Improved estimates of ‘critical’ phosphorus concentration from considerations of plant phosphorus chemistry

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Abstract
Phosphorus concentration in soluble, lipid and residue P fractions were measured in white clover (Trifolium repens) and Stylo (Stylosanthes hamata cv. Verano) over a 10 day period. The P concentrations to produce 90% of maximum yield ('critical level') was found to vary least in P fractions that have a metabolic role (lipid and residue). Soluble P and hence total P concentration was most variable. These data suggest that measures of nutrient content of metabolically active forms of P should be used to determine ‘critical’ levels as these fractions vary least over time.