Overcoming phosphate deficiency in flooded rice in Madagascar

Phosphorus (P) is the major limiting nutrient for irrigated rice grown on highly weathered soils in Madagascar. In these soils, the total P is often very large but strongly fixed on the large amount of iron (Fe) oxyhydroxides. These soils have in addition a very high capacity to sorb soluble P fertilizer. Soil flooding is known to increase the P availability because of reductive dissolution of P-bearing Fe oxyhydroxides. In non-reduced system, addition of organic materials (OM) can improve efficiently the availability of P in such soils. The aim this study is to understand the mechanisms of P solubilization in flooded soils in order to elaborate a fertilization strategy in low-input farming system for a sustainable productivity. More focus will be on the understanding of OM input and P availability interactions, and soil factors explaining the extent of P solubilization. Soil incubations and greenhouse experiments with various soil samples will be done to study the underlying mechanisms and soil factors involved. Their results will be used to explain rice responses to treatments in field experiments and on farm trials.Acknowledgements i Summary iii Samenvatting vi List of abbreviations x List of symbols xii Table of contents xiv CHAPTER 1 The role of organic matter management in phosphorus availability to rice: introduction and research objectives 1 CHAPTER 2 Soil flooding and rice straw addition can increase isotopic exchangeable phosphorus in P-deficient tropical soils 21 CHAPTER 3 Effects of organic matter addition on phosphorous availability to flooded and non-flooded rice in a P-deficient tropical soil: a greenhouse study 39 CHAPTER 4 Effects of soil flooding and organic matter addition on plant accessible phosphorus in a tropical paddy soil: an isotope dilution study 57 CHAPTER 5 Pronounced rhizosphere mobilization of phosphorus by rice plants in flooded and non-flooded phosphorus deficient soils 75 CHAPTER 6 Farm yard manure application does not increase phosphorus availability to flooded rice grown in highly weathered soils: field experiments 93 CHAPTER 7 General conclusions and suggestions for future research 109 Bibliography 116 List of publications 126nrpages: 142status: publishe