MULTIPURPOSE TREE PRUNINGS AS A SOURCE OF NITROGEN TO MAIZE UNDER SEMIARID CONDITIONS IN ZIMBABWE .1. NITROGEN-RECOVERY RATES IN RELATION TO PRUNING QUALITY AND METHOD OF APPLICATION

When multipurpose-tree (MPT) prunings are used as a source of nitrogen to annual corps in agroforestry systems, it is important that high le vels of N recovery are attained. In order to test the effect of prunin g quality and method of pruning application on N-recovery rates, a fie ld experiment was conducted using prunings of five MPT species and two methods of application (surface versus incorporation). There was an i nteraction of pruning quality and method of pruning application on N r ecovery and maize grain yield. For most MPT species used, incorporated prunings gave high nitrogen recovery compared to surface applied prun ings. In order to achieve synchrony between N supply from prunings and N demand by a maize crop, relatively large amounts of N should be rel eased into the soil before peak N demand by the maize crop. This could be achieved by incorporating adequate quantities of high quality prun ings such as those of Cajanus cajan and Leucaena leucocephala.