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 (Pheum 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 (N(c)) in older swards (>4 yr; N(c) = 37.0 x SB(-0.35); R(2) = 0.79), expressed as a function of shoot biomass (SB), was less than previous reports on younger swards (N(c) = 48.0 x SB(-0.32)). Critical P concentration (PC) was also less in older swards (Pc = 3.27 x SB(-0.20); R(2) = 0.68) than in younger ones (PC = 5.23 x SB(-0.40); R(2) = 0.63). Critical P concentration can also be expressed as a function of N concentration (P(c) = 1.07 + 0.063N; R(2) = 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.