Changes in yield response to major nutrients and in soil fertility under intensive rice cropping

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.