The interaction of nitrogen and potassium nutrition on dry matter and nitrogen yields of the graminae: Perennial ryegrass (Lolium perenne)

Nitrogen and potassium uptake and yields of perennial ryegrass, grown in controlled environments in three experiments, are examined in relation to the N and K status of the soil medium, defined by a (N/K)soil atom ratio. Maximum dry matter yields (DMY) were obtained at (N/K)s values ranging from 4–40 for four “high K” soils and 10–90 for a “low K” soil. The N concentration in the crop increased approximately 50% with a ten‐fold increase in the (N/K)s value although, in the “low K” soil, the relationship suggests that K deficiency may be limiting N uptake. No rational explanation can be given why the percentage N value in grass in one experiment is significantly less than in the other two. Exchangeable Na in “low K” soils is known to substitute for K during crop growth. Organic N in soils mineralises under favourable environmental conditions to provide N for crop growth. The possible contribution of these two factors to the observed relationships between N concentration and (N/K)s, and between (N/K)plant atom ratio and (N/K)s values, is discussed briefly.