Phosphorus Relationships in Flooded Rice Soils with Low Extractable Phosphorus
- H. Shahandeh,
- L. R. Hossner and
- F. T. Turner
Rice (Oryza sativa L.) grain yields on some flooded soils are not increased by P fertilizer despite their low soil test P status determined on air-dry soils by soil test methods such as Bray, Olsen, or Texas A&M. Conventional soil test methods apparently do not accurately assess the capacity of these soils to supply P to rice grown under flooded conditions. To test the possibility of an Fe-P association, oxalate extractant (which would extract noncrystalline Fe and its associated P) was used to provide a more accurate measure of available P in flooded soil. Phosphorus response experiments were conducted on 10 rice soils under greenhouse and field conditions and related to the oxalate-extractable Fe and P, P uptake, P adsorption, and P desorption under aerobic and anaerobic conditions. Oxalate-extractable Fe significantly increased under anaerobic conditions in all soils including the soils that were nonresponsive to P and had low levels of extractable P using conventional extractants. Phosphorus adsorption and desorption data confirmed the importance of oxalate extractant in predicting P availability following anaerobiosis. The inability of Bray 1, Olsen, and Texas A&M soil test methods to accurately predict P availability in flooded rice soil appears to be due to their failure to extract the active reductant-soluble P fraction. The measurement of P associated with poorly crystalline Fe by oxalate extractant is a useful soil test method for predicting the availability of P in flooded rice soils.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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