By Top Crop Manager
October 10, 2014 - Fall is an ideal time to take soil samples in preparation for developing a fertility program for the next growing season. According to OMAFRA's Bonnie Ball, soil fertility specialist, and Nicole Rabe and Doug Aspinall, OMAFRA land resource specialists, recently there have been some significant changes in technology to increase the accuracy of soil sampling.
A soil sample is a composite of cores collected from an area no larger than 10 ha. There are a number of methods to divide fields into sampling zones: past field boundaries; management differences; variation in topography, soil type, and/or the crop. This is referred to as Directed Sampling. Examples are given below.
Ideally sample zones are based on more than one parameter. Sensors, mounted on ground-based (ATV etc.) or aerial vehicles e.g. satellite, aircraft, Unmanned Aerial Vehicle (UAV), usually equipped with GPS, can help delineate sample zones. These include: optical sensors, which measure how light is reflected or absorbed and get used to indicate crop characteristics such as mass, health, moisture, and nutrient status via vegetation indices e.g. Normalized Difference Vegetative Index (NDVI), and soil properties such as organic matter (OM), CEC and moisture; and soil electrical conductivity sensors, which get used to indicate soil CEC, salt, clay, OM and water content. Other sensors exist and more will come to market in future. While sensors can show where variation exists in the field, they may not tell what causes it, or how to manage it. In some cases, calibration of sensor output is not well established for Ontario conditions, so it is important to check if sensor output matches reality in the field by soil or plant sampling.
Soil test results from directed sampling can be used to generate nutrient level maps and to define management zones for variable nutrient and lime applications.