Evaluation Of Changes In The Properties Of Soils Under Agroforestry Systems In The Sub-Humid Zone Of Ghana

ABSTRACT The properties of similar soils under two agroforestry systems, a sole Leucaena leucocephala woodlot and alley (one with cassava and the other maize in between Leucaena leucocephala hedgerows) were compared at two depths (0 - 15 cm and 15 30 cm) to assess the relative changes in soil properties due to the agroforestry practices. Similar soils under conventional tillage, adjacent natural fallow and forest reserve were also used in the study for comparison of data and evaluation of improvements in soil quality. All the plots occur at the same topographic site in the sub-humid forest-savanna transition zone of Ghana. The soil properties determined included organic carbon, total nitrogen, available phosphorus, cation exchange capacity, bulk density, aggregate stability and available water capacity. Generally, all the surface soils showed greater concentration of the properties determined except for the bulk density. The forest reserve was the best restorer of soil fertility in terms of soil chemical properties. The organic carbon content in the 0 15 cm depth of the forest reserve soil was 34.1 g/kg and 14.8 g/kg for the 15 -30 cm depth. The woodlot soils showed the next greatest accumulation of organic carbon of 18.6 g/kg and 10.9 g/kg for the two respective depths. The soils under woodlot contained more available phosphorus than the rest of the plots and was next to the natural fallow in terms of cation exchange capacity. The soils under the natural fallow plot, however, showed greater available phosphorus concentration than the soils under the two alley systems and conventionally tilled plots. Among the farming practices, soil aggregates under the woodlot were more  stable with a mean weight diameter value of 1.78 mm because of better canopy protection, binding action of roots and organic carbon accumulation. Soil aggregates were more stable under the cassava-alley and maize-alley than under the conventional tillage and the natural fallow due to the pronounced cultivation, of the conventionally tilled plot and the dual reduced organic carbon and cementing action of biotic life by burning in the natural fallow plot. The subsoil bulk density was higher than the bulk density of the surface soil in all the farming practices apparently due to greater organic carbon accumulation and greater volume of biopores as a result of higher root density in the surface soils. The soils under the cassava-alley and conventionally tilled plots recorded higher subsoil bulk densities than the rest of the farming practices and this is attributable to the compaction effect of tillage. The soils under the woodlot, alleys, conventional tillage and natural fallow were similar to each other with regard to total nitrogen; probably due to either a generally low rate of mineralisation and/or the destruction by burning of organic matter and micro-organisms which are responsible for mineralisation. The available water capacities of the soils under these practices were quite similar because of similar macroporosity. There were also no differences in the bulk density values of the surface soils. The organic carbon content of the soils and the mean weight diameter of the soil aggregates under woodlot were more uniformly distributed and more related to each other than those of the alleys. A multi-criteria analysis using different weighting scenarios namely (i) equal sustainability and economic (ii) sustainability and (iii) economic indicated that the woodlot was the best farming practice and there was little to choose between the alleys and the conventionally tilled plots.