CHANGES IN YIELD RESPONSE TO MAJOR NUTRIENTS AND IN SOIL FERTILITY UNDER INTENSIVE RICE CROPPING

Abstract

Nutrient balance is a critical factor in maximizing grain yield of a single crop and in crop intensification. This paper summarizes data on rice yield and soil fertility parameters from long-term fertility trials, under a rice-rice cropping system, conducted at four sites in the Philippines: three rice experiment stations, namely, Bicol (Pili clay: Typic Pelludert), Maligaya (Maligaya silty clay: Vertic Tropaquept), and Visayas (Santa Rita clay: Typic Pelludert) from 1968 to 1986; and the IRRI farm (Maahas clay: Andaqueptic Haplaquoll) from 1964 to 1986. Large responses to complete NPK fertilizer were observed at all four sites, but response to individual nutrients (N, P, or K) differed greatly among sites. N-use efficieny did not change over time at IRRI and Bicol, but decreased substantially over time at Maligaya and Visayas. P-use efficiency, however, increased drastically over time both at Maligaya and Visayas, more so after 10 yr of cropping, but no change in P response over time was observed at Bicol. At IRRI, there were no appreciable responses to P or K throughout the 23-yr period; at the other sites where there were appreciable responses to P and K, interactions between P and K were large, i.e., response to P was larger when K was applied than when no K was applied, and response to K was larger when P was applied than when no P was applied. The difference in response between the dry and the wet seasons was more pronounced in responses to P and K than with response to N. Yields from organic + inorganic NPK fertilizer (tested only at IRRI) did not differ from those from NPK inorganic fertilizer alone at any time in the 23-yr period. Under intensive rice-rice croppings, the soil pH decreased, and the total N and organic matter content increased, with successive applications of one or more nutrients over time. Extractable P values in P-fertilized plots were higher than those in plots that did not receive P over time. The exchangeable K values decreased with successive application of P and increased with successive application of K, over time. The effects of successive application of fertilizers on CEC and exchangeable Ca in the soil were not consistent.