LEUCAENA PLUS MAIZE ALLEY CROPPING IN MALAWI .2. RESIDUAL P AND LEAF MANAGEMENT EFFECTS ON MAIZE NUTRITION AND SOIL PROPERTIES

Agroforestry systems involving leaf removal for animal fodder may resu lt in rapid depletion of soil fertility. The purpose of this research was to determine if the effects of leaf removal on soil fertility para meters and maize yield in a Leucaena leucocephala alley cropping syste m could be reversed. Three leaf management strategies in a Leucaena al ley cropping trial that had been in effect from 1987 to 1991 were inve stigated: 1) leaves returned, 2) leaves removed, and 3) leaves removed , with 100 kg inorganic N ha(-1) added. In the 1990/91 season, a 3(4) confounded factorial design was utilized to investigate the effects of leaf management strategy, N rate (0, 30 and 60 kg N ha(-1)); maize pl ant 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, leave s were applied equally to all plots, and no P was applied. The N rate and plant population treatments were continued, and the same confounde d factorial design was implemented to investigate residual leaf manage ment strategy, residual P rate, n rate, and plant population. The yiel d gap between the plots where leaves had been returned vs. removed nar rowed each season due to uniform leaf application. Application of N im proved yields during both seasons. Residual effects of the initial P a pplication decreased to only 10% of the total yield in 1992/93. Plant population affected yields only during the season of very good rainfal l. 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, b ut uniform leaf additions eliminated differences in K and Zn uptake. P lant 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 seas ons, and resulted in a relative improvement of several soil properties . The residual effect from P applications was not adequate to maximize yields.