10 cm of the soil in this experimental plot was 3.55 % (Moore 1960) and this value was regarded as the equilibrium level of derived savanna soils in this study. The mean organic matter contents of the savanna fallows of different ages were expressed as percentages of the derived savanna equilibrium level. Forest and savanna fallows of comparable ages are compared with respect to the extent to which they have approached the equilibrium level. This exercise will hope- fully shed more light on some of the factors that account

for the differential rates of orgamc matter build-up in forest and savanna fallows.

Tab 2 shows the mean organic matter contents of the fallow plots expressed as percentages of the equilibrium levels of forest and savanna soils. As should be expected, as the organic matter status of fallow soil improves over time, it gradually approaches the equilibrium level under the climax vegetation. With respect to forest fallows, it will be observed that at the end of the tenth year of the fallow period, the organic matter content of fallow soil in the Ijebu-Ode/Shagamu area has attained 77 % of the equilibrium level which is attained under the mature forest of 80 years.

This finding suggests that the build-up of soil organic matter in forest fallow is relatively rapid during the first ten years or so of the fallow period and thereafter it becomes much slower as the equilibrium level is approached. This situation may be attributed to the fact that the leaves of trees in forest fallows, the major source of organic materials that are converted into soil organic matter on decomposition, quickly attain their maximum development within the first few years of the fallow period (Vine 1968).

Although the mean organic matter contents of 1-year and 3-year savanna fallows in Oyo district are well below the equilibrium level of climax rain forest soil, they are closer to the equilibrium level of derived savanna soil than forest fallows of comparable ages are to the forest equilibrium level. It may well be therefore, that one of the reasons that account for the relatively slow rate of organic matter build-up in savanna fallows is the fact that their organic matter contents are closer to savanna equilibrium level than are forest fallows to forest equilibrium level at the beginning of the fallow period.

The rate at which the organic matter content of savanna fallows in Ibarapa division approaches the equilibrium level is slower than that of Oyo area. For instance, soil organic matter content of savanna fallows in Oyo area has attained 92.14 % of the equilibrium level at the end of the fifteenth


year of the fallow period while in Ibarapa division, fallow soil organic matter content has reached only 80.28 % at the end of the thirtieth year. This may be due to the fact that the fallows in Ibarapa division have been subjected to more severe burning as explained earlier.

The correlations between the length of the fallow period and soil organic matter content for the forest fallows, Oyo savanna fallows and Ibarapa savanna fallows are 0.77, 0.71 and 0.59 respectively. These results suggest that generally, an increase in the length of the fallow period tends to have greater effects in improving soil organic matter content in forest fallows than in savanna follows. The reasons for this phenomena have been explained earlier. Also, the reasons for the relatively low correlation between length of fallow period and soil organic matter build-up in Ibarapa division have been given.

2. Intercorrelations B e t w e e n Soil P r o p e r t i e s From Tab 3, 4 and 5 showing the correlation matrices between the soil variables in the three study areas it is possible to assess the relative importance of soil organic matter in the restoration of fallow soils, especially in comparison with such an intrinsic soil property as the particle-size distribution. First, we shall consider the case of the forest fallows (Tab 3). Except in two cases, pH and bulk density, the sand content is negatively correlated with all other soil properties. This, perhaps, is to be expected given the loose, inert nature of the sand faction.

The negative impact of the sand content on the restoration of the fertility of the soils under consideration can be judged from the fact that the sand fraction is over 70 % in all of them. The silt content has fairly significant positive correlations with the soil water holding capacity (0.72), cation-exchange capacity (0.68), exchangeable magnesium (0.62), potassium (0.56) and sodium (0.55) and total nitrogen (0.57). There are also weak positive correlations with organic matter and available phosphorus. The clay content is fairly significantly correlated with available phosphorus (0.62) and soil organic matter (0.45). Thus, it would appear that in these essentially sandy soils, the fine soil fraction, silt plus clay, exercises a substantial influence on other soil properties and, therefore, plays an important role in soil fertility restoration in the forest fallow soils.

However, the influence of soil organic matter appears to be dominant. It is significantly correlated with soil water-holding capacity (0.84), available phosphorus (0.83), total nitrogen (0.83), exchangeable potassium (0.82), cation-exchange capacity (0.80), porosity (0.71) and exchangeable sodium (0.66). There are also weak to moderate positive correlations between organic matter and exchangeable calcium (0.49) and magnesium (0.40). As to be expected, organic matter is significantly negatively correlated with sand content and bulk density.

