Nutrient Dynamics in Wetland Organic Vegetable Production Systems in Eastern Zambia

The aim of this study was to determine effects of organic inputs on vegetable crops and on a subsequent maize crop grown in wetlands. The following treatments were applied to cabbage (Brassica oleracea) and onion (Allium cepa) crops: Gliricidia sepium (Gliricidia) biomass (8 t ha-1), Gliricidia biomass (12 t ha-1), cattle manure (10 t ha-1) with half recommended fertilizer rate, and recommended fertilizer rate (800 kg ha-1 basal dressing and 250 kg ha-1 top dressing fertilizer). The residual effect of the treatments was tested on a subsequent maize crop. The soil at the sites had low organic matter content (average 2%) and it was acidic (average pH 4.4). Soil inorganic N increased significantly from 11 mg kg-1 in the unfertilized crop to 22 mg kg-1 in the Gliricidia treatments after cabbage, and from 10.3 mg kg-1 to 37.2 mg kg-1 after the onion crop. There were significant differences (P<0.05) in onion and cabbage yields and in subsequent maize yield in both cabbage and onion plots. This study concluded that the application of high quality Gliricidia prunings lead to rapid release of N and higher vegetable yields. However, there is a high amount of residual N that can be leached.