Consistent soil sampling equals consistent results

By Gary Sliworsky
OMAF/MRA Rep.

Occasionally there are questions about whether soil sample values can change depending on the weather in a given year, or the time of year the sample was taken. There is no easy answer. Soil test values vary over time, but for most soil tests the variation is relatively small, and the direction of change is not always predictable.
Sample Depth Influences Soil Test.
Most of the variation over time is NOT reflecting changes in the chemistry of the soil, but instead is a reflection of changes in what soil was sampled. We know that the fertility of the soil declines as we go deeper. This is particularly true in forage or no-till fields that have not been worked for many years. Even in conventionally tilled fields, the vertical mixing of nutrients is not perfect. Some stratification develops over the growing season as plants pull nutrients to the surface and then leave them in the crop residue. Because of this stratification, the depth of soil that is sampled can have a large impact on the soil test value for that sample.
Soil Conditions Can Affect Sample Depth.
So what does this have to do with variation in soil test over time? The depth of the sample is often related to how hard the ground is, so a sample from a dry, compacted field will likely be shallower than from a loose or moist field.
To illustrate the point, say a farmer was quite concerned about the increase in his soil test values after one application of liquid manure to his pasture fields. He had been collecting samples in August, when the weather had been very dry. When asked he admitted that the samples might have been 4 inches deep rather than 6 inches, and that he had sampled through any manure that was laying on the surface. A subsequent sample, taken later in the fall when the ground had softened, showed more reasonable soil test values.
Consistent Sampling Time Equals Consistent Results.
You can increase the consistency of your sample results over time by taking care in how the samples are taken. The advantage of always sampling at the same time of year is that soil conditions are usually consistent, so it is easier to achieve consistent depth. Watch out though, that a consistent depth doesn’t mean you are sampling into the subsoil in compacted or eroded areas of the field, or your sample will show lower results than it should!

Events to Remember
Sept. 28 – Cattle Sale, Stratton, 9:30 a.m. Contact Sales Barn Manager Philip Krahn, 807-271-0425, for more information.