Fractionation of Inorganic and Organic Phosphorus in Virgin and Cultivated Soils1
- A. N. Sharpley and
- S. J. Smith2
Amounts of phosphorus (P) in inorganic and organic pools were determined for three noncalcareous and five calcareous surface soils (0–150 mm) which had been cultivated for at least 15 yr, and their virgin analogues, to ascertain if relative pool sizes or soil P fertility are being changed by cultivation and associated fertilizer application. The soils were representative of major agricultural areas of the USA, with total P concentrations ranging from 200 to 1920 mg kg−1 (approximately 50% inorganic P) and P applications from 0 to 90 kg ha−1 yr−1. P forms in the pools and extractants used were as follows: inorganic P; loosely-bound (1.0M NH4Cl), nonoccluded (0.5M NH4F + 0.1M NaOH), occluded (citrate-dithionite-bicarbonate + 0.5M NH4F + 0.1M NaOH), and Ca bound (1.0M HCl) (calcareous soils only): organic P; labile (0.5M Na HCO3), moderately labile (1.0M H2SO4 + 0.5M NaOH), moderately resistant (0.5M NaOH/HCl soluble), and resistant (0.5M NaOH/HCl insoluble). Distribution of inorganic P in virgin noncalcareous soils was, on average, 2% loosely-bound, 52% nonoccluded, and 46% occluded. Cultivation resulted in an increase in nonoccluded and decrease in occluded P. Average distribution in the virgin calcareous soils was 2% loosely-bound, 11% nonoccluded, 9% occluded, and 78% Ca-bound, with no significant change in pool size with cultivation observed. On average, organic P in the virgin soils was 7% labile, 48% moderately labile, 33% moderately resistant, and 12% resistant, with no significant change in relative amounts due to cultivation. A mounts of fertilizer P applied and organic P mineralized and immobilized in stable inorganic P pools during cultivation, were related to P sorption index of the soils. The conversion of organic to inorganic P may be reversed by management practices allowing a build up of soil organic matter. Since little change in amounts of loosely-bound inorganic and labile organic P was observed during cultivation, the more stable pools may represent a better estimate of long-term soil fertility.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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