The wording in the source

Timing

• Soil testing is strongly suggested every 2-4 years if soils are at an optimum level of fertility, based on producers’ goals, and base applications have been calculated.

  How and When to Sample

• Sampling should be avoided within a few months after a lime or fertilizer treatment due to a higher values.

  How and When to Sample

• After harvest in the fall is an ideal season to sample because soil moisture conditions are generally more stable

  How and When to Sample

Area per sample

• This approach suggests sampling at the rate of one composite from each 2.5-acre area.

  Typical Soil Sampling Methods, Whole field uniform fertilizer applications

• This approach suggests collecting one sample from each 1.1-acre area that will provide a better representation of the actual field variability.

  Site-specific applications

• The size of zones can vary depending on field characteristics but seldom exceeds 10 acres.

  Zone sampling

Method

• Each sample should consist of five soil cores, 1 inch in diameter, collected to a 7-inch depth from within a 10-foot radius around each point.

  Figure 2A caption

• it is important to collect samples from precisely the same points each time a field is tested. Sample locations can be identified using a global positioning system (GPS) unit or by accurately measuring the sample points with a device such as a measuring wheel.

  Typical Soil Sampling Methods

Depth

• The fertilizer recommendation system in Illinois is based on crop response to fertility levels in the top 7 inches of the soil, which is the proper sampling depth for pH, phosphorus, and potassium.

  How and When to Sample

Cores

• Soil samples should be a composite of 10 to 15 cores (whether grid or zone sampling) from the intended management area within a field, depending on field characteristics such as soil types, land or fertilizer uses, and terrain. Collecting 15-20 cores per area can improve representation.

  How and When to Sample