Soil Test Interpretation

sample soil test results copy

By Harry Brook

Soil testing is done one of two ways. There is the old-fashioned way of wet chemistry. That is where chemicals are used to extract the nutrients in question. A newer, quicker, way is using near infrared reflective spectroscopy to get a reading. It works by shining a particular wavelength of light on a carefully prepared sample and reading the reflectance from the sample. Different nutrient levels will show different wavelengths of reflected light. The trick with NIRS is the vast library of samples that have been used to calibrate the reading. The more samples and the greater the variability of the samples, the more accurate is the test result. NIRS is used in many testing labs, including soil, feed and others.

So, what is important on the soil test to help you make informed, wise, fertilizer decisions? Of prime importance is the organic matter content in the soil. Organic matter is like a bank account for nutrients in your soil. If you skimp on fertilizer, a healthy organic matter level can minimize yield losses if growing conditions are good. Conversely, if organic matter levels are low, it leads to soil structure problems, crusting and a crop that depends almost entirely on the fertilizer to meet the yield potential. A rough rule of thumb is that there is about 0.7 lbs of nitrogen available from every .1% of organic matter. However, if you don’t adequately fertilize for the crop you are taking off, you are mining the organic matter and it eventually will cause problems in the soil and reduce yield.

The big four nutrients are nitrogen, phosphorus, sulfur and potassium with nitrogen being the single biggest nutrient used for the crop. Both nitrogen and sulfur are subject to being leached out of the soil by over-abundant rain. Phosphorus levels should be fertilized unless the field has been heavily manured. Western Canada has been blessed with large amounts of potassium in the soil but there are now areas where potassium supplementation is required to ensure adequate nutrition. Soil levels of potassium should be above 200 lbs/acre.

Soil pH is important as it is a measure of soil acidity. pH below about 5.5 starts to limit the crops that can grow successfully. It is expensive to try and change pH as it requires large amounts of soil amendments. pH’s above 7.0 are alkaline. Closer to neutral, at 7.0, is best.

A measure of soil salinity is the electrical conductance. The E.C. measures how readily electricity is conducted in the soil. If you have a lot of salts in the soil, that conducts more electricity and also can limit crop choices and crop growth. Ideally, the E.C. should be under 2.0 but it doesn’t affect crop choices until it starts to approach 4 or greater. There are tables for crop salinity tolerances.

Another measure of salinity is the SAR or sodium absorption ratio. It is a measure of the sodium ions compared to calcium and magnesium ion on clay particles. Once the SAR is greater than 13, you start to have soil structure problems and encounter sodic soils (gumbos).

The cation exchange capacity, C.E.C., is an indirect measure of the clay content of the soil. It measures how many cations can attach to soil particles and, since clay particles are finer than sand or silt, there is more surface area to attach cations to it. It is not a very useful measure.

Trace minerals such as copper, are useful as many of our soils are low in copper. Wheat is the most likely crop to demonstrate copper deficiencies. It often manifests this deficiency with high ergot levels. However, ergot can also occur from cool, wet conditions at flowering in wheat, too.

These are the basic test results that are useful for farming and planning the next year’s crop. Get the most out of your soil test by looking at the results and understanding the implications.

Harry Brook is Flagstaff County’s Agricultural Fieldman. He can be reached via email at: or by phone at: 780-384-4138.