Noise source evaluation of a real-time soil sensor, part I: Static elevation changes

The chemical application rates of many pre-emergence herbicides as well as fertilizers are sensitive to soil organic matter content. Research was cond ucted to examine the effects of static elevation changes on a real-time soi l organic matter sensor which operates on the basis of measuring the electr omagnetic reflectance of the soil surface. The soil organic matter sensor o perates beneath the surface of the soil, with the sensor mounted 25 mm from the bottom of the housing shank. The signal from this sensor currently con tains fluctuations which indicate differences of as much as 1% organic matt er content, or errors of as much as 25%. This study examined the effects of static elevation changes on the output of a sensor which uses electromagne tic reflectance to measure soil organic matter. This study used actual soil samples, and colour boards to examine the effects of static elevation chan ges on the sensor output. Regression analysis, using the data collected wit h the soil samples, showed that the strength of the relationship between th e sensor output and soil organic matter remains fairly constant for differe nt elevations; and the slope of the line describing this relationship varie d greatly with elevation changes. This indicates that care must be taken to ensure that laboratory tests conducted to determine the relationship betwe en the sensor output and soil organic matter content are performed at the e levation that will be present when the sensor is used in the field. Colour boards were used to evaluate the results from a study on dynamic elevation changes to be evaluated. They showed that the effect of changing elevations is much larger when the sensor is closer to the soil surface, indicating t hat an attempt to determine the optimal elevation of electromagnetic reflec tance sensors above the soil surface should be conducted during the design process. (C) 2000 Silsoe Research Institute.