Soil types and drainage

Abstract

At least 90 per cent of the phosphorus entering the Peel-Harvey estuarine system comes from land cleared for agriculture, most of it from the coastal plain. These soils are naturally deficient on phosphorus and sulfur and this is supplied in superphosphate, which contains about 10 per cent phosphorus and 1 percent sulphur. However, rain leaches some of this applied phosphorus from the land into drains and rivers which flow into the estuary. In 1981, farmers in the Harvey River-Mayfields Drain catchment lost the equivalent of 1,300 tonnes of superphosphate into the estuary. Between them they have in effect spent $120,000 to fertilise a crop of blue-green algae in the estuary. The amount of phosphorus lost from the land depends on a number of factors. They include the nature of the soils and the type of farming; the amount of land cleared and the effectiveness of drainage from it:and the amount and kind of fertiliser used and whether it is applied before, during or following winter rain