Soil fertility changes in the long-term experimental plots at Kybybolite, South Australia. II. Changes in phosphorus.

Abstract

The fate of phosphorus applied as superphosphate and rock phosphate to grassland plots at Kybybolite established in 1919 on solonetzic soils has been studied by soil analytical techniques. The additional effect of the amendments lime and gypsum has also been examined. From past records of phosphorus added as fertilizer, the probable amounts of phosphorus removed in animal products, and present phosphorus levels of the surface 8 in. of soil, it has been possible to draw up a balance sheet of gains and losses. The results show that considerable penetration of phosphorus applied in the water-soluble form has occurred. Less than half the phosphorus applied as superphosphate can be accounted for in the surface 8 in. The remainder is presumed to have been leached to lower soil horizons. In contrasting these results with those of other areas of similar rainfall where little movement of applied water-soluble phosphate has occurred, it is believed that the greater penetration at Kybybolite can be explained on the basis of the coarse texture and the low fixing capacity of the surface soil. Where acid-soluble phosphate has been applied, less movement through the soil is apparent, and 75–89 per cent. of the residual phosphorus applied as rock phosphate can be accounted for in the surface soil, mostly in the top 2 in. Lime has resulted in an increased retention of phosphorus in the surface soil. Gypsum, on the other hand, has not resulted in increased retention. The need for a reappraisal of the present widespread practice of continuous mono-application of superphosphate to pastures after residual levels in the soil have been raised is discussed. In particular, the possible role of acid-soluble phosphate is examined.