Effect of phosphorus on three perennial Stylosanthes species in tropical Australia. II. Phosphorus and nitrogen within the plant and implications for grazing animals

Abstract

This paper describes the effect of phosphorus application on concentrations and distribution of phosphorus (P) and nitrogen (N) in three Stylosanthm specics (S. scabra cv. Scca, S. guiunensis cv. Schofield and S. viscosa CPI 34904) grown in the field on a soil of low acid-extractable (0.005 M H2SO4) P status (4 mg/kg). Over a two-year period (May 1978 to September 1979), regular measurements were made of P and N in whole plant shoots, leaves, stems, inflorescence, seed and litter.Phosphorus application increased P concentrations in all three legumes, but did not affect its pattern of seasonal change throughout the year. The P (and N) concentrations in most plant parts decreased through the wet season to reach minima in the late dry season. However, with seed there was little seasonal variation. In the second year, Seca was more efficient at taking up P and utilizing it for growth under low soil P conditions than Schofield. CPI 34904 was less efficient than Seca at taking up P, but also used P efficiently for growth. The efficient utilization of P in the leaves and stems of Seca and CPI 34904 meant that P levels were often below the level (0.12%) considered adequate for growth of grazing cattle. Dry season P concentrations in all legumes were inadequate. When P was applied, P concentrations in leaf and green stem of all species were higher than 0.12% during the wet season, but green stem of Seca and CPI 34904 fell below this level during the dry season.Applied P slightly increased N concentrations in leaves, green stems and whole plants of all species. Nitrogen concentrations were adequate (> 1.1% N) for maintenance of cattle weight at most times. Only in the late dry season did green stem of Seca contain insufficient N.Phosphorus application had little effect on the distribution of P and N within the plants. All species held a large percentage (20-50%) of these nutrients in green stems, but CPI 34904 was exceptional in directing up to 60% of the P and N to inflorescence and seed at the expense of leaf and stem.