Soil Test Results (Horticultural Crops)

At the far left of the RESULTS section is a box labeled “Sample/Field Number” which contains the identifying name or number you attached to the sample on the Soil Sample Information Sheet when you sent it to the laboratory. This is blank for the Grapes Report and is on the Vegetables Report. The grape grower probably sent in only one sample and didn’t bother to give his field an identifier. However, if you send in more than one sample it is critically important that you give them names or numbers and maintain a list of the sample identifiers you attached to each field.

To the right of Sample/Field Number is “Estimated Soil Texture”. Texture is determined by an experienced lab technician on the basis of how a moist soil sample feels when it is manipulated between the thumb and fingers. In the Grapes Report, soil texture is classified as Medium and if you go up to the Soil Texture Code box in the INTERPRETATION section you will see this means that your soil is a loam or silt loam. The “Estimated Soil Texture” is also Medium for the Vegetables Report.

The rest of the categories in the RESULTS section are the numerical measurements of laboratory analyses that were performed on your soil sample. For the Grapes Report “Organic Matter” is 3.0%, “pH” is 5.5, “Buffer Index” is 6.5, “Bray 1 Phosphorus” is 45 ppm (parts per million), “Potassium” is 110 ppm, “Zinc” is 0.5 ppm, “Calcium” is 1500 ppm, and “Magnesium” is 70 ppm. For the Vegetables Report “Organic Matter” is 3.0%, “pH” is 6.0, “Bray 1 Phosphorus” is 4 ppm, “Olsen Phosphorus” is 6 ppm, and “Potassium” is 75 ppm. If you want to know more about the laboratory procedures used to obtain these measurements, see Our Methods on the University of Minnesota Soil Testing Laboratory web site.

The amount of soil organic matter is important, because it is one of the main factors determining nitrogen fertilizer recommendations. The higher the organic matter content, the lower the nitrogen fertilizer recommendation. This is because decomposition of organic matter and the associated release of plant-available nitrogen is a significant source of this nutrient for plants. Examples showing the effect of soil organic matter levels can be seen in the nitrogen recommendation tables for different crops in the University of Minnesota Extension bulletin Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota.

A soil test that is often confusing to people is the Buffer Index. The Buffer Index is used to determine how much lime is required when soil pH is too acid. The Buffer Index is only run if the pH of a mineral soil is less than 6.0. The box will be blank if the soil is organic or if the pH is 6.0 or higher. Soils differ in their buffering capacity, or ability to resist a change in pH, so soils with the same pH may need different amounts of lime to achieve a similar pH change. The pH measurement tells you whether you need to apply lime and the Buffer Index tells you how much lime will be required to accomplish the desired change in pH.

The two boxes for phosphorus soil test results, “Olsen Phosphorus” and “Bray 1 Phosphorus”, can also be confusing. The Grapes Report only has numbers in the Bray 1 P box, but the Vegetables Report has values for both Olsen P and Bray 1 P. As noted above in the INTERPRETATION OF SOIL TEST RESULTS section, the Olsen P measurement is the one used for the Vegetables Report. The reason for the difference in the two reports is that different laboratory methods are used to determine plant-available P, depending on the pH of the soil. For calcareous soils with a pH greater than 7.4, the Olsen test is used. If soil pH is 7.4 or less, the Bray 1 test is used. The pH of the soil in the Vegetables Report is 7.6, so Olsen P is the appropriate measurement.

When the Olsen P test is run, there will be numbers in both the Olsen P and Bray 1 P boxes. This is because the Bray 1 test is initially run and its results are recorded on all soil samples. For samples with a pH above 7.4, the Olsen test is then run and that result is also recorded. If there are numbers in both the Bray 1 and Olsen boxes, the Olsen P value is always the one used for interpretation and P fertilizer recommendations.

The numerical laboratory measurements are not very useful to many people, because unless you work with them a lot it is not clear what the numbers mean (except for pH). That is why the INTERPRETATION section comes first. Laboratory tests for the amounts of plant-available nutrients are indexes of relative availability, rather than absolute measurements of availability, and different laboratory methods give results that have varying numerical scales. The INTERPRETATION section tells you whether the laboratory measurement is low or high in terms of the need for fertilizer application. If the soil test is low, it means that the crop is likely to respond positively to the addition of fertilizer. If the soil test is high, it means that additional fertilizer is much less likely to improve crop growth. The two phosphorus tests discussed above, Olsen and Bray 1, provide an example of how different laboratory methods give results that have varying numerical scales. For many vegetables an Olsen P test of 28 ppm is high, but a Bray 1 P test of 28 ppm is medium.