Critical Levels of P in Subclover ( Trifolium subterraneum L.) 1

Phosphorus deficiencies are widespread in California annual grasslands and in other areas of the world where subclover ( Trifolium subterraneum L.) is an important legume. Therefore, the effects of clipping frequency and stage of growth on critical levels of P in subclover were studied. Critical P level was defined as percent P in the clover part when lack of P limited clover yields to 95% of those obtained with adequate P. In pot experiments subclover was grown in P‐deficient soils fertilized with increasing amounts of P until no additional increase in yield was obtained. When leaves were sampled from plants 48 to 151 days of age, the critical level decreased from 0.61 to 0.11% P, but changes after 120 days were not statistically significant. At day 120, leaves from plants previously defoliated 0, 1, 2, or 3 times had critical values of 0.11, 0.18, 0.23, and 0.28% P, respectively. The last three determinations were for leaves of the same physiological age harvested 13 days after the previous clipping. At 120 days critical P values in stem tissue also increased with increasing frequency of defoliation.