Lowland rice response to thermophosphate fertilization

Use of adequate rates of phosphorus (P) in crop production on high-P-fixing acid soils is essential because of high crop response to P fertilization and the high cost of P fertilizers. Information on lowland rice response to thermophosphate fertilization grown on Inceptisols is limited, and data are also lacking for soil-test-based P fertilization recommendations for this crop. The objective of this study was to evaluate response of lowland rice to added thermophosphate and to calibrate P soil testing for making P fertilizer recommendations. A field experiment was conducted for two consecutive years in central Brazil on a Haplaquept Inceptisol. The broadcast P rates used were 0, 131, 262, 393, 524, and 655 kg P ha(-1), applied as thermophosphate Yoorin. Rice yield and yield components were significantly increased with the application of P fertilizer. Average maximum grain yield was obtained with the application of 509 kg P ha(-1). Uptake of macro- and micronutrients had significant quadratic responses with increasing P rates. Application of thermophosphate significantly decreased soil acidity and created favorable macro- and micronutrient environment for lowland rice growth. Across 2 years, soil-test levels of Mehlich 1-extractable P were categorized, based on relative grain yield, as very low (0-17 mg P kg(-1) soil), low (17-32 mg P kg(-1) soil), medium (32-45 mg P kg(-1) soil), or high (> 45 mg P kg(-1) soil). Similarly, soil-test levels of Bray 1-extractable P across 2 years were very low (0-17 mg P kg(-1) soil), low (17-28 mg P kg(-1) soil), medium (28-35 mg P kg(-1) soil), or high (> 35 mg P kg(-1) soil). Soil P availability indices for Mehlich 1 extractant were slightly higher at higher P rates. However, both the extracting solutions had highly significant association with grain yield.