In the Ibarapa derived savanna study area, the intercorrelations between the soil properties are, with a few exceptions, generally weak. Available phosphorus in par- ticular, is uncorrelated with any other soil property. Unlike

Tab 3 Intercorrelations between soil properties for Ijebu-Ode/Shagamu forest fallows

BD = bulk density; Poro = total porosity; N = total nitrogen; P = available phosphorus; K + = potassium; Ca ++ = calcium; Mg ++ = magnesium; Na + = sodium; CEC = cation -- exchange capacity; O.M. = organic matter; Water = water-holding capacity

Correlation coefficients that exceed /0.32/are significant at the 5 % confidence level

GeoJournal 6.2/1982 189

1 ("i::: :::::: :::::: : : ::: : : : : : : ; : : .

:~;5: :::I: i:i::: ; : : : . . . . . . . . . .

Tab 4 Intercorrelations between the Soil Properties for Ibarapa savanna fallows

,: t4 ,S ::::::;:::::: 8 o.m ..... :mo

': '::i::;;:: ': : ; : - 0 ;4 ] - -0 :26 )~0:46: ::: --0:07 i :~143 : ~0.1:5 11-0:09 ,0,20 -0:33 ;::~::::: ; : : :1,00:: : ~ : ! 3 8 : ~ : 3 0 ~042 ~:: 0.34:: 0 .4 i : 0 ,26 : :0 46 ) 0;07 0,43 0, i5 0.20 :0;33 '::.3::.:: : : : : :: ::.i:' :: : : . ' :: 0,0:9:: : :~i20 Z0!04 --0.0:5:--0107 : - 0 .00 0:07 0:.15 0,18 '0.12

:: : : : : : i !::i0o ~oio9 0 : 2 0 ~:o:oo olo4 o o 7 0,00 -o.o7 -o,ts -o.18 -0.i2 5 : ; :;:::: : :: :::::: : l:.o0:: :~.60: : :~:0 8 8 --0,18 ±o.sg: : 0.05 ~0.56 --0.50 '0 ,14 --0.30 --0.68

i::::):::: :i::::::: : :::: ::: :: : : : : ] ! 00 : : 0 .27 0 i 6 0.42 - - 0 . 0 7 : 0 . 2 7 :0.17 " 0 . 0 0 0.09 0.21 : ;: : 1~00 : 0,71 : :0,82 0,01 052 0 42 0.09 0,23 0.63

;:: ~::::I ::::: : : : : ::;: :: : : : :: : ::::: :: : : : : : : , : i~00 0:83 :--003: 040 ) 0,52 0.23 0.45 0.65 ::: : : : : 1 , 0 0 0:05 0,61 0.37 0,08 0.31 0.71

, : 1 . 0 0 0.02 0.12 -0.08 0.16 -0.03 1.00 0.32 0,04 0.21 0 3 7

1:00 0.17 055 0.6,0 1.00 0.60 0.19

1,00 0.35 1.00

16 pH

0.06 0.06

--0.12 0.1 2

-0,08 0.!0 0.08 0.03 0. I I 0.15 0:09 0.16

--0.90 -0,38

0.06 1.00

BD = bulk density; Poro = total porosity; Map = macro-porosity; Mip = micro-porosity;CS = coarse sand; FS = fine sand; N = total nitrogen; P = available phosphorus; K + = potassium; Ca ++ = calcium;CEC = cat ion- exchange capacity;O.M. = organic matter; Moi = moisture content.

Correlation coefficients that exceed/0.50/are significant at the 5 % confidence level

Tab 5 Intercorrelations between soil properties for Oyo savanna fallows

K÷

.0.35 0.27

: 1.00 --0.24 0.37

1.00 0:25i00

:6 ̧ :/: 7 : :Ca ++ Mg÷÷

8 9 I0 11 12 Na + GEE O.M: Water pH

"0,64 --0.87 --0.37 0 54 0,1 g 0.38

0.47 0.40 0.08 0,37 - 0 ; i 8 0 . 1 4 039 0.17 0.52

1 0.66 ) 030

1.00

:i:;i/:iiii: !'::::: ̧ : :: L ; ' ! : :-::LI:;/Z:~: ~::i :i :;Z": :: /-:::.: i : : :::'::? i-! ::¸:¸ . . . . : : :

