The wording in the source

Timing

• Most fields should be sampled every three to four years.

  AGR-16, Sample Timing, p. 1

• High-value crops, such as tobacco, commercial horticultural crops, alfalfa, red clover, and corn silage, should be sampled annually so that plant nutrient levels can be monitored more closely.

  AGR-16, Sample Timing, p. 1

• Application of manure can change soil test phosphorus, potassium, and zinc levels dramatically, so sampling manured fields each year is also recommended.

  AGR-16, Sample Timing, p. 1

• Soil samples can be collected through much of the year, although fall (September to December) or spring (February to April) are the best times.

  AGR-16, Sample Timing, p. 1

• In either case, a field should always be sampled the same time of the year in order to make historical comparisons.

  AGR-16, Sample Timing, p. 1

Area per sample

• An individual sample should represent no more than 20 acres except when soils, past management, and cropping history are quite uniform.

  AGR-16, Collecting Field Crop Samples, p. 1

• Currently the industry standard grid size is 2.5 acres, but Kentucky research shows that variability within areas as small as one acre can be as great as the variability within the entire field.

  AGR-16, Grid Soil Sampling, p. 2

Method

• The most representative sample can be obtained from a large field by sampling smaller areas on the basis of soil type, cropping history, erosion, or past management practices (Figure 2).

  AGR-16, Collecting Field Crop Samples, p. 1

• If a few years of yield maps are available, these can help identify areas of the field that should be sampled separately.

  AGR-16, Collecting Field Crop Samples, p. 1

• Because soil variability is so high, it is important to treat each grid cell as a field.

  AGR-16, Grid Soil Sampling, p. 2

Depth

• Take soil cores to the depth of the tillage operation (usually 6 to 8 inches).

  AGR-16, Sample Depth, Tilled Areas, p. 3

• Take soil cores to a depth of 3 to 4 inches for pastures, no-till planting (where fertilizer or lime remains on the soil surface), and minimum-till planting (where fertilizer is incorporated only in the surface 1 to 2 inches).

  AGR-16, Sample Depth, Non- or Reduced-Tilled Areas, p. 3

Cores

• Collect at least 10 soil cores for small areas and up to 30 cores for larger fields.

  AGR-16, Collecting Field Crop Samples, p. 1

• At least 10 random samples should be collected across the entire grid cell, rather than a few cores from the center of the grid (Figure 3).

  AGR-16, Grid Soil Sampling, p. 2

Field identifiers

• Keep good records indicating where each sample was taken.

  AGR-16, Collecting Field Crop Samples, p. 1