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DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

Changes in Phosphorus Fractions in Soils under Intensive Plant Growth

Dep. of Tropical Plant and Soil Science, Univ. of Hawaii, 1910 East West Rd., Honolulu, HI 96822 USA

rsyost{at}hawaii.edu

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 and 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 HCl-P and residual P seemed to act 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.

Abbreviations: DAE, days after emergence • i (subscript), inorganic • o (subscript), organic • T (subscript), total

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