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.