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Declining soil fertility is a key problem faced by farmers in eastern Zambia. This chapter assesses farmers' experiences of testing improved tree fallows in participatory on-farm trials to increase soil fertility. It also highlights the development of an adaptive research and dissemination network of institutions and farmer groups for testing and disseminating improved fallows. Sesbania sesban and Tephrosia vogelii performed well, but Cajanus cajan was discontinued because it was browsed heavily by livestock. The economic analysis compared a 2-year improved fallow, followed by maize cropped for 3 years, with fertilized and unfertilized continuously cropped maize. Over a 5-year period, farmers used 11% less labour on the improved fallow plot than on unfertilized maize, but harvested 83% more maize. Improved fallows had higher returns to land and to labour than continuously cropped unfertilized maize; returns compared to fertilized maize were mixed. Farmer interest is strong, as the number of farmers planting improved fallows has increased from under 20 in 1993-94 to roughly 10 000 in 2000. Key elements contributing to the progress made thus far include: (1) effective diagnosis of farmers' problems and screening of potential solutions; (2) farmer participation in the early stages of testing of improved fallows; (3) testing of a range of management options by farmers and researchers, and encouraging farmers to innovate; and (4) development of an adaptive research and dissemination network.

Maize production in Malawi is limited by high costs and sub-optimal use of chemical fertilizers under continuous cultivation. A long-term gliricidia/maize trial was undertaken on a Ferric Lixisol from 1991/92 to 2001/02. The purpose of the study was to assess the performance of a gliricidia/maize intercropping system as a low-input soil fertility replenishment option in southern Malawi. The experiment was a 2 × 3 × 3 factorial design with three replications. Treatments included two maize cropping systems (with and without gliricidia trees), and three rates of inorganic N fertilizer (0, 24 and 48 N kg ha−1 representing 0, 25 and 50% of the national recommended N rate), and three rates of P fertilizer application (0, 20 and 40 P ha−1 representing 0, 50 and 100% of the recommended rate). No effect of P was detected on yield early in the trial, and this treatment was discontinued. The gliricidia pruning biomass did not decline after 10 years of intensive pruning, with strong correlation between tree biomass production and years after establishment (r = 0.91, p < 0.001). Application of gliricidia prunings increased maize yields by three times compared to the yield of unfertilized sole maize. Maize yield from the unfertilized gliricidia pruning treatment was superior to the yield from sole maize supplemented with a quarter or half the recommended N rate. The study confirmed that a gliricidia/maize intercropping system is a promising soil fertility replenishment option in southern Malawi and elsewhere in southern Africa.

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Trees grown for 2 to 3 yr in rotation with crops (tree fallows) are a potential technology for increasing soil fertility in maize‐based cropping systems in sub‐Saharan Africa. The objective of our study was to determine the effect of contrasting fallow options, compared with continuous maize (Zea mays L.), on light fraction soil organic matter (SOM), inorganic N (NO3 and NH4), and N mineralization. Soil was collected 2 and 3 yr after the establishment of fallow and maize systems on a sandy clay loam (Ustic Rhodustalf) in Zambia. Total soil C, N, and P were not different among the fallow and maize systems. Soil inorganic N and amount of light fraction N (150–2000 µm, <1.13 Mg m‐3) were higher for the mean of five N2‐fixing trees than the one non‐fixing tree. Inorganic N, anaerobic and aerobic N mineralization, and light fraction N were (i) higher for the mean of the two trees with lowest (lignin + polyphenol)/N ratios in leaf litter than the two trees with highest ratios in leaf litter and (ii) higher for Sesbania sesban (L.) Merr. than the mean of other trees. Sesbania and fertilized maize monoculture resulted in similar soil inorganic N, but N mineralization and light fraction N were greater after sesbania. Comparable effects of the systems on light (<1.13 Mg m‐3) and light + intermediate fraction (<1.37 Mg m‐3) SOM suggest that light and intermediate fractions can be combined to simplify the fractionation procedure. Tree species vary greatly in effect on N availability and hence their suitability for soil fertility replenishment.

In this chapter, we discuss the principal uses of baobab (Adansonia digitata), detar (Detarium microcarpum), nere (Parkia biglobosa), tamarind (Tamarindus indica), shea tree or karite (Vitellaria paradoxa) and ber (Ziziphus mauritiana) in Sahel, Africa. These species were preferred by farmers because of their nutritional, medicinal and income-generating values and their potential value. The current and future plans for their domestication are discussed.