Soil Test Results (Lawn & Garden)

At the far left of the SOIL TEST RESULTS section is a “Sample/Field Number” line 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 Example Home Lawn Report and is 1B on the Example Vegetable Garden Report. The homeowner who submitted the Lawn Report probably sent in only one sample and didn’t bother to give that sample 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 sample location.  

Below the Sample/Field Number is a row of boxes that begins with “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. Soil texture will be one of three categories: “Coarse” (includes sands, loamy sands, and sandy loams), “Medium” (includes loams and silt loams), and “Fine” (includes clay loams, silty clay loams, and silty clays). In the Example Home Lawn Report soil texture is classified as Coarse, so you know it is a sandy soil that is well drained but may need frequent watering. The “Estimated Soil Texture” is Medium for the Example Vegetable Garden Report, so that soil will hold water better.

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 Example Home Lawn Report “Organic Matter” is 2.8%, “pH” is 7.6, “Olsen Phosphorus” is 45 ppm (parts per million), “Bray 1 Phosphorus” is 40 ppm, “Potassium” is 89 ppm, and “Sulfur” is 6 ppm. For the Example Vegetable Garden Report “Organic Matter” is 3.5%, “Soluble Salts” are 0.5 mmhos/cm (the units stand for millimhos per centimeter, which is a measure of electrical conductivity), “pH” is 5.6, “Buffer Index” is 6.5, “Bray 1 Phosphorus” is 15 ppm, and “Potassium” is 89 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.

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 (so it was run for the Example Vegetable Garden Report). The box will be blank if the soil is organic or if the pH is 6.0 or higher (so it is blank for the Example Home Lawn Report). 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 Example Vegetable Garden Report only has numbers in the Bray 1 P box, but the Example Home Lawn Report has values for both Olsen P and Bray 1 P. The reason for the difference in the two example 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 Example Home Lawn 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). The meaning of the numbers will become clearer when we go through the INTERPRETATION section. 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 plants are 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 plant 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 a vegetable or flower garden an Olsen P test of 10 ppm is high, but a Bray 1 P test of 10 ppm is medium.

Soil texture, organic matter, pH, buffer index, phosphorus, and potassium are part of the Regular Soil Test Series and are the usual categories tested. Soluble salts and the rest of the plant nutrients listed on the report are most usefully tested for under specific circumstances where a problem is suspected or likely to occur. These circumstances include certain soil types and plants that are more prone than others to develop deficiencies of specific nutrients. Most soils in Minnesota contain adequate amounts of these nutrients to meet plant needs and keeping pH in the optimum range through liming will maintain adequate calcium and magnesium levels (dolomitic lime will supply magnesium). The addition of organic nutrient sources like compost and manure by gardeners will also supply micronutrients and help keep deficiencies from occurring. See the Soluble Salts (PDF), Secondary Macronutrients (PDF), and Micronutrients (PDF) sections in the University of Minnesota Extension bulletin Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants (PDF) for more information on these topics.

Lead is not a plant nutrient, but it is the last element listed in the row of possible Soil Test Results. Lead can be a human health concern in some situations. For more information on lead and whether you should test your soil for it see Lead (PDF) and Lead in the Home Garden and Urban Soil Environment (PDF).