DETERMINATION OF BIOAVAILABLE PHOSPHORUS IN SOIL

Available soil P (AP) is usually based on inorganic P (P(i)) soil-test levels, a practice that ignores the potential contribution of organic P (P(o)) mineralization. Our objective was to develop a P-availability index that included both P(i) and P(o) dymanics. Fourteen Kansas soils, ranging in bicarbonate-extractable P(i) from 1.5 to 21.5 mg P kg-1, were used. One set of soils was untreated and another enriched with C + N source to raise microbial activities and create a biological sink for P. The treated soil was designated as bioactive. After 7 d of incubation, we measured the quantity of NaHCO3-extractable P(i), total P (P(t)), and inorganic plus microbial P (P(i + m)) in both soils and used these data to describe the size of the labile P(o), microbial P (P(m)), immobilized P(i) (P(immob)), potentially mineralizable P(o) (P(min)), total bioavailable P, and bioavailable P(o) pools. Bioactive soils always had less P(i) than untreated soils after incubation, probably, because of microbial immobilization. Labile P(o) in the bioactive soils averaged 2.4 times greater than in untreated soils. Labile P(o) after incubation averaged 2.8 times greater than the P(i) level prior to incubation in bioactive soils and remained essentially the same in untreated soils. Clearly, microbial activity enhanced P(o) availability, an increase in AP that would not be apparent in soil tests where chemical extractions measure only P(i). We combined the P(i), P(o), and P(m) pools to create a bioavailable-P index. This index differed from the typical soil-test P index (preincubation P(i)) more for the bioactive soil (r2 = 0.50) than for the untreated soil (r2 = 0.74). These data suggest this extraction scheme can enhance understanding of the P-supplying power and P nutritional dynamics of certain soils.