Changes in phosphorus fractions in soils under intensive plant growth

The total quantity of P and plant-available P often differ greatly in soils of the tropics, which typically range in weathering intensity. Assessing available P is fundamental to managing P in many of these soils. Phosphorus availability in some soils has been inferred from the Hedley sequential extraction assuming that each P fraction reflects similar plant availability in different soils. However, experimental measurements of plant P availability were either of short duration or involved multiple P applications, which complicates assessment of the plant availability of P fractions. The objectives of this study were to examine the changes in P fractions under exhaustive cropping on diverse soils end to discern the differences in plant availability among P fractions. Eight soils ranging in weathering from Vertisols and Mollisols to Ultisols and Oxisols were amended with Ca(H2PO4). H2O to raise soil solution P to 0.2 mg L-1 and planted for 14 crops to remove available P. The results indicated that the Fe-impregnated strip-P and inorganic NaHCO3-P (NaHCO3-P-i) decreased the most in response to plant P withdrawal in all soils. The inorganic NaOH-P (NaOH-P-i) also declined with plant P uptake in all soils. The HCI-P and residual P seemed to art as a buffer for the strip-P and the NaHCO3-P-i in the slightly weathered soils, whereas NaOH-P-i seemed to act as a buffering pool for strip-P and NaHCO3-P-i in the highly weathered soils. Residual P in the slightly weathered soils was plant-available on a relatively short time scale. In contrast, residual P in the highly weathered soils accumulated in the presence of intensive plant P removal, indicating that it was unavailable to plants. Organic P (NaHCO3- and NaOH-P-o) fractions were not significant contributors to available P in these soils that received high levels of inorganic P. Phosphorus fractions separated by the same sequential method were not of equal availability to plants in all soils.