Integrating leguminous trees and shrubs in cropping systems of Southern Africa

Soil and water management is an essential element in food security, agriculture sector growth and sustainable land management of sub-Saharan Africa (SSA). The increased land degradation and declining fertility of SSA soils contribute to food insecurity and poverty. Previously, agroforestry researchers tended to focus mostly on soil nutrient replenishment as being solely responsible for post-fallow crop yield dynamics. Missing from many studies on soil fertility issues is the recognition of the important role of soil physical properties in agricultural productivity. However, many factors affect soil fertility and some agroforestry measures taken to correct soil nutrient deficiencies can also produce desirable soil physical effects. We hypothesized that planted tree fallows can potentially increase soil N status and improve soil physical properties, thus increasing subsequent crop yields. Field studies were conducted on infertile sandy clay loams at Msekera and Kagoro, Zambia, to determine the effect of contrasting fallows (natural fallow, planted non-coppicing and coppicing tree fallows) and no-tree no-fallow [maize (Zea mays L.) with and without fertilizer] on soil fertility and maize yields. This study attempted to address agricultural productivity by viewing soil fertility in terms of both chemical and physical properties. Hence, this report discusses the implications of improving the nutrient status of soils without correcting soil physical constraints. Data from both tree-(agroforestry) and non-tree-based systems have been used to illustrate important physical and chemical changes that occur in soils as a consequence of varying management regimes or cropping systems. Such data show that the concept of soil productivity refers to more than replacement of the lost nutrients. Other aspects include soil structure, soil water retention, water storage, infiltration and soil penetration resistance. The results imply that standard inputs such as mineral or organic fertilizers can maintain only some elements of soil productivity. Therefore, a broader view that incorporates the role of soil physical properties and water in influencing productivity is appropriate. This research does not attempt to provide a comprehensive treatment of all aspects of soil fertility in agroforestry systems. For example, it does not address the role of soil biota diversity in soil productivity.

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