An alternative hypothesis to account for the generally low phosphate content of Australian soils

Abstract

It is postulated: (1) That the generally low phosphate content of Australian soils is due to the occurrence over extensive areas either of phosphate-low rocks which are largely sedimentary and built up from coarse particles which are left behind after the finer particles (which are richer in phosphate) were removed by water or wind, or of unconsolidated parent materials derived from such rocks or from other rocks with an average or low phosphate content. (2) That the process of sorting, resulting in the removal of the finer particles, is more important as a phosphate-depleting process than is leaching. (3) That significant phosphate losses may also result from the removal of surface soil and of organic matter during the process of erosion; and that this phosphate accumulates in valleys and swamps, in drainage basins in the interior, or it may he washed into the ocean. Laterite formation does not appear to be a phosphate-depleting process but, during the formation of laterite, phosphate may move within the profile, being ultimately fixed in bands rich in iron. Under conditions of impeded drainage, loss of phosphate is regarded as improbable over large areas, and it is suggested that apparent losses, such as have been reported for small areas, may be accounted for by removal of the phosphate from the soil by plants. The more obvious factors responsible for the removal of phosphate are briefly discussed, and the values for leaching obtained by certain authors are queried, chiefly on the grounds that the authors have not taken into consideration the amount of phosphate that is held in the vegetation. These views are opposed to those put forward by Wild, who suggested (1958) that the generally low phosphate values in Australian soils are due to "losses by leaching, especially under poor drainage conditions" rather than to low phosphorus in the parent rocks. This view was somewhat modified (1961) to read: "over most of the continent leaching, if this was the cause of the loss, had taken place in wetter periods of the past".