Phosphorus: its efficient use in agriculture

Changes in understanding the behavior of soil and fertilizer phosphorus (P) during the last 150 years are presented and recent concepts have been linked with agronomic data to produce a model that considers four pools of inorganic soil P related to their plant availability and extractability by chemical extractants. The stronger the bonding of phosphate ions to soil components, the lower the plant availability. P-use efficiency in agriculture is related to soils reaching and being maintained at a critical level of readily plant-available P, and factors affecting the critical level are discussed. Efficiency can be assessed by the direct, difference, and balance methods. The latter, calculated as the P output/input ratio, shows that P-use efficiency can exceed 80–90%. Combined data from controlled experiments in England and derived “statewide” aggregate information in the United States relating output/input ratios to changes in plant-available P could best be described by a single, simple function, making a powerful and convincing statement suggesting that there is an underlying “simple rule” for the behavior of plant-available inorganic soil P that is related to the four-pool concept discussed.