A routine call we get involves a person who takes a soil sample this year and submits it for analysis. The data do not match the results of the last sample that was taken from the field three years ago. The logical question is, "why?"
The usual reason is that soils vary in pH and nutrient content across the field, so the results will be different if subsamples were taken from different parts of the field in each sample. However, there are other possible reasons, and those are the ones I want to analyze in this article.
The depth at which the samples are taken is critical. Soil labs assume the sample is taken from a depth of 0 to 6 inches unless they are told otherwise. Sometimes, real-world samples are not taken from a 0- to 6-inch depth. We primarily work in pasture and hayfield settings. When the soil is dry, it is difficult to get a probe in the ground deeper than 2 inches without breaking or bending it. If the soil is wet, we may go considerably deeper than 6 inches simply because we can.
Why is sampling depth critical?
Nutrient levels will usually be higher in the upper part of the soil in pasture and hayfield situations, as well as in no-till situations. Fertilizer and manure are placed on top of the ground, and plant roots cycle nutrients from deeper in the soil profile to the soil surface when the plants decompose. Therefore, if the sampling depth is shallower than 6 inches, your report will show higher nutrient levels than are actually present because you are sampling the more nutrient-rich portion of the soil. If you collect soil deeper than 6 inches, your report will show lower nutrient levels than are actually present because you are including portions of the soil that are less nutrient rich in the sample.
Some soil test data can vary by season of the year. Soil pH is usually higher when soil is collected in wet seasons and lower when collected in dry seasons. The soil test pH value can vary by as much as 0.5 units between seasons of the year. If the sample is collected in the spring, it is likely to show a higher pH value than if it is collected in the summer.
Soil test phosphorus (P) can also vary by season and soil moisture content. Phosphorus is more soluble in wet conditions than in dry, so soil test P levels can be higher when samples are collected in the spring than in the summer. If the soil is very wet, the differences can be substantial.
Soil test potassium (K) can vary according to time of year if certain crops are grown. Hay crops remove a great deal of potassium, so soil test K levels will often be lower in the fall after a season of hay has been removed than in the spring before haying commences. Grain crops contain a lot of potassium in the stalks and stover. This potassium is temporarily bound up in the plant residue but will be released when the residue decomposes. Potassium can be lower shortly after grain harvest (early summer for wheat and fall for corn, sorghum, cotton, etc.) but will increase up to planting time unless the stalks are removed from the field.
What can be done to manage for soil test variation? Collect soil samples at the same season of the year. You can collect samples at any time during the year, just do it in the same season each year to minimize the effect of seasonal variation. Collect soil samples at exactly 0 to 6 inches each time. I have a mark on my soil probe at 6 inches and press the probe into the ground to that depth each time. If the soil is so dry that you cannot get the probe 6 inches into the ground, don't take soil samples that day.
Following these steps should help you develop a better nutrient management program through use of soil testing.