Effects of NPK fertilizer combinations on yield and nitrogen balance in sorghum or pigeonpea on a vertisol in the semi-arid tropics

A long-term experiment was carried out on a Vertisol from 1986 to 1992 to e xamine the combined effects of NPK fertilizers on yield using sorghum (Sorg hum bicolor L. Moench cv. CSH 5) and short-duration pigeonpea (Cajanus caja n L. Millsp. cv. ICPL 87). The fertilizer treatments were as follows: 0 (no fertilization), N (150 kg N ha(-1)), P (65.5 kg P2O5 ha(-1)), K (124.5 kg K2O ha(-1)), and all possible combinations (NP, NK, PK, and NPK). In this s tudy we continued this experiment during the period 1993 to 1994 and analyz ed the crop yield response to fertilizers and the N balance. The amount of N derived from the atmosphere and fertilizer was estimated by the N-15 natu ral abundance method and N-15 isotope dilution method, respectively. A comb ined application of N and P fertilizers gave the highest grain yield for th e two crops under the 8th and 9th continuous croppings, unlike the applicat ion of K fertilizer. The values of total N for the two crops were significa ntly higher in the NP and NPK plots. These crops took up N mainly from soil . There was a significant positive relationship between the uptake of N-dff and N-dfs by each crop. Pigeonpea or sorghum took up more N from the soil in the N fertilizer plots than in the plots without N, suggesting that soil N fertility was enhanced and the amount of N supplied from soil increased in the plots with consecutive application of N fertilizer for 7 y. Even pig eonpea, which fixes atmospheric N inherently, needed N fertilizer to achiev e high grain yield, suggesting that N fixation by the nodules was not alway s sufficient to meet the N requirements of the crop under these conditions. Although fertilizer N exerted a beneficial effect on plant growth and yiel d in the two crops, the values of fertilizer N recovery (FNR) by the two cr ops were considerably low. Therefore, it is suggested that the development of N fertilizer management which could maximize FNR of each crop should be promoted.