Many large fields have considerable variation in soil types and fertility levels. Site-specific application of crop nutrients and lime using variable-rate technology may be economically and environmentally advantageous
for these situations. Soil pH levels, P, and K often show considerable variability across a large field capacity because of non-uniform application of fertilizers and manures, natural variability, and differing crop yields. Soil N levels may also show some variation, but sitespecific management of this nutrient is not warranted if the entire field has the same cropping and manure application history.
Site-specific management requires the collection of multiple soil samples within the field, which are then analyzed separately. This is most useful when the sampling and application are performed using precision agriculture technologies such as global positioning systems, geographic information systems, and variable-rate applicators. However, use of conventional application technology can also be effective.
Three to 5-acre grid sampling (every 350 to 450 feet) is generally recommended, especially for fields that have received variable manure and fertilizer rates. The suggested sampling procedure is called unaligned because in order to get a better picture of the field as a whole, grid points do not follow a straight line. Grid points can be designed with the use of precision agriculture software packages or by insuring that sampling points are taken by moving a few feet off the regular grid in random directions (figure 21.3). Grid sampling still requires ten to fifteen individual cores to be taken within about a 30-foot area around each grid point. Sampling units within fields may also be defined by soil type (from soil survey maps) and landscape position, but fertility patterns do not always follow these features.
Grid soil testing may not be needed every time you sample the field—it is a time-consuming process—but it is recommended to evaluate site-specific nutrient levels in larger fields at least once in a rotation, each time lime application may be needed, or every five to eight years.