Godsey Ag

Grid Sampling


Figure 1. Figure illustrates variable rate lime application varying from 0 lb ECC/ac (white area) to 4000 lb ECC/ac (yellow area).

Grid soil sampling is perhaps the oldest precision agricultural technology around. The benefits are clear and by accurately placing fertilizer you increase the probability of seeing a response to applied fertilizer. Almost all fields have a tremendous amount of variability in pH, phosphorus (P), and potassium (K). This variability often means the difference between being deficient and sufficient in various parts of the field (Table 1). Fields with elevation changes, soil texture changes, or different historical management practices will often have more variability.

If soil pH is a problem in a field, the use of grid soil sampling will often save a significant amount of money in lime and it will prevent over applying lime in high pH areas. In the case of P and K, the prescription maps generated from grid sampling will allow the producer to apply fertilizer where it is most needed.

Grid Soil Sampling vs. Zone Sampling

Grid sampling and zone sampling will depict the variability in the field but at different resolutions. Most of the time less than five samples are taken within a zone sampling scheme on a 100 acre field while 40 samples are taken from the same field (2.5 acre grid) using grid sampling. In most situations the grid sampling will give you a higher resolution of the nutrient variability in the field.  Grid size is dependent on the amount of variability that you think is in the field. Typically, the grid size is between 2.5 and 5 acres in size. The more variability that you believe you have, the smaller the grid size should be.

From the example in Table 1 you can see the range of soil test values from grid soil samples. Phosphorus ranged from 10 to 204 lb/ac, which resulted in recommendations from 0 to 50 lb P2O5/ac. If a producer applies a uniform rate based on one composite sample they would under apply P to 70% of the field and over apply 30% of the field. Grid sampling allows variable rate application of P placing it where it needs to go.










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