The Impact of Phosphorous Applications and Incubation Periods on P- Desorption Characteristics with Successive DMT-HFO-P Extractions on P Fixing Soils

Aim: The overall objective of this study was to establish the rates and how much of the transformed or fixed Phosphorus can be available to plants over time, having determined the extent of Phosphorus fixation by both Rustenburg and Loskop soils.

Methodology: Therefore, the impact of applied Phosphorus rates (0, 25, 50, 100, and 200 mg kg-1) and incubation period (1, 120, and 240 days) on the Phosphorus desorption rates of a red-sandy clay soil (high P fixing) and a red-sandy loam soil (low P fixing), were investigated. Cumulative and sequential P extractions/desorption were carried out for 1, 7, 14, 28, and 56 days per treatment combination to determine Phosphorus (P) transformations.

Results: About 30 – 60% of Phosphorus added was transformed into less stable P pools within one day and 80-90% after 120 days. The transformation of P applied to less labile P pools was faster in the red-sandy clay soil (high P fixing) than in the red-sandy loam soil (low P fixing). The cumulative DMT-HFO-P extraction curves of the Rustenburg and Loskop had not reached plateaux, indicating that desorption rate could proceed for a much longer period than the 56 days. This is important for crops such as tobacco due to residual effects of the added Phosphorus fertilizer and thus lower application rates.

Conclusion: Much as Rustenburg soil is reflected to be a high P fixing, the P release rates are still more able to meet most of crops requirements; however, importantly is may depend on how much of the soil volume is exploited by the root system of a particular crop.