Effects of agriculture on soil quality in northeastern Ghana
Gana, Bertha Kweley
The effects of agriculture on soil quality were assessed in northeastern Ghana. The farming practices are governed by the farmer's socio-economic conditions and interviews and questionnaires were used to assess these conditions. The effects of the farming practices were evaluated at sixteen sites using soil quality indicators, and soil redistribution was assessed using 137 Cs redistribution. Food production is dominantly household-based subsistence agriculture. Farmers have small compound farms and nearby bush-fallow farms. Eighty percent of respondents noted worsening soil conditions and soil fertility. Although private ownership of land is commonly suggested to improve soil conservation, 75% percent of respondents were satisfied with the traditional method of land allocation. The complex nature of the soil-landform relationship confounded attempts to group the sites based solely on farming practices. Nine upper slope sites had high concretion contents, dithionite-extractable iron contents and magnetic susceptibility values. Four lower slope sites had mottled soils with low concretion contents and oxalate/dithionite iron ratios and magnetic susceptibility values indicative of poorly drained conditions. The final three soils occurred on an active erosional/depositional surface in a small catchment. The reference inventory of 137Cs was 830 Bq m-2 (coefficient of variation of 25%). The highest soil losses (averaging 19 Mg ha-1 yr-1) occurred on two upper slope, compound farms. Average soil loss from the bush farms was 7 Mg ha-1 yr-1. Negligible soil gain occurred on lower slope sites, indicating net export of eroded soil. The assessment of soil quality changes focused on the fine soil fraction. Soil P, base saturation, and pH were not appreciably affected by cultivation. Upper slope sites had, on average, lost 67% of the pre-cultivation soil organic carbon (SOC). The loss of SOC lead to decreases in total soil N and cation exchange capacity. Less than 50% of the observed SOC loss could be attributed to erosion, indicating the importance of SOC loss through residue removal, burning, and mineralization. Although erosion control would be an important contributor to soil quality maintenance, a broader effort involving many facets of the farming system is required.