Grid vs Zone Soil Sampling
Grid Soil Sampling
Grid soil sampling divides the field into areas of a defined size (1, 2.5 and 5 acre grids). Samples are taken from within each grid with each soil sample having a distinct sample ID and GPS location. This allows the assessment of nutrient variability present in each field with high resolution. This higher resolution identifies any "hot spots" or trouble areas where nutrients amounts are low or deficient. Fertilizer application is targeted to where nutrients are needed and over-application is minimized. Multiple years of data provides historical changes and trends in nutrient levels.
Zone Soil Sampling
Zone soil sampling divides the field into regions based on a measured characteristic or past performance of the field such as soil type or texture, VERIS data (soil variability measured through electrical conductivity) , spatial yield data, etc. A single composite sample is taken within each zone or multiple samples are taken for each zone depending on its area. Each soil sample has a unique sample ID and GPS location in the field. Zone soil samples classify spatial variability in the field and reduce the number of sample points . However, there is lower resolution than grid soil sampling, thus less accuracy of fertilizer placement and rate.
Which Method is Correct?
In general, grid soil sampling is preferred with its higher resolution where zones can overlook areas of the field that greatly affect the amount and placement of fertilizer and the size/shape of the zones can change year-to-year. That is why the use of each method should have a sound agronomic reasoning behind it. If there is little to no soil data for a field, grid soil sampling is the way to go to capture as much information as possible. Zones can be established based on at least three years of spatial yield data per crop or soil characteristics (texture, type, etc.) that are proven to be highly correlated to yield variability.