An Assessment of Spatial Variations of Some Soil Properties under Different Land Uses in South-Western Nigeria

The research was carried out by author MA under the supervision of author BAS. All the other authors assisted in the write up, statistical analysis, proof reading and final editing of the work. ABSTRACT Land use and soil management affect the distribution of soil properties thereby resulting in changes in the fertility status. Soil physical and chemical properties on the surface (0-30 cm) of five land uses (cassava/maize, oil palm, cowpea, secondary forest and building site) on a sandy loam Alfisol (Typic paleudalf) were assessed for spatial variations. All the land uses differed significantly (P<0.01) from consideration in order to reduce soil fertility decline and enhance proper soil management. There must therefore, be a careful choice of appropriate use of land if we are to optimise production since loss of soil and fertility status may be difficult to replace in one’s lifetime.


INTRODUCTION
It is widely known that land use types influence soil properties differently [1]. However, the extent to which land use types influence soil properties need to be studied to greater details. This is because soil varies with land uses from location to location [2]. The spatial variation in soil properties with land use types could not be over emphasised since it is an important aspect in soil fertility decline studies. This is also necessary because spatial variation in soil properties has not received the same amount of research attention as soil erosion, possibly because soil fertility decline is less visible, less spectacular, and more difficult to assess. Although, few studies had been conducted on soil spatial variation as reported by [3,4] and [5], yet the increasing rate of soil degradation owing to inappropriate land usage calls for more attention especially on land use studies. There is therefore need to know the extent to which soil properties vary under different land uses, so as to provide better understanding for effective soil fertility management and contribute significantly to the careful choice of appropriate land use. This is the premise upon which this study is based with the aim of assessing the differences in soil properties resulting from the different agricultural land use types.

MATERIALS AND METHODS
The study was carried out at the Olabisi Onabanjo University, Ago-Iwoye, which is located in the rainforest belt of south-western Nigeria. Ago-Iwoye is located between the latitude 3 0 45' and 4 0 05'E. The area is associated with long rainy season, March to October with peaks between June and July and September and October. The mean annual rainfall is 1350mm with about 80% of it being concentrated within the months of April and October. Mean relative humidity of the area is generally high (80%) with the peak between May and October. The mean temperature of the area is 29ºC and this low range accounts for its constantly high temperature. The mean annual sunshine hours recorded for the area is 2000 and the mean net radiation for the study is 70kg/cm (Ikenne Meteorogical Station). The soil is generally sandy especially at the top horizons. The soils are highly deficient in mineral reserves due to the high intensity of weathering and leaching of the soil. The clay content is of the kaoline type with low water and nutrient holding capacities [6], while the cation exchange capacity (CEC) ranges from as low as 1.3cmol/kg to 17.3cmol/kg in the top soil. The values of the exchangeable cations are generally low which is due to their susceptibility to leaching as a result of heavy rainfall and sandy nature of the soils. The pH of the area also decreases with depth [6].
Five land use types occurring on the same soil type within a distance of 30-60 m were studied. These include:

1.
Cassava/Maize Cropping (CM): The land was under fallow for a period of 7 years before conversion to arable production. Mechanized method of clearing is often practiced in this site.

2.
Oil palm (OP): This was a 20 years old oil palm production at the time of study. This was basically established for commercial purposes, with minimal soil management practices.

3.
Cowpea (CO): This site was cultivated on the adjacement plot of the arable crops (CM). This site has been under the cultivation of arable crops for a period of seven years, prior to cultivation with cowpea.

4.
Under Brushed Secondary Forest (FO): This land use consists of big trees, shrubs, climbers and smaller weeds. The under story is opened and the soil surface is covered by a thick litter layer of dead leaves and twigs. This forest has been in existence since the inception of the University in 1982. It was used as a control to serve as undisturbed soil.

