Soil Properties Along the Toposequence of an Inland Valley Watershed under Different Land Uses in the Ashanti Region of Ghana

ABSTRACT

Inland valleys (IVs) of Ghana offer a very high potential for sustainable agricultural production due to their relatively high soil fertility and water availability. Within the IVs, different land use systems can be identified along the toposequence. The land uses selected for this study were: traditional rice farming in the valley bottom (TR); mixed cropping (MC), young fallow (YF), and cocoa plantation (CP) in the lower and upper slopes; and primary forest (PF) at the crest or summit. Soil reaction, cation exchange capacity (eCEC), and levels of major soil nutrients were determined for each. Results showed that the pH of the soils was moderately to slightly acid and that MC had the lowest pH. The observed topsoil mean values were TR, 5.5; MC, 4.9; YF, 6.4; CP, 6.1; and PF, 6.2. With the exception of TR, pH generally decreased with depth. Exchangeable cations, especially potassium, calcium, and magnesium, and effective cation exchange capacity were higher in the upland systems than in the valley bottom, showing the degradation caused by traditional slash-and-burn rice cultivation. Recorded mean topsoil eCEC (cmol (+)/kg) was TR, 7.2; MC, 6.1; YF, 11.1; CP, 14.1; and PF, 14.7. Total nitrogen (TN) levels ranged from 0.2% in TR to 0.4% in PF. Vegetation cover, litter fall, and organic matter accumulation seemed to influence the distribution of total carbon (TC) and TN. Available phosphorus (Bray 1), on the other hand, was higher in TR than in the upland land use systems, possibly due to the hydromorphic and relatively coarse nature of the soils. Sustainable lowland farming systems, such as the Asian type of sawah system, need to be established. At the same time, because effective management of the upland is reflected in the valley bottom soils, proper management of the upland soils could help increase food production and conserve the environment.

Keywords:

• toposequence
• inland valley watershed
• sawah
• hydromorphic
• cocoa plantation

ACKNOWLEDGMENTS

The authors are grateful to the Ministry of Education, Science and Culture of Japan (Monbusho) and the Japan International Cooperation Agency (JICA) for providing the necessary support for soil sampling and laboratory analyses.