Soil nitrogen and physical properties and maize yields after mixed planted fallows of tree and herbaceous legumes

Abstract

An experiment was conducted at Msekera Research Station from 1998 to 2002 to determine effect of planted fallows of tree species mixed with herbaceous legumes and those of single species fallows on biomass production, soil properties and subsequent maize grain yields. After 2 years of fallow the highest total above ground biomass of 9.2 t ha−1 was recorded in sole Gliricidia. Pre-season soil total inorganic N in 0–20 cm soil depth under tree fallows was highest in Sesbania + Macrotyloma axillare mixture. Sesbania + M. axillare mixture had the highest cumulative water intake at fallow clearing. Among the 7 mixtures and 3 sole species treatments, Sesbania + M. axillare mixture produced the highest maize yield of 2.7 and 1.9 t ha−1 in 2001 and 2002 season, respectively, after 2 years of fallow. The results of the study indicate the complementarity of herbaceous legumes and tree mixtures in improving soil quality. We conclude that mixed species have the potential to increase the amount and quantity of total N added to the soil and increase nutrient cycling. However, maize grain yields were not significantly improved as compared with single species due to low biomass production of the trees. The coiling nature and the added labour requirements for weed control would limit the use of these mixtures by smallholder farmers who are labour constrained. Agroforestry trees and non-twining legumes could be promoted instead.