Changes On Phosphorus Functional Fractions of Two Louisiana Alluvial Soils Grown with Corn At Different Phosphorus Rates.

Soils in the Mississippi River alluvial plains constitute one of the seven distinct soil areas in Louisiana commonly under crop production occupying approximately more than 2 million hectares. A pot experiment was conducted in 2011 to evaluate the changes on Mehlich-3 (M3) extractable P and P functional fractions of two alluvial soils (Commerce silt loam and Perry clay) in Louisiana which are commonly cultivated for corn production. Pots containing approximately 22 kg air-dried soil were mixed with different P rates (0, 34, 67, 101 and 134 kg P₂O₅ ha^-1). Treatments were replicated four times and arranged in a randomized complete block design. After 30 days, seeds of corn were sown and grown until maturity. Mehlich-3 extractable P and different inorganic P fractions (P_i) of soil samples collected at pre-planting and harvest were quantified. The P fractions included labile P, Al-P, Fe-P, reductant-P, and Ca-P which correspond to sequentially extracted NH₄Cl-P_i, NH₄F-P_i, NaOH-P_i, NaHCO₃-P_i, and H₂SO₄-P_i. The application of P fertilizer significantly raised M3-extractable P of both soils (P<0.05). A high correlation between P rates and M3-extractable P was obtained at pre-planting for both Perry clay and Commerce sl with coefficient of determination (r²) values of 0.983 and 0.956, respectively.  At harvest, the high correlation between P rates and M3-extractable P was only observed for Commerce sl (r²=0.984). Labile P and Al-P fractions of samples collected before planting increased with increasing P rate while the relationship of Fe-P with P rates became observable only at harvest for both soils. With time across P rates, both soils showed build-up of less readily-available reductant-P. There was no clear effect of P fertilization on growth and development of corn on Commerce sl. Perhaps the increasing levels of M3-extractable P and readily-available labile P and Al-P fractions with P application rates can explain the significant increase in grain yield of corn grown on Perry clay. Our findings demonstrate that P fertilization depending on soil properties can promote certain P fractions in the soil where potential contribution to corn P nutrition would likely have to come from enhanced level of readily available P fractions.