--0,64 0 5 t 0.57 0.22 0.75 0.91 0.62 0.66 Iio0

-0:44 ~0.17 -0.68 0.4! -0 .16 0.58 0,12 0.27 0,42 0.99 0.41 0.47 0.22 0.47 0.41 0,36 0.28 0,74

4 . 1 7 0.17 0.20 0.12 0.22 0.47 0,21 0.05 0,67 1.00 0.37 0.47

1 , 0 0 0 . 4 4

1.00

N = to ta l n i t rogen; K + = potass ium; Ca ++ = ca lc ium; Mg ++ = magnesium; Na + = s o d i u m ; C E C = cat ion -- exchange c a p a c i t y ; O . M . = organic

ma t te r ; Water = water - ho ld ing capac i ty .


in the forest fa l low soils just described the inf luence o f soil organic matter is not as glaring and dominant over that o f the soil physical characteristics. Soil organic matter is only fa i r ly h ighly correlated wi th cation-exchange capacity (0.6) and exchangeable calcium (055). I t also has a weak posit ive correlat ion wi th clay content (0.45). By contrast, clay content has fa i r l y high posit ive correlat ions wi th total nitrogen (0.83), moisture content (0.65), and exchangeable

calcium (0.52). The si l t content, also, is fa i r ly highly correlated wi th total nitrogen (0.82), moisture content (0.63) and exchangeable potassium (0.52). Thus, again, as wi th the sandy forest soils and even more so, the f ine soil f ract ions are crucial for the restorat ion of the fe r t i l i t y o f these savanna fa l low soils.

The intercorrelat ions between most soil properties appear even weaker in the Oyo derived savanna area than in


Ibarapa. But the general pattern of interrelations is the same. Organic matter is only highly correlated with total nitrogen (0.99). It also has a weak positive correlation with pH (0.47). The silt content is fairly highly correlated with pH (0.58), exchangeable calcium (0.54) and cation-exchange capacity (0.51) while clay has moderate to weak positive correlations with cation-exchange capacity (0.57), exchangeable calcium (0.47) and magnesium (0.4) and pH (0.42).

3. Improvements in Soil Condition with Age of Fallow

A major point of interest in the study of soils under fallow is the improvement in soil conditions with the increasing age of fallow. Tab 6, 7 and 8 show the correlation coefficients between age of fallow and individual soil properties in each of the three study areas. In the case of the forest fallows, the results of the correlation analysis are to a large extent as one would expect. There are marked improvements in soil morphological characteristics with increasing age of fallow. The restoration of soil structure with age of fallow is reflected in the fairly high negative correlation

between age and bulk density (-0.50) and the positive correlation with total porosity (0.51), both of which suggest a fair degree of particle aggregation and structural stability. Organic matter and available phosphorus show the highest positive correlations with age of fallow. The build-up of soil organic matter with the age of fallow probably accounts for the marked improvements in total nitrogen, water holding capacity, cation-exchange capacity and available phosphorus. Only soil pH is not significantly correlated with age of fallow.

With regard to the derived savanna fallow soils in Ibarapa, the correlation of soil characteristics with age of fallow is generally weak; although the picture is somewhat similar to that of the forest fallow soils. There is evidence of improvement in soil structural status as revealed by the negative correlation between age and bulk density and the positive correlation with porosity. Of all the soil properties, organic matter content, exchangeable potassium, cation- exchange capacity and moisture content are the most significantly correlated with age of fallow.

The relatively weak correlations between soil properties and age of fallow can be explained in terms of the frequent

Tab 6 Correlation coefficients between age of fallow and soil properties for ljebu-Ode/Shagamu forest fallows

Tab 7 Correlation coefficients between age of fallow and soil properties for Ibarapa savanna fallows

Tab 8 Correlation coefficients between age of fallow and soil properties for Oyo savanna fallows



disturbance of the fallow vegetation by man through intense grazing and annual bush burning. The weak negative correlation between age of fallow and available phosphorus contrasts sharply with the situation in the forest region.

The pattern of age-soil relationships is less clear with regard to the derived savanna soils in eye area. Only soil organic matter and total nitrogen are significantly correlated with age of fallow. The correlations between exchangeable calcium, sodium, potassium and cation-exchange capacity on the one hand and fallow age, although positive, are very weak indeed. The correlation between age of fallow and exchangeable magnesium, also very weak, is negative and this suggests that magnesium content of the soil tends to decrease with an increase in the length of the fallow period. It is important to point out however, that the age differences between the classes of fallows in eye area are not as marked as in the Ibarapa example. The oldest fallows studied for eye area are only 15 years old as compared to 30 years for Ibarapa division.

