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This article in SSSAJ

  1. Vol. 56 No. 5, p. 1510-1515
    Received: Aug 8, 1990

    * Corresponding author(s):
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Phosphorus Fractions and Availability in Sri Lankan Alfisols

  1. R. R. Sattell and
  2. R. A. Morris 
  1. Dep. of Crop and Soil Science, Oregon State Univ., Corvallis, OR 97331-7306



Phosphorus is the second most limiting element in the Alfisols of tropical Asia and Africa. The relationships between P fractions in 12 Sri Lankan Alfisols and P taken up by plants in a greenhouse experiment and P removed by five chemical extractants were examined. Soils were selected to represent different land uses and catena positions. Sufficient P to elevate soil solution concentrations to 0.2 µg P g−1 was applied to one set of samples. A second set received 75% of the P applied to the first and a third received no P. Phosphorus uptake by foxtail millet [Setaria italica (L.)] was determined after 25 d of growth in a Neubauer experiment. Soil P was fractionated sequentially into labile (removed by 0.5 M NaHCO3) and moderately labile (removed by 0.1 M NaOH) inorganic and organic P as well as more stable soil P components. On average, 45% of total soil P was in the NaOH fraction and, of this, 72% was in an organic form. Plant P was significantly related to labile inorganic (NaHCO3-Pi) and moderately labile organic (NaOH-Po) fractions. Double-acid P (extracted with 0.5 M HCl plus 0.0125 M H2SO4) was significantly related to the same fractions. Moreover, double-acid P was better correlated with plant P than was P removed by those extractants highly correlated only with labile inorganic P. The study suggested that the moderately labile organic P fraction as well as the labile inorganic P fraction contributed significantly to P uptake from tropical Alfisols and that the double-acid extractant was superior to soil P extractants that are uncorrelated to the moderately labile organic P fraction.

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