A Model of Critical Phosphorus Concentration in the Shoot Biomass of Wheat

Critical nutrient concentrations are required for assessing the level of crop nutrition. Our objectives were to validate an existing model of critical phosphorus concentration (Pc = 0.94 + 0.107N) in the shoot biomass (SB) of wheat (Triticum aestivum L.) and to assess the alternative approach of expressing Pc as a function of SB rather than shoot N concentration (N). We applied four rates of P fertilizer (0, 10, 20, and 30 kg P ha−1) on soils with a low to medium available P concentration at four locations in three countries (Normandin [Canada; 2010, 2011, 2012], Brandon [Canada; 2010, 2012], Ylistaro [Finland; 2010, 2011], and Beijing [China; 2012]) for a total of 8 site‐years. Shoot biomass, and N and P concentrations were measured on five dates with 1‐wk intervals from vegetative to late heading stages of development, and grain yield was measured. Increasing P fertilization did not increase grain yield at any of the 8 site‐years and had little effect on SB. Under nonlimiting P conditions achieved in most cases with no applied P, the allometric relationship between Pc and SB differed among locations, while the relationship between Pc and shoot N concentration (Pc = −0.677 + 0.221N − 0.00292N2, R2 = 0.82, P < 0.001) was independent of locations. This model of critical P concentration predicts lower Pc values than that previously reported, mostly for high shoot N concentrations. This predictive model of Pc can be used to quantify the degree of P deficiency during the wheat growing season.