Critical P Concentration of Forage Grasses.

Plant critical nutrient concentrations are required for nutrient diagnostic. The process of concomitant dilution of P and N with increasing shoot biomass has led to a model that predicts critical P concentration (P_c) as a function of N concentration during the growing season. Our objective was to test the general value of a model established for timothy (Phleum pratense L.) in eastern Canada (P_c = 1.07 + 0.063N). Four rates of P (0, 10, 20, and 40 kg P ha^-1) were applied for two or three years (2010-2012) at five sites for a total of 14 site-years in four countries [Normandin (CA), Québec (CA), Maaninka (FI), Toulouse (FR), and Changins (CH)] on soils with a low available P content. Timothy was the predominant species in Canada and Finland, while old multi-species swards were used in Switzerland and France. Shoot dry matter (DM) yields, and N and P concentrations were measured on four dates with one-week intervals from the vegetative to late heading stages of development of the primary growth. Although three different soil tests suggested a positive response to P fertilization, increasing P fertilization increased shoot DM yield at only six of 14 site-years. Data of P and N concentrations for nonlimiting P conditions were extracted and they confirmed the validity of the model for timothy under other environmental conditions. The model, however, cannot be generalized to multi-species swards. This model of critical P concentration can be used to quantify the degree of P deficiency during the grass growing season.