Green Manure and N Fertilizer Rate Effects on Cassava and Sweet Corn Sequential Cropping in a Sandy Typic Plinthustult

Abstract

Appropriate N fertilizer rate for cassava and sweet corn sequential cropping grown after different green manures was investigated on a Typic Plinthustult in the tropical climate of Thailand. Split-plot experimental design was used, and three treatments, ruzi grass (Brachiaria ruziziensis), sun hemp (Crotalaria juncea), and local weeds (e.g., jungle rice (Echinochloa colana), crabgrass (Digitaria biformis), and coat button (Tridox procumbens)) grown as green manure, were chosen as the main plot. Nitrogen fertilizer levels, 0, 0.5, 1.0, and 1.5 times the rates recommended for cassava (94 kg N ha) and sweet corn (191 kg N ha), were tested as subplots. Comparing average yield over all fertilizer treatments, ruzi grass statistically had the highest dry biomass and the lowest N content and released the highest NH4-N but gave the lowest tuber yield and peeled sweet corn grown after cassava. Incorporation of sun hemp, having the highest N content and the highest NO3-N released to the soil, significantly gave the highest yields of cassava (25.1 t ha) and peeled sweet corn (3.5 t ha) and aboveground biomass. Incorporated sun hemp and N applied at recommended rates significantly gave the highest cassava fresh tuber yield of 27.2 t ha. Either crop grown sequentially showed no response to N fertilizer rates except for cassava starch content that significantly decreased with increasing N fertilizer rates. In a separate experiment, sun hemp decomposition resulted in a slightly higher available N production than did ruzi grass. The sun hemp decomposition was greater in most weeks during the experiment.