Suppression of superphosphate-phosphorus fixation by farmyard manure II. Some studies on the mechanisms

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Abstract
Studies were conducted on a calcareous loam soil to find out the effect of premixing superphosphate (SUP) with farmyard manure (FYM) at a 1 to 4 ratlo on the NaHCO3 extractable phosphorus, lime, and phosphate potentials, and the conversion of SUP to various phosphatic forms. SUP was applied at rates of 98, 245, 490, and 2453 kg P/ba. The soil was incubated for various time intervals from 0 to 96 days. At all the time intervals, NaHCO3 extractable phosphorus remained at a higher level in the premixed treatments than that found in the unmixed or SUP treatment only. The soil solution in the premixed treatments contained Ca phosphates of higher solubility than those in the unmixed treatment. The content of Al, Fe-P, and NH4Cl extractable-P (soluble-P) in the soils of premixed treatments were found to be greater, occurring at the expense of Ca-P. Octocalcium phosphate appeared to be a fairly stable form in all these soils. The implications of these studies on the availability of phosphorus to plants have been discussed.