Phosphorus and Nitrogen relationships during spring growth of an aging timothy sward
Plant-based methods of determining P and N deficiencies require critical concentration values. We studied the response of timothy (Phleum pratense L.) to P and N fertilization over 7 yr with the objective of using relationships between P and N concentrations and dry matter (DM) yield to determine critical P and N concentrations. Timothy, seeded in 1998, was harvested during spring growth from 1999 to 2005 in an experiment with four sampling dates (stem elongation, early heading, late heading, and early flowering), four P applications (0, 15, 30, and 45 kg P ha–1), and four N applications (0, 60, 120, and 180 kg N ha–1). Maximum DM yield and, therefore, nonlimiting P and N conditions, were obtained with 120 kg N ha–1 without applied P. Under these nonlimiting conditions, relationships between P and N concentrations and DM yield changed with production years. Critical N concentration (Nc) in older swards (>4 yr; Nc = 37.0 x SB–0.35; R2 = 0.79), expressed as a function of shoot biomass (SB), was less than previous reports on younger swards (Nc = 48.0 x SB–0.32). Critical P concentration (Pc) was also less in older swards (Pc = 3.27 x SB–0.20; R2 = 0.68) than in younger ones (Pc = 5.23 x SB–0.40; R2 = 0.63). Critical P concentration can also be expressed as a function of N concentration (Pc = 1.07 + 0.063N; R2 = 0.71), a relationship independent of sward age and shoot biomass but affected by N deficiency; the slope is the same but intercept values vary from 1.31 to 1.41 when N limits shoot growth. Critical P and N concentrations can then be used to quantify the degree of P and N deficiency in young and old timothy swards.