Forms of soil phosphate in some genetically-related New Zealand soils

Abstract

Phosphate fertilisation of soils is essential for the maintenance of high levels of agricultural production. This is especially true in New Zealand, where repeated topdressings with superphosphate are essential for the maintenance of the grass-clover pastures on which the nation’s economy is founded. The quantity of superphosphate fertilisers used by New Zealand farmers during 1964 was approximately 1.5 × 10⁶ tons.

Unfortunately, as Way (1850) first observed, a large part of the applied phosphate is retained, or fixed, by the soil in forms that are unavailable or only slowly available to plants. It has been frequently suggested that only about 20 per cent of the applied phosphate is recovered in succeeding crops in normal British farming practice. Although some workers have suggested that the recovery of applied phosphate may greatly exceed this figure under some circumstances, nevertheless, it is generally agreed that fixation of phosphate by soils constitutes a major agricultural problem.

After half a century of investigation, it may be claimed that the mechanisms by which a soil fixes phosphate are now largely understood. Following the development of procedures for the determination of different forms of soil phosphate, attention has recently turned to the task of investigating the consequences of soil-phosphate interactions.

The processes occurring in virgins soils are of interest to soil scientists because they offer a yardstick, or control, against which the effect of man on soils may be evaluated. However, the very great variability of virgin soils presents difficulties for soil scientists attempting to work by the classical scientific method of isolating individual factors and examining the effect of each of these factors in turn of the system under investigation.

In New Zealand, a number of soil sequences have been recognised in which soils formed from the same or similar parent materials possess different properties due to differences in the impress of the other soil-forming factors: some of these sequences have been described by Walker (1965).

The aim of the present study was to gain further information concerning the transformations of native phosphate during the formation and development of virgin soils. The investigation commenced with an evaluation of published procedures for the fractionation of soil inorganic phosphate. The procedure finally adopted by the present author was based on that of Chang & Jackson (1957): however, a number of modifications were introduced in an attempt to overcome some of the errors inherent in the procedure of these authors. The modified fractionation procedure was used for the investigation of the forms of phosphate in one rock and three soil weathering sequences.