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Fall Soil Sampling

By Sara Bauder

With fertilizer prices on the rise, it’s more important than ever to understand your soil test levels and crop response to applied fertilizers. Fall is a great time to soil sample before freeze up. The non-mobile nutrients, phosphorus (P), potassium (K), pH, soluble salt content (EC) and most micro and secondary nutrient soil tests are minimally affected by sampling time. However, try to avoid soil sampling after an extreme dry period, as soil K levels will be lower due to the effect of shrinking clay mineralogy trapping K and, therefore, making it unavailable to plants. Soil biological activity affects nitrogen (NO3-N) and sulfur (SO4-S) soil tests. It is recommended to wait until soil temperatures reach less than 50 degrees Fahrenheit in the fall before sampling. Above this temperature, nitrogen and sulfur are released from organic matter and crop residue. Other factors, such as warm winters with early springs, sampling small grain stubble with excessive regrowth and amount of snow cover can lead to changes in NO3-N soil test levels before planting the following spring. Soil sampling for nitrate and sulfate is probably best done in the spring immediately before fertilizer applications.

Fall Soil Sampling

Generally, soil tests for P, K, pH, EC, calcium (Ca), magnesium (Mg) and micronutrients will not change immensely from year to year; these can be analyzed every two to three years. Conversely, levels of NO3-N, SO4-S and Chloride (Cl-) can change from year-to-year and should be analyzed every year a non-legume crop is planted.

Soil samples should represent a uniform area of your field; take time to assess fields and split them up by differences, such as texture, color, slope, amount of erosion, drainage and pest management. Other options for producers using precision technologies may be small grids (two to five acres) or productivity zones based on soil differences. Individual samples would be submitted to the lab from each grid point/area or management zone. This data can be used to generate variable-rate fertilizer and seeding application maps. Regardless of what sampling method you use, some basic guidelines should be followed.

                                            Sampling Guidelines

    Consistency

    • When sampling with a hand or hydraulic probe, carefully remove crop residue from the soil surface, then insert the probe (as straight as possible) to the desired depth.
    • Take 10 to 15 samples from each management zone (or six to eight per grid) and mix thoroughly, as soils can be highly variable from one point to another.
    • Sampling depth should be consistent from year to year so that nutrient values can be comparable over time.

    Depth

    • Sampling depth for the non-mobile nutrients (P, K, pH, OM, EC, zinc, iron, manganese, copper, and boron) is 0-6 inches. A deeper sample (0-24 inches) is recommended for mobile nutrients like NO3-N, chloride and sulfur.
    • The best practice is to separate samples into two sub-sample depths, 0-6 inches and 6-24 inches. Keep in mind that deep samples (0-24 inches or, in some cases, over-vulnerable aquifers, 0-48 inches) must be taken if the field is a part of a concentrated animal feeding operation (CAFO) manure plan.
    • In tilled fields, sampling prior to tillage operations is preferred for most-accurate results to ensure the correct sampling depth.

    Submission

    • Use a labeling system that works for you and the lab in which you intend to submit samples.
    • It is best to air dry samples before mailing (do not use heat) or freeze and ship as quickly as possible, avoiding weekend layovers.
    • For a list of known soil testing labs in and near South Dakota, view our Soil Testing Labs page.
    Source : sdstate.edu

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