The problem is not N deficiency: Active canopy sensors and chlorophyll meters detect P stress in corn and soybean

Leaf nitrogen (N) concentrations and N nutrition sufficiency are positively related to both leaf chlorophyll and Normalized Difference Vegetation Index (NDVI) measurements. However, NDVI is also well related to canopy biomass and other nutritional deficiencies, including phosphorus (P), that result in stunted crop growth and would cause lower NDVI values that could be wrongly attributed to N deficiency. The objective was to evaluate the sensitivity of chlorophyll meter readings and canopy sensor derived NDVI values to corn (Zea mays L.) and soybean (Glycine max L. Merrill) P nutritional status, using existing field trials exhibiting yield responses to P nutrition. Besides three NDVI and chlorophyll measurements, leaf N, leaf P and grain yield were determined for both crops. The NDVI and chlorophyll measurements detected P stress at early vegetative growth stages in both corn and soybean. Corn exhibited a strong positive correlation between both NDVI and leaf chlorophyll and both leaf P and yield. Soybean NDVI and leaf chlorophyll were significantly related to grain yield and both leaf N and P. Both crops exhibited significant reductions in growth with increasing P stress. The strong relationships among yield, leaf P, NDVI values and chlorophyll meter readings indicate that P nutrition stress could easily be confounded with N nutrition stress, in both crops, when only sensors are used for diagnosis.