Using Active Canopy Sensors to Monitor Corn and Soybean Phosphorus Stress: Interactions with Crop Nitrogen Status.

Leaf chlorophyll is positively correlated with leaf N and N nutrition sufficiency. Minolta SPAD-502 and Normalized Difference Vegetation Index (NDVI) measurements are strongly related to leaf chlorophyll and are used to indicate crop N status. However, other nutritional deficiencies can confound crop N status. This study evaluated the sensitivity of SPAD and NDVI as indicators of corn and soybean phosphorus (P) nutritional status, and associated leaf N and P levels. In 2011, NDVI (active proximal sensor) and SPAD measurements were made on soybean (at V2, V4, V7 and R1) and corn (at V6, V8 and R1) in field trials exhibiting yield responses to P nutrition. P deficiency was detected with the first NDVI measurement in both corn and soybean (though visible differences were not discernible), and soybean leaf P and N were positively related. Surprisingly, corn leaves exhibited a strong, positive correlation between SPAD values and both leaf P and grain yield. However, soybean leaf SPAD measurements were significantly related to grain yield and leaf P at V7. The SPAD and NDVI sensors were able to detect soybean and corn growth responses to improved P nutrition. These results suggest an opportunity for the use of such sensors for the diagnosis of P deficiency. However, this opportunity presents a potential challenge in corn fields where N deficiency is assumed but may be confounded with P deficiency.