The wording in the source

Area per sample

• Even after careful consideration to the field landscape and variability, no individual sample zone should represent more than 25 acres.

  AGF-513, Taking a Representative Soil Test

• A 360 X 360 foot grid results in a systematic sampling pattern for each 2.5 acres in a field.

  AGF-513, Systematic Grid Sampling

Method

• Systematic grid sampling and zone management are the two primary methods of dividing field areas for soil sampling using precision applications (Table 2).

  AGF-513, Soil Sampling with Precision Agriculture

• Follow a zigzag pattern over the sample area when zone sampling (Figure 2).

  AGF-513, Taking the Soil Cores

• Georeferenced sample points may give a better opportunity to compare sample trends over time by returning to near the same point in future years.

  AGF-513, Zone Management

Depth

• Most soil tests are calibrated using an 8-inch sampling depth, which corresponds to the plow depth. Management in no-till systems may benefit from a 0–2 inch sample for pH, especially where pH-sensitive herbicides are used.

  AGF-513, Taking the Soil Cores

Cores

• For a standard soil sample, take at least 15 sample cores from each field area of 25 acres or less into one composite sample.

  AGF-513, Taking the Soil Cores

• If the field has a history of banded fertilizer applications, increase the number of samples to 20–25 cores.

  AGF-513, Taking the Soil Cores

• Sample points should be taken randomly with 10–15 cores per sample area of up to 25 acres.

  AGF-513, Zone Management

• Soil samples are collected at each sample point within a 10-foot diameter circle, with two cores pulled from each quadrant, for a total of eight cores.

  AGF-513, Systematic Grid Sampling

Laboratory and analysis

• Standards for laboratory procedure and extraction solutions have been established by land-grant universities in the publication Recommended Chemical Soil Test Procedures for the North Central Region. It is a good practice to ask if the laboratory utilizes these procedures in their analysis process.

  AGF-513, Sending the Sample to the Testing Lab