Effects of pasture improvement with subterranean clover and superphosphate on the availability of trace metals to plants

Abstract

The effect of pasture improvement by the use of superphosphate and subterranean clover on the availability of trace metals to plants was studied in pot culture. Increased availability of manganese, zinc, copper, iron and aluminium was found in soils that had been under continuous improved pasture for 30 or more years, but no change in the availability of molybdenum could be determined. The effects of these availability changes on plant growth differed according to the plant species grown. Oats was little affected but substantial increases in the uptake of metal cations, especially of manganese, occurred with subterranean clover and lucerne, leading to severe manganese toxicity in lucerne. No significant differences could be measured between the amounts of manganese and copper extracted from virgin and fertilized soils using several reagents, but increases in zinc extracted by hydrochloric acid, EDTA and ammonium chloride, in iron extracted by EDTA, and in aluminium extracted by EDTA and ammonium chloride were found in the fertilized soils. It is concluded that the increased metal uptake was largely due to the lower pH of the fertilized soils. However, in the case of zinc, additions of fertilizer impurities could also have contributed. The increase in EDTA extractable iron appeared to be associated with the increased organic matter content of the fertilized soils.