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 assigned to the sample when you sent it to the laboratory.  There is no "name" for the Example Home Lawn Report sample,  and the sample is identified as "1B" on the Example Vegetable Garden Report.  The homeowner who submitted the Lawn sample probably sent in only one sample and didn’t bother to give it an identifier.  However, if you send in more than one sample it is extremely important that you retain a list of the identifiers you attached to each sample.  

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 five categories: “Coarse” (includes sands, loamy sands, and sandy loams), “Medium” (includes loams and silt loams), “Fine” (includes clay loams, silty clay loams, and silty clays), and (rarely, in the case of organic soils), "Peat", or "Muck".  In the Example Home Lawn Report soil texture is classified as Coarse, so you know it is a sandy soil (likely well-drained and 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 results 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 measurement that may be confusing to people is the Buffer Index. The 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 (as is the case on the Example Vegetable Garden Report).  The box will be blank if the soil is organic or if the pH is 6.0 or higher (like 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 is used to calculate how much lime will be required to affect the desired pH adjustment.

Clients often wonder why there are two boxes for phosphorus: “Olsen Phosphorus” and “Bray 1 Phosphorus”.  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 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 lower, 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 results shown in both the Olsen P and Bray 1 P boxes.  This is because the Bray 1 test is run on all Soil Testing samples as a matter of routine.  If pH is found to be above 7.4, the Olsen test is then run and that result is also reported.  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.  These two measurements also provide an example of different laboratory methods giving results on different scales:  For a vegetable or flower garden an Olsen P result of 10 ppm is high, but a Bray 1 P result of 10 ppm is medium.

The numerical laboratory results on a soil test report may not very useful to many people on their own.  The meaning of the numbers will become clearer when we go through the INTERPRETATION section.  Laboratory measurements of 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 measurement is low or high in terms of the need for fertilizer application.  If the soil test is low for a given nutrient, it means that plants are 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 plant growth.

Soil texture, organic matter, pH, buffer index, phosphorus, and potassium make up the Regular Series Soil Test. Soluble salts and the rest of the nutrients listed on the report are often measured under specific circumstances in which a problem is suspected or likely to occur.  These circumstances include certain soil types and plants that are prone to develop deficiencies of specific nutrients.  Most soils in Minnesota contain adequate secondary macronutrients and essential micronutrients to meet plant needs, and keeping pH in the optimum range through liming will help maintain adequate availability.  The addition of organic nutrient sources like compost and composted manure 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).