Effect of phosphorus supply on three perennial Stylosanthes species in tropical Australia. III. Potassium, calcium, magnesium and sodium concentrations and implications for grazing animals

Abstract

Thc effect of phosphorus application (0 and 50 kg P ha-1 as triple superphosphate) on potassium (K), calcium (Ca), magnesium (Mg) and sodium (Na) concentrations in three Stylosanthes species (S. scabra cv. Seca, S. pianensis cv. Schofield and S. viscosa CPI 34904) was examined in a field experiment. Regular measurements were made of these cations in the leaves, stems, inflorescence, seed and litter of established plants during the wet and dry seasons of 1978-79.Phosphorus supply did not affect K, decreased Ca, increased Mg, and in some fractions increased Na concentration. It decreased the ratio of Ca to P. Applied P had no effect on the changes in cation concentration through time. With increasing plant age, K decreased in leaves of all species, Ca decreased in Seca and Schofield (but remained constant in CPI 34904), Mg was relatively steady in all species, and Na decreased in Seca and CP134904 but not Schofield. A comparison of cation concentrations in intact leaves and leaf litter suggests there was remobilization of K, and to a lesser degree, Na and Mg, but not Ca from old leaves prior to senescence.Potassium and Ca in leaf and green stem were generally considered adequate for cattle requirements throughout the year. However, Mg and Na often fell below the level required for cattle growth. The wide Ca/P ratio combined with low P concentration in all species suggests that cattle grazing these pastures would respond to P supplementation.