Phosphorus toxicity in Subterranean clover and oats grown on Muchea sand, and the modifying effects of lime and nitrate-nitrogen

Abstract

Phosphorus supplied as mono-sodium phosphate depressed the growth of subterranean clover and oats grown on Muchea sand in pot culture. In both plants the detrimental effects appeared at phosphorus levels equivalent to about 2-4 cwt. per acre superphosphate, provided both nitrogen and lime were in very short supply. Adequate nitrogen, as sodium nitrate, invariably overcame the toxic effects of phosphorus within the range of levels examined. With the clover, nodulation was usually poor, but some evidence is presented in which "heavy" inoculation with Rhizobium markedly increased nodulation and alleviated the toxic effects. Nodule numbers in the clover were increased by addition of lime and in most instances the beneficial effects resulted from enhanced nitrogen supply. Lime was advantageous to the growth of oats, also, wherever applied nitrogen was low. Possible explanations of this are suggested. Phosphorus toxicity symptoms are described for both plants and the relation to percentage total phosphorus in the leaves is discussed. Figures in excess of 1.4 per cent. and 3 per cent. total phosphorus were found in affected leaves of subterranean clover and oats respectively. Identical symptoms were observed in clover with the mono- and dibasic phosphates of sodium, calcium, and potassium.