5.
Building Site (BS): The bush regrowth around WAPCO building site in the main campus of the University was used because of its potential for backyard farming and was opened for construction purposes in recent times from secondary forest. Tree species found in the study area include Pentaclethra macrophylla, Basqueria angloansis, Guarea spp., Mimusop spp., Cola gigantean, and there is a wide spread of Elaeis guineensis and these were dominated by the presence of Chlomolaena odorata, a perennial weed found in fallow lands in south western Nigeria.
Using a systematic sampling procedure, composite samplings were taken with the aid of soil auger at the surface (0-15cm) and sub-surface (15-30cm) depths from both the long standing forest region which was chosen to serve as the control and the other four land-use types. The samples were then air dried, crushed and sieved for chemical and physical analyses in the laboratory.
The soil particles size analyses were determined using the hydrometer method [7] with sodium hexametaphosphate as the dispersing agent. Soil pH was determined potentiometrically in both water and 0.01M potassium chloride solution (1:1). Exchangeable bases were extracted with neutral ammonium acetate solution; [8] Ca and Mg were determined by atomic absorption spectrophotometry, and K and Na by flame photometry. Exchangeable acidity was determined by the KCl extraction method [9], total nitrogen was determined by the macro-kjeldahl digestion method of [8], available phosphorus was determined by the [10] method. Organic carbon was determined by the method of [11]. ECEC was determined as the sum of exchangeable bases and exchangeable acidity. Available micronutrients were determined by Atomic Absorption Spectrophotometer (AAS) method after leaching with NH 4 Cl [12].
The data were subjected to analysis of variance (ANOVA) and means were separated using least significant difference (LSD). Co-efficient of variation (CV%) by [13] was used for grouping of soil properties.

RESULTS AND DISCUSSION
From the data in Table 1, the cowpea plot (CO) was significantly (p=0.05) more fertile than other land use types. It contains more exchangeable bases and organic carbon and is similar to other land uses in other properties. The presence of higher value of organic carbon could be attributed to both the fallow land that has just been opened up and the conserving nature of cover crops like cowpea which does not only fix atmospheric nitrogen, but prevent the removal of top soil and organic residue from the soil through any agent of degradation such as erosion. There was more extractable potassium as well as nitrogen in secondary forest (FO) than any of the other land use types. This was probably because of the longer period of their accumulation than any other land use where erosion due to soil exposure as well as the cultivation of soil depleting crops like maize and cassava had limited their accumulation. The sandy texture of the soils, particularly in the oil palm (OP) resulted in the excessive drainage and low moisture thereby leading to the leaching of essential nutrients especially potassium which is needed for high yield in oil palm production, as reported by [14,7,15]. It is interesting to discover that cassava/maize cropping system which was planted on the same plot with cowpea after the opening up of the fallow and showed decline in soil fertility as reflected in Table 1 compared with other land use types. This was because of the depleting nature of such crops as far as nutrients are concerned, coupled with the exposure of land to erosion which ultimately washes away the fertility and fragile topsoil. It is important to note that inappropriate allocation of land for any use hinders the optimization of agricultural productivity as could be seen around the building site which showed high fertility status compared with cassava/maize based cropping system and oil palm. This therefore calls for the appropriate use of the land for the purpose it is best suited for as reported by [2]. Table 2 shows that the differences among land uses were statistically significant for 9 of the 15 soil properties considered. The differences in silt, clay, calcium, carbon, nitrogen and Zn were not significant. The results of the L.S.D. tests (Table 3a) are summarized in Table 3b. The cassava/maize land use differs from the secondary forest in all thirteen properties, from the oil palm in seven properties, from cowpeas land use in five properties and from building site in four properties.
The least different pair differs in at least three soil properties. The cassava/maize land use is different from all the other land use types in P, oil palm is different from others in pH, K, H + , CEC, C and N. The cowpeas land use from others in silt, clay, Mg, K, H + , CEC, C and N. However, there is no property distinguishing the building site from the others. The changes in soil properties resulting from different land uses or plant species are large, though it is difficult to explain some of these results. If for future uses some of the land uses need to be merged with little or no fertility decline, they can be grouped rationally according to the number of significantly different properties. From Table 3b, it can be seen that the land uses that can be combined with least fertility decline are building sites and cowpeas, cassava/maize crops and building site, cassava/maize crops and cowpeas. At the other extreme is the cassava/maize and secondary forest pair with more than twelve properties significantly different, merging these two types is not advisable because of the high degree of variability. The result emphasised the need for caution in assuming uniformity of soil properties between areas previously or currently used for different crop combinations of land uses, even if they occur on the same soil type, because land uses influence soil properties differently consequently leading to various degree of fertility decline. Silt, Clay, Mg, K, Na, H+, CEC, C, N, P Grouping of soil properties by co-efficient of variation (C.V.%) [18] showed that cowpeas land use was more homogenous than the others ( Table 4). The soil properties (exchangeable Ca, K, and P) were highly variable (group III) in the cowpeas land use, but 4-8 properties were highly variable in other land use types. This suggests that cultivating the land to cowpea will not only improve the soil fertility (i.e. degrade the land less), but also decrease soil variability and this is desirable for both practical and experimental agriculture, since it does not expose the land to any agent of degradation.