Detecting Spatial Relationships In Corn Nitrogen Sufficiency Through Crop Reflectance.

Spatial variability of physical and chemical properties of soil presents many significant obstacles in optimizing fertilizer N use efficiency. A more thorough understanding of the sources of variation responsible for shifts in plant available N could increase N use efficiency. A trial was conducted at an on farm site containing high field variability in soil properties near Yazoo City, MS. A 0.2 ha grid size was employed to sample an 18 ha field. Eighty sample locations resulted and were geo-referenced using GPS. Prior to planting, soil samples were taken from each location at three depths, 0- to 15 cm, 15- to 30-cm, and 30- to 60 cm. Samples were analyzed for extractable available N, pH, routine soil testing, and texture. Extractable soil N values ranged from 56.49 kg N/ha^-1 to 114.44 kg N/ha^-1 with a mean of 80.81 kg N/ha^-1. Soil texture samples ranged across clay, silty clay, silty clay loam, loam and silt loam classes.  Whole plant samples were taken at V5 for total N, and at V8-9, and V17-18 for leaf N% determination. Total N at V5 ranged from 3.23 to 5.32%. Leaf N% at the V8-9 and V17-18 stages ranged from 1.81 to 4.19% and 1.36 to 3.60%, respectively.  At the V5 and V8-9 stage, crop reflectance with a tractor mounted passive sensor was taken. Preliminary data suggest a positive relationship among shifts in GNDVI and leaf N% as soil texture changed across sites. This study will be replicated in 2011 to provide a more extensive database to derive statistical significance of data.