A refresher on soil sampling

As harvest progresses, fields become ready to be soil sampled.

The objective of soil sampling and analysis is to inventory soil nutrient levels and develop fertilizer recommendations for future crops. Soil sampling can occur any time there is not a growing crop in the field. In-season sampling is typically done to diagnose problems.

Uniform areas should be sampled—uniform by soil type and past management practices. Samples should never represent more than 10-15 acres if a field is uniform and has been under a similar management regime throughout.

Sample using a soil probe or auger. These tools are generally available at your Michigan State University Extension county office or fertilizer dealer. Samples should consist of 20 cores from the sampled area at the same depth throughout the field. Sampling depth is particularly important in reduced-tillage and no-till fields because of nutrient stratification due to limited incorporation. Samples should be taken to the plow depth or 8 inches in reduced or no-till systems.

In long-term no-till systems or other systems where nitrogen is surface applied, consider taking a second soil sample 3 inches deep to have tested separately for pH. Nitrogen fertilizers decrease soil pH, and the pH of the second sample is important for determining the proper lime rate and efficacy of herbicides.

A representative composite sample is best made when a field is sampled in a zig-zag sampling pattern. Cores should be placed in a clean plastic bucket. The individual cores should be broken up, well-mixed and sub-sampled to make the final sample.

Soils should be sampled at least once every three years. Intense cropping systems, where large amounts of fertilizer are applied and removed, should be sampled more frequently. Many growers utilize grid sampling to map the nutrient status of their fields. Grid sampling is typically done in 2.5 or less acre squares across the field.

Michigan State University offers soil testing services at the Soil and Plant Nutrient Laboratory. There are also a number of private labs that can test soil.

For a more in-depth reading on soil sampling, please see “Sampling Soils for Fertilizer and Lime Recommendations,” MSU Extension bulletin E498.

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