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8: Soils

Materials and method
Analyses
Results and discussion
Conclusion
References

Emmanuel Owusu-Bennoah

The southern forest-savanna transition zone of Ghana is a major producer of food crops, mainly cassava, Manihot utilisima, maize, Zea mays, and vegetables for the nearby urban areas and other settlements in Ghana. There is strong evidence of accelerated environmental change (Gyasi 1976; Benneh and Agyepong 1990; Dorm-Adzobu et al. 1991). This change is associated with increased population growth and with increasingly inappropriate and unsustainable farming practices. In many areas within the zone, the traditional system of food production through the forest-based shifting cultivation is now virtually eliminated, there being little forest land left to sustain this land extensive system. Most areas are now under continuous cultivation of maize-cassava intercrop. The creeping effects of a shortage of good agricultural land are becoming insidious. Among the biophysical environments affected by this production pressure is the soil.

In the humid tropical forest areas, much of the soil nutrient is stored in the trees, whose canopy protects the soils against the impact of the heavy rainfall and other weather elements in a delicate ecological relationship. Therefore, exposure of the soils by deforestation and bush burning leads to impaired nutrient status of the soil, soil structure degradation by erosion, dessication, compaction and other degradational processes. These have negative implications for the fertility and productivity of the soils. So far there has been little study into the effect of production pressure and environmental change on the erosion of the biota and the intrinsic qualities of the soils in the zone. Such a study is extremely important because it could form a basis for planned interventions to counter or control the deterioration of the biophysical environment.

This paper presents the results of investigations into soils as part of the multidisciplinary study of changes in the biophysical environment of the southern sector of Ghana's forest-savanna ecotone.

Materials and Method

The field studies focused on the following specific sites within the larger study sites in the forest-savanna zone:

• Amanase and Whanabenya in the study area centred on Amanase;
• Adenya, Gyamfiase and Kokormu in the study area centred on Yensiso;
• Osonson and Sekesua in the study area centred on Sekesua (see figs. 5.1, 6.1-6.3, pp.39, 44, 45, 46).

At each site, an undisturbed virgin forest grove, fallow land (2-3 years) and a farm under current cultivation were identified along a path line measuring up to 2 km.

Virgin, fallow and cultivated soils were sampled by taking 10 cores at each site to a 20 cm depth. Cores were composited and packed into polythene bags and transported to the laboratory. Soil samples were air dried, sieved (2 mm mesh) and stored in air-tight glass bottles for analysis.

Analyses

Particle size distribution was analysed using the hydrometer method (Day 1965) following H₂O₂ oxidation of organic matter and dispersion with sodium hexametaphosphate. The soil pH was measured in 1:1 (w/v) of veil: wafer with glass electrode on a pH meter. Organic carbon was determined by the wetoxidation method (Walkley and Black 1934). Exchangeable contents of the soils were determined by extraction with 1N ammonium acetate (NH₄OAc) at pH 7. Exchangeable Ca and Mg in the extract were determined by an atomic absorption spectrophotometer and K and Na by flame photometry. Cation exchange capacity (CEC) was calculated as the sum of the exchangeable potassium, calcium, magnesium and sodium. Total N and total P were determined by digestion of the soil with H₂SO₄ and H₂O₂. For all samples the concentration of P was determined colorimetrically in filtered samples by the molybdenum-blue method (Murphy and Riley 1962), while total N was determined by the Kjeldahl method. All analyses were carried out in duplicate.

Results and Discussion

The selected physical properties of the soils under different vegetation cover from the six sites within the zone are presented in table 8.1. The particle size analysis showed that, with the exception of the Amanase site, cultivation and cropping generally decreased the sand contents in the topsoils while the clay contents were increased, giving rise to sandy clay loam to sandy clay texture. In Amanase, cultivation did not modify the texture of the soils because of their sandy nature. The increased fine fractions observed in the topsoil of the cultivated soils might have caused clogging of the macropores and hence the uncontrolled sheet erosion observed on the farms. A similar observation had been made by Cunningham (1963).

Table 8.1 Some Physical Properties of Soils under Different Vegetation Cover at Sample Points in the Study Area

Table 8.2 shows some chemical properties of the soils. The data indicated a consistent decline in soil pH with cultivation except at the Osonson site. The range of decrease, however, was not significant: from 0.1 to 1.1 units. The general trend of decrease in soil pH could be attributed to the leaching of bases from the topsoil, which agrees with the observation made by Nye and Greenland (1964) and Lal (1973). Again, with the exception of Amanase and Adenya/Gyamfiase, there was a gradual decrease in the exchangeable cations and the cation exchange capacity due to leaching and crop uptake. In general, the results show that the decrease in the CEC is reflected in the decrease of the pH and organic matter contents in the soils.

