Soil Inorganic Phosphorus Fractionation and Availability under Greenhouse Subsurface Irrigation

Inorganic phosphorus (P) fractions and their availability under subsurface irrigation were investigated in a greenhouse planted for 5 years with tomato. Irrigation was applied when soil water conditions reached the predefined maximum allowable depletion (MAD) for different treatments, e. g., -10 kPa, -16 kPa, -25 kPa, -40 kPa, and -63 kPa. Concentrations of five inorganic P fractions, which include of soluble / loosely bound P, aluminum (Al) P, iron (Fe) P, calcium (Ca) P, and occluded P, were obtained by following a sequential chemical fractionation procedure. Results showed that the effect of subsurface irrigation and schedules on inorganic P fractions was more pronounced in topsoil layers than in deep soils. The concentrations of soluble / loosely bound P, Al P, and Fe P generally decreased with soil depth, having the largest values at the depths of 0-10 cm and 10-20 cm. In all the irrigation treatments, Al P and Fe P were the dominant fractions at the depths of 0-10 cm, 10-20 cm, and 20-30 cm, whereas Ca P and occluded P were most predominant at the depths of 30-40 cm and 40-60 cm. Soluble or loosely bound P, Al P, and Fe P were the main sources contributing to plant-available P, whereas Fe P and Al P were the two most important sources for contribution to plant-available P. Frequent irrigation with small amounts of water (e. g., irrigation with MAD of -16 kPa and -25 kPa) yielded larger concentrations of soluble / loosely bound P, Al P, and Fe P, which are the main sources of plant-available P. However, infrequent irrigation with larger amounts of water in each irrigation event led to greater concentrations of Ca P and occluded P, which are relatively less available to plants.