Response to fertilizer nitrogen and water of post-rainy season sorghum on a Vertisol. 1. Biomass and light interception

In parts of peninsular India, sorghum (Sorghum bicolor L.) is grown during the dry season using water stored in the root zone. The optimum application of nitrogen is difficult to assess because no comprehensive model exists f or the interaction of water and N. To explore this system as a basis for mo delling in the first instance and ultimately for better management, sorghum (cv. SPH-280) was grown in the post-rainy season at ICRISAT (Andhra Prades h, India) with and without irrigation and at six rates of nitrogen from zer o to 150 kg/ha applied before sowing. The biomass of top components was mea sured weekly and of roots every 2 weeks. Interception of solar radiation wa s monitored continuously in all treatments. Leaf expansion was strongly influenced both by water and by N, whereas spec ific leaf area was almost independent of treatment. In the irrigated treatm ent, the Biomass Radiation Coefficient (e) for the main growth period was a lmost independent of N application at 1.3-1.4 g/MJ and was also independent of leaf N. In consequence, the main source of differences in yield was a d ecrease in radiation interception with decreasing N. In contrast, without i rrigation, biomass, yield, e and leaf N were all maximal at 60 kg/ha N. At 33 days after emergence (DAE), root mass was almost independent of N whe ther water had been applied or not, but was somewhat smaller with irrigatio n. Later, root, leaf, and panicle mass all responded to N and to water, but stem mass was unresponsive to N with irrigation. There was evidence of tra nslocation from stem to grain in most treatments. With irrigation, a maximu m grain yield of 4.8 t/ha was obtained at 150 kg/ha N and without irrigatio n the maximum was 3.2 t/ha at 90 kg/ha.