Critical phosphorus concentrations in parts of Macroptilium atropurpureum cv. Siratro and Desmodium intortum cv. Greenleaf as affected by plant age

Abstract

Critical phosphorus concentrations were determined for different parts of Macroptilium atropurpureum cv. Siratro and Desmodium intortum cv. Greenleaf at several plant ages in order to establish an appropriate sampling technique for chemical analysis of phosphorus status. Critical phosphorus concentrations were derived by using a non-rectangular hyperbola functional relationship which allowed calculation of variance associated with critical values. When expressed relative to tissue dry weight, critical phosphorus concentrations in whole shoots declined from 0.30¦0,03% (95% confidence limits) at 41 days from sowing to 0.09¦ 0.01 % at 77 days for Siratro and from 0.33 ¦ 0.14 % at 45 days to 0.16 ¦ 0.03 % at 73 days for Greenleaf. Similar declines in critical phosphorus concentration with plant age were measured for combinations of all parts of the upper shoot back to the fourth expanded leaf. Thus plant age effects and variability associated with each determination of critical phosphorus concentration would limit the practicability of phosphorus analysis in detecting marginal phosphorus deficiency. This especially applies to plants growing as perennials in pastures where identification of plant age is not possible. Plant age effects can be lessened to some extent when critical phosphorus concentrations are calculated relative to tissue water, but it is suggested that less empirical techniques of plant analysis are required if plant age effects on critical phosphorus concentrations are to be overcome.