Yield performance and agronomic efficiency of wetland rice under long term fertilization in an entisol

A study was initiated in 1977 on an entisol soil at the Cropping Systems Research Centre, Karamana, Kerala, on a rice rice cropping system under wetland conditions during kharif and rabi seasons of 2005 2007. The experiment was laid out in a partially confounded factorial design with three levels of N (40, 80 and 120 kgha⁻¹), 3 levels of P₂O₅ (0, 40 and 80 kgha⁻¹), and two levels of K₂O (0 and 40 kgha^"1). After 30 years of cropping, without P fertilization a decrease in plant height, total and productive tillers was observed. In contrast to N, consecutive increase in P up to the third level was found to stimulate growth and tiller production significantly. Grain yield and dry matter production were markedly reduced in treatments completely depleted of P but supplied with adequate amounts of N and K. The per cent increase in grain yield over control increased with incremental additions of N and P but the magnitude of increase was more for P levels. The agronomic efficiency was higher at lower N and P levels. Skipping P significantly decreased N, P and K uptake. Though phosphorus is not generally a limiting nutrient in the acid entisols of India, it is equally important for rice as N and K and its long term depletion is found to affect crop response to N as well. Thus imbalanced and inadequate fertilization affects yield response of wetland rice to nutrients.