Brian J. Wienhold and Gary E. Varvel, Agroecosystem Management Research Unit, USDA-ARS, Lincoln, NE
Fertilizer N recommendations often include an adjustment for soil organic matter (SOM). In most fields SOM varies spatially and methods for rapidly determining this variability for use in in-season fertilizer recommendations are needed. Optical sensors that can be mounted on a planter or tillage implement and used to collect georeferenced SOM represent an approach for meeting this need. This study compared sensor based measurements of SOM to laboratory measurements of SOM for dry and moist soil under ridge-tilled, disk tilled, or plow tilled plots in a continuous corn or corn-soybean rotation. Sensor output included visible, near infrared (NIR), and four manufacture derived indexes. All output values were sensitive to soil moisture. Visible and NIR were different between the two rotations when the soil was dry and similar when the soil was moist.