Regrowth and nutrient composition of different plant organs in grass-clover canopies as affected by phosphorus and potassium availability

Regrowth after cutting and the distribution of nitrogen (N), phosphorus (P) and potassium (K) in different plant organs of white clover and perennial ryegrass growing in pure or mixed swards were investigated under field conditions in a soil with a low-to-moderate availability of P and K. In all treatments, white clover constituted more than 70 % of the above-ground biomass in the mixed swards. The petioles were the dominant pool of dry matter throughout regrowth and contained the greatest amounts of N. P and K. Increased supply of P and K increased the growth of ryegrass, but not that of white clover in the mixed swards. The increased competition from ryegrass led to a decline in the yield of white clover laminae as well as in the N content per unit of dry matter in laminae, petioles and stolons. The P content of all white clover organs also declined following P application to the mixed swards. whereas K application increased their K contents. In the pure swards of ryegrass and whits clover, yields and contents of N. P and K in the dry matter were either not affected or increased following P and K application. It was concluded that commonly-used defoliation heights may remove 80% or more of the nutrient and dry matter pools located in the petioles but the remaining quantities of dry matter and nutrients in the petioles will normally exceed the corresponding quantities in the stolons. (C) 2001 Annals of Botany Company.