The organic carbon of the virgin soils varied from 2.2% to 4.5% (average 2.7%), whereas the organic C contents of the cultivated soils varied from 0.8% to 2.8% (average 1.7%), a mean drop of approximately 37%, which could be attributed to the exposure of the soils as a result of deforestation and cultivation. This seems to support the view by Nye and Greenland (1964) and Cunningham (1963) that the most serious effect of forest removal is the rapid depletion of soil organic matter. As shown in table 8.2, cultivation of long duration has reduced by approximately 46% the concentrations of total N.

Table 8.2 Selected Chemical Properties of Soils under Different Vegetation Cover at Sample Points in the Study Site

The reduction in the total N is entirely accounted for by the decline of the organic carbon through the continuous cropping of the soils. The C: N ratio, which is also an index of fertility (Tisdale et al. 1990) is shown in table 8.2. The ratio of the virgin soils varied from 10 to 15 (mean 12.7), whereas the range for the cultivated soils was 13-16 (mean 14.5). The wide C:N ratio of the cultivated soils is an indication of decline in soil fertility (Lal 1973). Under the present management of low fertilizer inputs, P shows a very consistent decline with cultivation. This again may be associated with increased erosion losses resulting from lowered organic C. Soils under fallow between two and three years, however, had high levels of N and P. This is a reflection of the biological-biochemical mineralization processes during which organic matter is mineralized. It is also a reflection of biocycling of P through deeper plant roots causing a relative enrichment in the topsoil (Barber 1979).

Most interestingly, soil pH tended to increase with fallow under C. odorata, the prolific herbaceous species associated with deforestation. The effect of C. odorata in increasing the basic cations is dramatic, particularly the exchangeable Ca and hence the CEC contents in the fallow soils. This may account for the corresponding increase observed in the pH. The improved contents of organic C, total N and P under the dominantly C. odorata fallow for a relatively short period may perhaps indicate that fertility is regenerated under this vegetation, which is widely regarded as a weed. However, further studies are needed to confirm this, and to determine the proper management of C. odorata in the farming system.

Conclusion

The findings from the study confirm the view that changes have taken place in the soils of the southern forest-savanna transition zone. The decline in the soil pH and plant nutrients with continuous cultivation is caused by increased erosive losses associated with lowered organic matter in the soils. There is a possibility of an extension of the zone of depletion into lower soil horizons. It is, therefore, necessary in future to sample the entire solum if changes in the concentrations of the soil organic C and plant nutrients with continuous cultivation are to be described adequately. Information such as total solum depth, horizon thickness and bulk density would be required to evaluate the total organic matter budgets under each vegetation cover. According to Tiessen et al. (1982), the incorporation of values for horizon thickness into such data gives a measure for the total amounts of organic matter lost from a soil due to the combined effects of mineralization and erosion processes.

The decline in soil fertility in the zone could be controlled through the application of suitable fertilizers or through growing soil improving crops such as legumes in suitable rotations. Finally, agroforestry methods may be an im portent option to be considered as a means of halting the threat to soil fertility losses in the zone.

References

Benneh, G. and Agyepong, G.T. 1990. Land Degradation in Ghana. London: Commonwealth Secretariat, and Legon: Department of Geography and Resource Development, University of Ghana.

Cunningham, R.K. 1963. The effect of clearing a tropical forest. Soil Science 14: 334-45.

Day, P.R. 1965. Particle fractionation and particle-size analysis. In: C.A. Black, ea., Methods of Soil Analysis, 545-67. Madison, WI: American Society of Agronomy.

Dorm-Adzobu, C., Agyepong, G.T., Amoako-Nuama, C.E., Oduro, W., Oteng-Yeboah, A.A. and Sackey, E. 1991. Ghana Biodiversity Review. Accra: USAID.

Lal, R. 1973. Soil erosion and shifting agriculture. Shifting Cultivation and Soil Conservation in Africa, FAO Soils Bulletin 24: 48-71.

Murphy, J. and Riley, J.P. 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27: 31-36.

Nye, P.H. and Greenland, D.J. 1964. Changes in soil after clearing in a tropical forest. Planf and Soil 21: 101-12.

Tiessen, H., Stewart, J.W.B. and Bettany, J.R. 1982. Cultivation effects on the amounts and concentration of carbon, nitrogen and phosphorus in grassland soils. Agron. J. 74: 831-35.

Tisdale, S.L., Nelson, W.L. and Beaton, J.D. 1990. Soil Fertility and Fertilizers. New York: Macmillan.

Walkley, A. and Black, I.A. 1934. An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci. 37: 29-38.

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