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Soil Science Society of America Journal Abstract -

Determination of Bioavailable Phosphorus in Soil


This article in SSSAJ

  1. Vol. 56 No. 3, p. 814-818
    Received: Mar 18, 1991

    * Corresponding author(s):
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  1. S. J. Thien  and
  2. R. Myers
  1. Dep. of Agronomy, Throckmorton Hall, Kansas State Univ., Manhattan, KS 66506



Available soil P (AP) is usually based on inorganic P (Pi) soil-test levels, a practice that ignores the potential contribution of organic P (Po) mineralization. Our objective was to develop a P-availability index that included both Pi and Po dymanics. Fourteen Kansas soils, ranging in bicarbonate-extractable Pi 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 Pi, total P (Pt), and inorganic plus microbial P (Pi+m) in both soils and used these data to describe the size of the labile Po, microbial P (Pm), immobilized Pi (Pimmob), potentially mineralizable Po (Pmin), total bioavailable P, and bioavailable Po pools. Bioactive soils always had less Pi than untreated soils after incubation, probably, because of microbial immobilization. Labile Po in the bioactive soils averaged 2.4 times greater than in untreated soils. Labile Po after incubation averaged 2.8 times greater than the Pi level prior to incubation in bioactive soils and remained essentially the same in untreated soils. Clearly, microbial activity enhanced Po availability, an increase in AP that would not be apparent in soil tests where chemical extractions measure only Pi. We combined the Pi, Po, and Pm pools to create a bioavailable-P index. This index differed from the typical soil-test P index (preincubation Pi) 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.

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