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Soil Test Results: What Do They Mean?

Posted Dec. 1, 2000

If you have ever received either soil test results or a soils recommendation report from the Noble Foundation, perhaps you wondered what all those numbers meant. I will attempt to explain them in this article.

First of all, there are several parts to the report. The top section has information that identifies the soil sample and information you provide us. The middle section contains the results from the laboratory analysis. At the bottom is the recommendation section in two parts, a fertilizer recommendation on the left and written comments on the right.

Let's look at the results from left to right. Keep in mind that a soil test is a chemical way of estimating the nutrients available to the plant. The pH is a measure of soil acidity. Generally 6.6 or lower indicates acidic soil, 6.7 to 7.3 means neutral soil, and a reading higher than 7.3 means the soil is basic. If the pH is 6.0 or lower, a buffer index will be done to indicate how much lime will be needed to raise the pH to 6.8.

The nitrogen test measures the amount of nitrate in the soil for the next crop. Things to remember are:

  1. nitrate is water-soluble and can move out of the root zone,
  2. the test does not account for a previous legume crop,
  3. the test does not measure recently applied anhydrous ammonia, and
  4. the test will not show nitrogen from manure until the manure has broken down.

Phosphorus and potassium are relative amounts of the nutrient available in the soil. The soil may contain much more than what is shown, but plants cannot extract and use it. Test results higher than 40 and 220 for phosphorus and potassium, respectively, are sufficient for most crops.

Calcium is associated with soil pH. Soils with a good pH generally have adequate calcium, and soils low in calcium generally need lime. A test result of 500 or higher is adequate.

Magnesium is similar in availability to potassium. The result is an indicator of sufficiency. Look for results higher than 100.

Sodium can be an indicator of a sodic soil. Excessive amounts of sodium may be present when the test is higher than 920. With math and some knowledge of chemistry, this number can be calculated into the amount of exchangeable sodium percentage.

Soluble salts will be shown when the pH is higher than 6.0. Soils with soluble salts at 2,600 ppm or more are classified as saline.

Organic matter is the organic content percentage of the soil. This number would ideally be around 5, but look for a range from 1 to 3.

Cation exchange capacity depends on soil organic matter, texture, and clay content. It is an indicator of the relative ability of the soil to supply potassium, calcium, magnesium, and sodium.

Additional tests can check iron, zinc, manganese, sulfur, copper, and boron levels. These tests are not standard but may be used to ensure that high-value crops have proper nutrition or to identify the cause of some production problems. High pH soils may have iron deficiencies, not because they don't contain iron, but because it is unavailable to the plants. Zinc is needed in very small quantities. Values above 0.3 are sufficient for all crops except corn and pecans. Manganese will generally not be a concern if the pH range is normal. Sulfur is usually not a concern because it is supplied in sufficient quantities by the soil and air, and copper is seldom a problem in our area. Boron may occasionally be needed on alfalfa and peanuts.

The fertilizer recommendation at the bottom left shows the amount of actual nutrient to apply in pounds per acre. The written recommendation and comments at the bottom right will be similar. The recommendation may give an example of how the nutrients could be applied. Comments will pertain to information you provided us on the soil sample entry form. Adjustments may be noted, depending on criteria such as whether the crop will be hayed or grazed, whether soil fertility should increase or stay at maintenance levels, how and when the fertilizer will be applied, and whether the crop will be irrigated. Always feel free to ask us additional questions.

All the recommendations we give are only as good as the sample itself. For information on how to take a good soil sample, consult OSU fact sheet 2207 or with your county extension agent.