The rather low to moderate correlations between soil nutrients and age of fallow for the two derived savanna areas suggest that the build-up of nutrients in the topsoils of savanna fallows during the fallows period is rather slow and fortuitous. Two reasons may be adduced to explain this.

Firstly, unlike in the forest zone, fallow vegetation develops very slowly in the savanna zone and it may take several years before a dense vegetation that adequately protects the soil is established. Consequently, some nutrients may be lost from the topsoil through leaching and erosion. In the eye study area, for example, it seems that the rate of nutrient loss from the topsoil roughly approximates the rate of nutrient recycling back to the topsoil; hence there is no appreciable build-up of nutrient in the topsoil. In the case of magnesium, it would seem that the rate of loss is slightly greater than that of recycling back to the topsoil. This explains why magnesium is negatively correlated with age of fallow.

Secondly, a significant proportion of the nutrients that accumulate in the toplayers of fallow soils are derived from the subsoil (Nye and Greenland 1960;Vine 1968). Nutrients are absorbed from the topsoil by plant roots and later recycled to the topsoil through the fallow and mineralisation of litter. Since savanna fallows are dominated by grasses which are shallow rooted, it may well be that most of the nutrients leached to the subsoil during cropping would not be transferred to the topsoil until many woody plants (notably trees and shrubs) with deep rooting habit have become established and this may take 10 years or longer in savanna fallows which are burnt annually.

Conclusion

In each of the three study areas, there is an increase in the organic matter content of fallow soil with an increase in

the length of the fallow period. With respect to forest fallows, the relationship between soil organic matter build- up and the length of fallow period is not strictly linear. Organic matter tends to build-up fairly quickly during the first ten years following the cessation of cropping; thereafter the rate of organic matter build-up becomes much slower as the equilibrium level is approached. In savanna fallows, soil organic matter builds up slowly but more or less evenly towards the equilibrium level although the rate appears to be faster during the first 15 years of the fallow period in Oyo area.

The rate and extent of organic matter build-up in forest fallows are generally higher than in savanna fallows. This is largely due to the fact that the biomass of forest fallows is greater than that of savanna fallows. Furthermore, savanna fallows are burnt annually. Burning not only destroys litter and soil organic matter but also results in a slower rate of fallow vegetation regeneration.

The implication of savanna burning is that it would generally take a longer period for savanna fallows to restore their ferti l ity status to levels attained by forest fallows, other things being equal. This points to the danger of burning savanna fallows at least from the point of view of managing and manipulating fallow ecosystems in order to improve the ferti l ity status of fallow soils for crop production.

Although burning prior to cultivation at the end of the fallow period may be beneficial to the crops in that it releases into the soil nutrients immobilised in fallow vegetation, occasional or recurrent annual burning of the savanna vegetation during the fallow period is unprofitable since it reduces the rate and extent of organic matter and nutrient build-up in the soil. An essential step for improving the capacity of savanna fallows to regenerate soil ferti l ity status would therefore be to protect them against burning. This measure would ensure not only that fallow vegetation develops more rapidly to protect the soil against leaching and erosion but also that the litter generated by fallow vegetation are not burnt but are available for convertion to soil organic matter.

In the forest fallows, soil organic matter exercises a decisive influence on the extent of soil ferti l ity restoration during the fallow period. The results of the intercorrelations between soil properties suggest that organic matter exercises an overwhelming influence on the nutrient status of fallow soil in the forest belt. In addition, improvements in the physical status of fallow soils particularly their porosity (and hence soil aeration and permeability to water and plant roots) and water-holding capacity are strongly in- fluenced by the organic matter status of forest fallows. In fact organic matter tends to exercise greater influence on the extent of improvement in soil nutrient and physical status than does soil clay content. This is not to underrate the influence of soil texture however. It has been shown that in these essentially sandy soils, the silt and clay frac-


tions exercise considerable control on soil structural and fer t i l i ty regeneration during the fa l low period.

In the savanna fallows, intr insic properties of fa l low soil, part icularly soil textural composit ion, assume greater significance in the restoration of the nutr ient and physical status of fa l low soils. In the two savanna study areas, soil clay content tends to exercise greater influence on the extent o f nutr ient build-up in fa l low soil than does organic matter. This situation sharply contrasts wi th that o f forest fal lows where organic matter exercises a dominant influence on the fer t i l i ty status of fa l low soils. Organic matter exercises a comparatively weak influence on the fer t i l i ty status of savanna fallows because of the relatively small organic matter build-up.

