4.3 Article Proceedings Paper

Chemical changes of applied and native phosphorus during incubation and distribution into different soil phosphorus pools

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TAYLOR & FRANCIS INC
DOI: 10.1081/CSS-200043281

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phosphorus adsorption; desorption; distribution; fractionation; sequential extraction; transformation

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The chemical transformations of applied and residual soil phosphor-us (P) into different pools in two soils [Alfisols], a red sandy clay soil [Haplo-Palcustalfs] from Rustenburg, which is a high P fixing, and a red sandy loam soil [Pale-Xerults] from Loskop, a low P fixing were examined after treatments with different P rates (0, 25, 50, 100, 150, and 200 mg kg(-1)) and incubation periods (1, 60, 120, 180, and 240 days) under laboratory conditions. A sequential P extraction procedure was carried out on each treatment combination to determine the changes in 1) soluble and labile, 2) adsorbed, and 3) occluded and residual P pools in each soil. By fractionating soil P into different pools, it was possible to observe the transformation and distribution of added and initial soil P into different pools after each incubation period. Although solution and labile P decreased with time of incubation, there were corresponding increases in adsorbed, occluded, and residual P. Loskop soil had more marked increases in solution and labile P than the Rustenburg soil, which showed higher values for the adsorbed, occluded, and residual P forms. These differences could explain the reportedly higher levels of P fixation (adsorption and/or precipitation) by the Rustenburg than by the Loskop soil. Approximately 30-60% of the added P was transformed into less labile P pools in 1 day and 80-90% of the added P was transformed to the less labile P pools after 60 days of incubation. This transformation was very well marked in the higher than in the lower P fixing soil. A major proportion of the P transformation was to the NaOH-extractable P pool (adsorbed/nonlabile pool). The added P desorbed from this pool was fairly constant (about 30%) for the different incubation periods. Thus, to maximize fertilizer P efficiency, especially in the Rustenburg soil, band placement at planting time should be recommended, because the findings show that after 1 day of incubation 60% of the added P was transformed into less labile P. When band placed in the soil, the soil in the vicinity of the band will be saturated with P, and some P will remain in more labile forms and thus available for a longer period of time. The use of plants with well-developed root systems could also be recommended to ensure that the roots explore the soil volume more effectively. It may also be noted that although the added fertilizer P is transformed to more stable (immobile) P forms, it could be seen as long-term residual P pools for plants.

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