Soil Test Results (Agronomic Crops)

At the far left of the RESULTS section is a box labeled “Sample/Field Number” which contains the identifying name or number you assigned to the soil sample when you sent it to the laboratory.  This is "5" on the Example Report.  If you send in more than one sample, it is very important that you keep a list of the sample identifiers you used for each of them.  As noted above in the INTERPRETATION section, the categories reported in the RESULTS section are those in the Regular Series Soil Test. 

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 information in the RESULTS section are the numerical results of the laboratory measurements 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 results, see Our Methods on the University of Minnesota Soil Testing Laboratory web site.

Organic matter content is one of the factors used to determine the nitrogen fertilizer recommendation for many crops.  Soils with high 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 making nitrogen recommendations for corn.

A soil test that is often confusing to people is the Buffer Index.  Buffer Index is used to determine how much lime is required when soil pH is too low.  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 above 6.0.  Soils differ in their buffering capacity, or ability to resist a change in pH, so soils with the same pH may require different amounts of lime to cause a similar pH change. The pH measurement tells you whether you need to apply lime and the Buffer Index is used to calculate how much lime will be required to affect the desired pH adjustment.

The numerical laboratory results in a soil test report are generally not useful to clients by themselves.  That is why the INTERPRETATION section comes first. Laboratory tests for plant-available nutrients are indices of relative availability, rather than measurements of absolute content, and those indices are expressed on varying scales. The INTERPRETATION section tells you whether the laboratory result is low, medium, or high in terms of the need for fertilizer application.  If the soil test is low for a given nutrient, it means that the crop is likely to respond favorably to the addition of fertilizer.  If the soil test is high, it means that additional fertilizer is less likely to improve crop growth.

Clients 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 methods give results on different scales.  For corn, an Olsen P result of 12 ppm is considered high, but a Bray 1 P result of 12 ppm is medium.

When the Olsen P test is run, there will be results shown in both the Olsen P and Bray 1 P boxes. This is because the Bray 1 test is run on every sample, while the Olsen test is only run on samples with a pH above 7.4.  If there are numbers in both the Bray 1 and Olsen boxes, the Olsen P value is used for interpretation and P fertilizer recommendations. The example soil test report for vegetables (linked below and on the Horticultural Crops page) gives an example in which the pH is 7.6 and the Olsen P test is used.