Distribution of phosphorus in sandy soils of coastal fynbos
The levels of soil phosphorus (both total and plant available) were investigated at the Pella intensive study site and were found to be low. Resin extractable phosphorus fluctuated seasonally, peaking at the soil surface during summer and declining to a minimum in the winter months. Rates of microbial mineralisation were thought to be the major influencing factor. The profile distribution of all forms of phosphorus were similar to those predicted by Smeck (1973) and were thought to result from the removal of phosphorus by plant roots in the mid-zone (20-70 cm) and its subsequent deposition at the soil surface as litter. The majority of insoluble phosphorus (60%) was organic and the remaining insoluble inorganic compounds were mainly iron bound. The input from precipitation to the site was 194 g ha⁻¹ yr⁻¹ occurring chiefly during the winter months. The different soil forms present at the site contained similar total phosphorus levels and underwent an increase in the size of the resin extractable phosphorus pool with progression from less to more weathered soils. This progressive change was linked directly to the decrease in total iron content in the soils. With decreasing iron content the number of binding sites within the soil declined. Fire returned a large amount of readily available phosphorus to the soil which had the effect of immediately elevating the soil resin extractable phosphorus levels. This effect was a transient one. After twelve months the levels of resin extractable phosphorus had declined to values only slightly above those found in the prefire soil.
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