EFFECT OF PLANT-AGE ON THE CRITICAL INORGANIC AND TOTAL PHOSPHORUS CONCENTRATIONS IN SELECTED TISSUES OF SUBTERRANEAN CLOVER (CV TRIKKALA)

Subterranean clover (cv. Trikkala) was grown in field and glasshouse experiments to calibrate the critical concentrations of inorganic P in selected plant tissues, for maximizing vegetative growth. Critical concentrations of total P in the various tissues were also calibrated from the field experiment. Critical concentrations of inorganic P derived in the glasshouse, for both youngest open leaf (YOL) and next oldest leaf (YOL+1), were similar at any one sampling time, but declined markedly with plant age, i.e. critical concentrations for YOL declined from 196 mg kg-1 at the first sampling to 96 mg kg-1 month later. Similarly, critical inorganic P concentrations for YOL derived in the field declined with time, i.e. from 187 mg kg-1 to 124 mg kg-1 from the first to second sampling. Critical inorganic P concentrations for YOL+1 were less age dependent, though much lower at any one sampling time. As such, inorganic P is not recommended for diagnostic laboratory use. Critical concentrations of total P for YOL and YOL+1 did not decline significantly over the sampling period, while those for whole plant tops (WT) declined from 0.35 to 0.30% over the same period. In the YOL tissue, the proposed critical nutrient range for total P is 0.40 - 0.45% while that for WT, sampled early in the growing season, is 0.35 - 0.40%. Results from this study support the use of total P in diagnostic plant testing services.