Chemical Changes of Applied and Native Phosphorus During Incubation and Distribution into Different Soil Phosphorus Pools

Abstract The chemical transformations of applied and residual soil phosphorus (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.