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

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Abstract
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.

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