The response of subterranean clover (Trifolium subterraneum L.) to foliar applications of phosphorus

Subterranean clover plants were grown in river sand or in culture solutions with and without phosphorus. Some phosphorus-deficient plants were sprayed once every 2–3 days with solutions (pH 2.5) of several phosphorus compounds at various concentrations, with and without wetting agents. Dry weights of tops and roots were significantly increased by most spray treatments compared with control plants grown without phosphorus. Foliar applications of 50 mM phosphoric acid solutions, containing little or no wetting agent, generally gave the greatest response. In the first experiment the plant dry weight in the best spray treatment was twice as high and in the second experiment 3.5 times as high as in the controls without applied phosphorus. In these spray treatments plant dry weight was no more than 40% of that in the control treatment receiving root phosphorus. Two days after application of 32P (solution pH 5.5, 0.01 mM phosphate) to the centre leaflet of either the first or the fourth trifoliate leaf, 70% of the tracer could be removed by washing with 20 ml water. Seven days after application of the tracer the treated leaflet of the first or the fourth trifoliate leaf still contained 77 or 70% respectively of the amount absorbed by the plant. In another experiment, in which 32P was applied in a 30 mM H3PO4solution at pH 2.5 or 5.0, washing of the treated leaflet 2 days after application removed 42 and 60% of the tracer respectively. Seven days after application the treated leaflet contained 28 and 34% respectively of the residue after washing. Relatively slow rates of uptake of phosphorus applied to the leaves were considered to be at least partly responsible for the poor growth responses compared with phosphorus applied to the roots. It was concluded that foliar applications of phosphorus offered little scope as a practical means of hastening the recovery from phosphorus deficiency.