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OSU Extension

College of Food, Agricultural, and Environmental Sciences

November 29, 2016 - 10:03am -- Anonymous

Fall is a good time to pull soil samples from harvested fields.  A regular, consistent program of soil sampling provides a useful record of farm field fertility.  Soil test results are an important component in a nutrient management plan as they will be a driver in determining if a nutrient application is needed and if so what quantity of nutrients should be applied.  The reliability of the soil test results is very much dependent upon the quality of the sample submitted.  Taking a good representative sample takes some time and effort and deciding how and where to sample needs to be part of a soil sampling plan.  The two most commonly used soil sampling methods are grid sampling and zone sampling.  Each method requires different sampling techniques and influences how the results are used and how nutrients are applied.  In a recent OSU Extension CORN newsletter, OSU Extension agronomy field specialists Elizabeth Hawkins, Greg LaBarge and Harold Watters along with Precision ag specialist John Fulton and soil fertility specialist Steve Culman wrote an article that explains the two different sampling methods and how and when to use each one.  I am including some excerpts from the article here and the entire article can be read at: .

Grid sampling should be used when there is little information available about the variation in nutrient levels across a field. Grid sampling may be useful in fields where variability is expected but the field history is not well known, topography is uniform but differences in soil type occur, varied management patterns have been used in the past or manure applications have occurred. Proper grid sampling makes it possible to identify variation within a field and is an important data layer when determining future management zones for fertilizer applications. 

Grid sampling involves taking samples at regular intervals across the landscape of a field. Grid size is selected to provide the desired data resolution. A 2.5-acre grid size is commonly used (360 by 360 feet); however, choosing a grid size that matches up to spreader equipment widths is recommended. The goal when grid sampling is to determine the best estimate of each soil test value near the center of the gridded area.  One recommended method to get this information is to collect subsamples within a 10 foot circle of each grid point.  Two cores are pulled from each quadrant of the circle; a total of eight cores for each grid point.

Zone sampling involves dividing the field into zones that are uniform enough to be managed as a whole and then sampling to determine the average soil test values for those zones. The success of the zone sampling relies on the amount and quality of the data used to determine the zones. Layers such as soils maps, aerial photos, yield maps, topographic maps, management history and personal field experience can provide valuable information about the variation in a field. This information can be used to define sample zones or management zones in a field. As the number of management zones in a field increase, the number of samples needed increase. If only a few zones exist, samples can be combined to reduce the cost of analytical expenses.

Management zones are a better choice than grids when the operator has a long history of working with the field, topography varies and can be used to define zones, where yield map data over time has defined high and low yielding areas, the soil type map represents yield zones or other remote sensing data is available to overlay with operator experience to define yield patterns in a field.

The goal when sampling by management zone is to determine the best estimate of the entire zone. If the data used to determine the zones is accurate, the soil test values should be relatively consistent. In this case, taking multiple soil cores is necessary to reduce the chance of pulling one from a “bad spot.”  Subsamples should be taken randomly (recommended to walk in a zigzag pattern) with 10-15 cores per sample area up to 25 acres. Georeferenced sample points may give a better opportunity to compare sample trends over time by returning to near the same point in future years. This can be beneficial to tracking soil fertility recommendation program effects on soil test levels over time.