Relationship between P and N concentrations in maize and wheat leaves

Simple plant-based diagnostic tools can be used to determine crop P status. Our objectives were to establish the relationships between P and N concentrations of the uppermost collared leaf (PL and NL) of spring wheat (Triticum aestivum L.) and maize (Zea mays L.) during the growing season and, in particular, to determine the critical leaf P concentrations required to diagnose P deficiencies. Various N applications were evaluated over six site-years for wheat and eight site-years for maize (2004–2006) with adequate soil P for growth. Phosphorus and N concentrations of the uppermost collared leaf were determined weekly and the relationships between leaf N and P concentrations were established using only the sampling dates from the stem elongation stage for wheat and from the V8 stage of development for maize. Leaf P concentration generally decreased with decreasing N fertilization. Relationships between PL and NL concentrations (mg g−1 DM) using all site-years and sampling dates were described by significant linear–plateau functions in both maize (PL = 0.82 + 0.089 NL if NL ≤ 32.1 and PL = 3.7 if NL > 32.1; R2 = 0.41; P < 0.001) and wheat (PL = 0.02 + 0.106 NL if NL ≤ 33.2 and PL = 3.5 if NL > 33.2; R2 = 0.42; P < 0.001). Variation among sampling dates in the relationships were noted. By restricting the sampling dates [413–496 growing degree days (5 °C basis) in wheat (i.e., stem elongation) and 1494–1579 crop heat units in maize (i.e., silking), relationships for wheat (PL = 0.29 + 0.073 NL, R2 = 0.66; P < 0.001) and maize (PL = 1.04 + 0.084 NL, R2 = 0.66; P < 0.001) were improved. In maize, expressing P and N concentrations on a leaf area basis (PLA and NLA) at silking further improved the relationship (PLA = 0.002 + 0.101 NLA, R2 = 0.80; P < 0.001). Predictive models of critical P concentration as a function of N concentration in the uppermost collared leaf of wheat and maize were established which could be used for diagnostic purposes.

► Phosphorus concentration of the uppermost collared leaf of wheat and maize generally decreased with decreasing N fertilization.
► Relationships between leaf P and N concentrations were described by significant linear-plateau functions.
► By restricting the sampling dates to a specific stage of development, leaf P and N relationships were improved.
► In maize, expressing P and N concentrations on a leaf area basis at silking further improved the relationship.
► Predictive models of critical leaf P concentration as a function of N concentration could be used for diagnostic purposes.