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Leucaena + maize alley cropping in Malawi; part 2: residual P and leaf management effects on maize nutrition and soil properties
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Agroforestry systems involving leaf removal for animal fodder may result in rapid depletion of soil fertility. The purpose of this research was to determine if the effects of leaf removal on soil fertility parameters and maize yield in aLeucaena leucocephalaalley cropping system could be reversed. Three leaf management strategies in aLeucaenaalley cropping trial that had been in effect from 1987 to 1991 were investigated: 1) leaves returned, 2) leaves removed, and 3) leaves removed, with 100 kg inorganic N ha−1added. In the 1990/91 season, a 34confounded factorial design was utilized to investigate the effects of leaf management strategy, N rate (0, 30 and 60 kg N ha−1); maize plant population (14,800, 29,600, and 44,400 plants ha−1); and P rate (0, 18, and 35 kg P ha−1). In the 1991/92 and 1992/93 seasons, leaves were applied equally to all plots, and no P was applied. The N rate and plant population treatments were continued, and the same confounded factorial design was implemented to investigate residual leaf management strategy, residual P rate, n rate, and plant population. The yield gap between the plots where leaves had been returned vs. removed narrowed each season due to uniform leaf application. Application of N improved yields during both seasons. Residual effects of the initial P application decreased to only 10% of the total yield in 1992/93. Plant population affected yields only during the season of very good rainfall. Leaf additions resulted in a relative increase in soil pH, total N,and exchangeable Ca, Mg, and K and a decrease C/N ratio in plots that had not previously received leaves. Leaves supplied more K and Zn to the upper 15 cm of soil than were being extracted by the maize crop, but uniform leaf additions eliminated differences in K and Zn uptake. Plant Zn uptake decreased with increasing P rate and plant population, and increased with increasing N rate and a history of leaf return. The results show that applying leaves equilibrated yields within two seasons, and resulted in a relative improvement of several soil properties. The residual effect from P applications was not adequate to maximize yields.