Soil Test Results (Agronomic Crops)

At the far left of the RESULTS section is a box labeled “Sample/Field Number” which contains the identifying number or name you attached to the soil sample when you sent it to the laboratory. This is 5 on the Example Report. If you sent in more than one sample, it is critically important that you maintained a list of the sample numbers you attached to each field. As noted above in the INTERPRETATION section, the categories reported in this RESULTS section are those in the Regular Soil Test Series.

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 this case, 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 rest of the categories in the RESULTS section are the numerical measurements of laboratory analyses that were performed on your soil sample. “Organic Matter” is 3.5%, “pH” is 5.5, “Buffer Index” is 6.2, “Bray 1 Phosphorus” is 10 ppm (parts per million), and “Potassium” is 90 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 organic matter level is one of the factors used to determine the nitrogen fertilizer recommendation for some crops. Soils with higher organic matter have lower nitrogen recommendations. This is because decomposition of organic matter and the associated release of plant-available nitrogen is a significant source of this nutrient for crops. Soil organic matter is not used in nitrogen recommendations for corn.

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 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 amount 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.

People often wonder why there are two boxes for phosphorus: “Olsen Phosphorus” and “Bray 1 Phosphorus”. This is because different laboratory methods are used 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. These two phosphorus tests provide an example of how different laboratory methods give results that have varying numerical scales. For corn an Olsen P test of 12 ppm is high, but a Bray 1 P test of 12 ppm is medium.

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 soil test report for vegetables Example Soil Test Report – Vegetables (PDF icon example_soil_test_report_vegetables.pdf) on the Horticultural Crops page gives an example of a soil where the pH is 7.6 and the Olsen P test is used.