Potential of Cowpea, Pigeonpea and Greengram to Contribute Nitrogen to Maize in Rotation on Ferralsol in Tanga – Tanzania

A glasshouse incubation of soil from field experiment where cowpea, pigeonpea and greengram were grown in rotation with maize was carried out at Mlingano Agricultural Research Institute. The objective was to study the N mineralization of the soil and its potential to supply N to the maize crop following legume rotation. The experiment comprised of eight treatments which included the three legumes in rotation with maize, with the maize stover removed or retained on the plots and continuous maize with the stover removed or retained. Soil sampling for the incubation was carried out before maize planting from treatment plots at 0–20 cm depth, the soil sieved through a 6-mm screen while fresh, and then 250 g of the soil incubated in 500 ml volumetric flasks at 60% water-holding capacity for 42 days. Destructive samplings were done at 14-day intervals and analysed for mineral N. The N mineralization increased with incubation time, with cowpea and pigeonpea having significantly higher quantities at the 42nd day sampling compared to those from greengram and continuous maize plots. The contributions of cowpea, pigeonpea and greengram to the soil N at this sampling time were equivalent of 47, 40 and 13 kg ha–1, respectively. Out of the total N mineralized during the entire period of incubation, 67, 51, 86 and 80% were on the 14th day for the cowpea, pigeonpea, greengram and continuous maize plots, respectively. Such early mineralization is not in synchrony with maize plants demand necessitating top dressing of N fertilizer as supplement for increased maize yields.