Use of Proximal Sensing Devices in the Estimation of Nitrogen and Water Stress in Corn

The objectives of this research were 1) to determine if reflectance measured by proximal sensing devices can differentiate between corn nitrogen (N) and water deficiency; and 2) to estimate corn water use in the form of evapotranspiration (ET) across varying degrees of nitrogen (N) and water stress and relate ET to total N uptake across the season. At a single location, an Unmanned Aerial Vehicle (UAV) was used to capture spatially sensitive canopy reflectance from experimental units differentiated by levels of water and N deficiency. Reflectance was measured across different growth stages and used to generate a Combined Index (CI) based on the Normalized Difference Red Edge (NDRE), Green Leaf Index (GLI), and Blue Reflectance Index (BRI) consisting of water stressed, high N stressed, medium N stressed, and non-stressed categories. The CI successfully identified 90% of treatments to the correct category within the experimental treatment structure but was not as successful at two validation sites. This study found that N and water-induced differences in corn productivity can be differentiated in-season by a combination of reflectance indices under narrow cropping system circumstances, but that NDRE alone provides superior information under a broader set of contexts. For the second objective, frequent soil moisture measurements and irrigation data was used to estimate ET using a soil water balance method. Despite differences in absolute N uptake and ET across treatments, the final cumulative ET relative to each treatment was highly correlated to the seasonal relative total N uptake, regardless of stress type or degree (r2=0.92, p < 0.001). This indicates that water use and N uptake are highly connected and should be integrated when making management decisions in corn. Decision-making tools used to schedule irrigation can be used to schedule N application timing and amount