NITROGEN REQUIREMENTS AT DIFFERENT GROWTH-STAGES OF SHORT-DURATION PIGEONPEA (CAJANUS-CAJAN L MILLSP)

The response to N fertilization of a short-duration pigeonpea genotype , ICPL 87, was studied in the field to assess the scope for geneticall y improving symbiotic N-2 fixation by pigeonpea. The field study was u ndertaken during 1985, 1986 and 1987 growing seasons on Vertisol and A lfisol at ICRISAT Center (peninsular India), Inceptisol at Gwalior (ce ntral India) and Entisol at Hisar (northern India) in as non-limiting environmental conditions as possible. Nitrogen fertilizer was applied to the soil at various growth stages to determine when N becomes most limiting. There was a significant response in grain yield to fertilize r N applied at flowering in Vertisol but not in Alfisol, Inceptisol or Entisol. This suggests that biological N-2 fixation by short-duration pigeonpea was not adequate to meet N requirements of the crop grown i n Vertisol but that it was probably adequate in the other three soil t ypes. These results are discussed in relation to the nodulation and ac etylene reductase activity of pigeonpea and also N mineralization pote ntial of different soils. It can be concluded that there is a need for genetic improvement of N-2 fixing ability of short-duration pigeonpea grown on heavy textured soils such as Vertisols.