Concentration of P in ear leaf samples at silking is widely used to detect P deficiency in maize. Ear leaf P concentration failed to indicate P deficiencies that were indicated by P concentration in shoots at the 4‐ to 6‐leaf stage in a field study in Ontario, Canada. To determine which samples were a better indicator of P deficiency, plant analysis data from Ontario field studies in which grain yield responded to P were interpreted using sufficiency ranges and DRIS methods. Ear leaf P concentration and P index usually failed to detect P deficiency and were only slightly affected by P fertilization. For shoots at the 4‐ to 6‐leaf stage, P concentration and P index usually correctly detected P deficiencies. Concentration of P and P index values for seedling shoots changed rapidly during early growth, resulting in diagnoses that varied with sampling time. The ear leaf at silking should be disregarded as an indicator of P deficiency for maize in Ontario. Reliable methods of assessing P nutrition of maize during early growth need to be developed.