A REEXAMINATION OF PHOSPHORUS-LIME INTERACTIONS IN PERENNIAL RYEGRASS

The results of a previous study had suggested that under conditions of limited P availability, Ca may be able to compensate for P in the shoot tissue of perennial ryegrass. To verify this preliminary finding, a factorial experiment was set up which simultaneously tested the effects of Ca and P fertilization on the yield and chemical composition of perennial ryegrass. Calcium was supplied as either lime or gypsum in order to differentiate between the effects of Ca and pH on the response of perennial ryegrass to P fertilization. In the final stage of the experiment a Zn treatment was included, to see whether altering the P/Zn ratios of plant shoots had any influence on the purported interaction between Ca and P. The results demonstrated that the P-sparing effect of lime occurs, at least partly, because Ca application improves the efficiency of absorbed P for DM production. However, it was reasonably clear that the site of the interaction between Ca and P was the soil-root interface, and not shoot tissue. It was suggested that under conditions of limited P supply, Ca stabilizes root membranes and thereby minimizes both the efflux losses of nutrients from root tissue, and the compensatory flow of photosynthates from shoots to roots. No interaction was observed between P and Zn treatments in this study. Instead, a positive interaction was found between lime and Zn treatments, which suggests that the stabilizing action of Ca on root membranes requires Zn as a co-stabilizing factor. It is proposed that chemical analysis of shoot tissue alone may not be sufficient to accurately diagnose the P, Ca or Zn status of whole plants, since the critical levels of these elements in shoots appear to bear little relation to their requirements in the rhizosphere.