The correlations between age of fa l low on the one hand and soil physical and chemical properties on the other are positive and generally high for the forest fal lows. This suggests that generally as the length of the fa l low period increases the fer t i l i ty status of the soil improves also. The pattern of correlations between age of fa l low and soil properties for Ibarapa savanna fallows is similar to that of

forest fal lows, except that the correlations for the savanna fallows are generally lower. This lends fur ther support to the assertion that savanna fallows generally have a lower capacity to restore soil fer t i l i ty status wi th in any given length of fa l low period.

For Oyo savanna fallows, age o f fa l low is signif icantly correlated with soil organic matter but not signif icantly correlated wi th soil nutrients except total nitrogen. The low correlations for Oyo area and the rather moderate ones for Ibarapa, suggest, that the build-up of nutrients in the topsoils o f savanna fallows during the fa l low period somewhat lags behind the accumulation of organic matter. This situation is mainly attr ibutable to the slow rate of regenera- t ion in savanna fallows of trees and woody perennials (mainly because of annual burning), whose roots 'transfer' the bulk o f plant nutrients from the subsoil to the topsoil. This again points to the need to protect savanna fallows against burning so that plants can regenerate fast to enhance the abi l i ty o f the fa l low vegetation to restore soil fer t i l i ty status.

R e f e r e n c e s

Areola, O.: Some problems and issues in studying savanna fallows. African Environment (in press).

Aweto, A.O.: Secondary succession and soil regeneration in a part of the forest zone of south western Nigeria. Unpublished Ph.D. thesis. Department of Geography, University of Ibadan 1978.

Blake, G.R.: Bulk density. In: Black, C.A. (ed.): Methods of soil analysis. Part I . A.S.A. Madison, Wisconsin p. 374--390, 1965.

Bray, R. and Kurtz, L.T.: Determination of total, organic and available forms of phosphorus in soils. Soil Science 39, 39-45 (1945)

Chapman, H.D.: Cation - exchange capacity. In: Blacks, C.A. (ed.): Methods of soil analysis. Part 2 A.S.A. Madison, Wisconsin. p. 891 - -901 ,1965 .

Charter, J.R. and Keay, R.W.].: Assessment of the Olokemeji fire control exper iment (investigation 254) 28 years after institu- tion. Nigerian Forestry Information 3 (1960)

Gbadegesin, A.S.: Secondary succession in the derived savanna zone of Oyo Local Government Area. Unpublished B.Sc. dissertation Depar tment of Geography, University of Ibadan 1979.

]ones, M.J. and Wild, A.: Soils of the West African savanna. Techni- cal communica t ion No. 55, Commonweal th Bureau of Soils, Harpenden (1975)

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Keay, R.J.W.: An Outline of Nigerian vegetation. Government Printer, Lagos, 1959.

Moore, A.W.: The influence of burning on a soil in the derived savanna zone of Nigeria. Trans. 7th International Congress of Soil Science 36,257--264 (1960)

Nye, P.H.: Soil analysis and the assessment of fertility in tropical soils. J. Sci. Food Agric. 14,277--280 (1963)

Nye, P.H. and Greenland, D.J.: The soil under shifting cultivation. Commonwealth Bureau of soils, Tech. Comm. 51, Harpenden (1960)

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Pereira, H.C., Chenery, E.M. and Mills, W.R.: The transient effect of grasses on the structure of tropical soils. Emp. J. expl. Agric. 22,148-160 (1954)

Reed, W.E.: Reconnaissance soil survey of Liberia. U.S. Department of Agriculture. Agric. Information bulletin 66 (1951)

Stephens, D.: Three rotation experiments, with grass fallows and fertilizers. Emp. J. expl. Agric. 28,165--178 (1960)

Vine, H.: Development in the study of soils and shifting agriculture in tropical Africa. In: Moss, R.P. (ed.): The soil resources of tropical Africa. Cambridge Univ. Press. 89--119, 1968.

Vomocil, J.A.: Porosity. In: Black, C.A. (ed.): Methods of soil analysis Part 1. A.S.A. Madison, Wisconsin p. 299-314, 1965.

Wilkinson, G.E.: The infiltration of water into Samaru soils, Samaru agric newsletter 1 2, 81 --83. Insti. Agric. Res., Samaru, Nigeria (1970)

organic matter and soil fertility restoration in forest and savanna fallows in southwestern nigeria

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