From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1996, No. 6


As farmers, dealers, consultants and researchers explore the implementation of site-specific, precision farming management systems on individual fields, the question of how to sample becomes more important. And it becomes more difficult to answer as we learn more about the sources of variability in the field.

For relatively flat fields, a grid pattern of sampling is probably the best choice. A systematic unaligned grid is recommended to avoid the influence of previous field operations, fertilizer banding, and other factors that tend to follow straight lines. This means the grid is laid out on a regular pattern, then each cell is subdivided into a smaller grid to determine the sampling point within the cell, so that its position is in a different location within the cell from that of any other sampling point in the same row or column.

Deviation from the planned grid may be necessary to be sure obvious field features are adequately documented. If there is a hill or depressional area that is not sampled with the normal grid pattern, take an extra sample from within that area. In areas where cropping is less intense and crop value is lower, larger grids may be more economically feasible. If contour planting, dramatic topographic differences, or major soil type differences are present and don’t line up well with a grid sampling pattern, select sampling sites to best characterize the field.

Regardless of the pattern used, careful documentation of the sample location is critical. If a global positioning satellite (GPS) system is available, use it to pinpoint exact sampling locations. If not, use row counts, a measuring wheel, or some other means to carefully document sample locations so that they can later be used to guide variable-rate fertilizer applications and future observations. It is not necessary to invest in GPS to take advantage of site-specific management systems. But GPS makes the job much easier and improves the accuracy of locating the samples.

Sample grid size is another difficult question. The answer is really “site-specific” and will depend on the value of the crop, prices of the input being measured, and amount of variability within the field. Most farmers find their fields are more variable than they expected. In the central Midwest, most research supports sampling on a one-acre grid size. The more common 2- to 3-acre grid may be adequate, but may need additional sample points to characterize special features—hills, wet spots, etc. Also take separate samples from old pastures, feedlots, or other known sources of variability.

Good sampling patterns are a key to successful soil sampling and site-specific, precision nutrient management systems that fit modern crop production.
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