August 31, 2014

Posts Comments

Understanding Your Soil Test Report for Lawn, Garden and Landscape Plants

The first step for a homeowner in accurately fertilizing a lawn, garden, or landscape planting is to collect a representative soil sample and send it to a laboratory for analysis. After completing the analysis, the laboratory sends the results back to you along with lime and fertilizer recommendations. The second step in accurate and efficient fertilization is to interpret the results correctly. Then you are ready to apply the necessary rates of fertilizer and lime to achieve optimum growth of your lawn, garden, or landscape trees and shrubs. The Soil Test Report from the University of Minnesota Soil Testing Laboratory is relatively straightforward, but it contains a lot of information and some technical terminology that can be confusing if you have limited experience using it.

The information that follows is designed to provide some simple guidelines you can use to effectively interpret your University of Minnesota Soil Test Report. The next section provides a list of definitions for technical terms and abbreviations used in the report. In the following sections we will go through two Example Reports step-by-step, review what is in the different parts of the reports, provide links to useful sources of related soil fertility information, and highlight the bottom line: what does the report tell you to do in your lawn, garden, or landscape?

It is also important to point out the kinds of things your Soil Test Report will not tell you. Sometimes people mistakenly think that the Soil Test Report will tell them if they have disease-causing organisms in their soil, plant attacking insects or nematodes, herbicide residues that might be harmful to their garden or landscape plants, or an analysis of the numbers of beneficial microorganisms that will indicate whether they have a “healthy” soil. The University of Minnesota Soil Test Report is focused on describing the fertility status of your soil and providing information that will help improve the mineral nutrition of your plants.

Definitions

Bray 1 Phosphorus – a soil testing procedure that is used to determine the level of plant-available phosphorus in soils with a pH of 7.4 or less

Buffer Index – a soil test measurement that is used to determine the amount of lime required to raise soil pH to the desired level; the buffer index is measured only when the pH of a mineral soil is less than 6.0

K – the chemical symbol for potassium

K2O – see Potash

LBS – abbreviation for pounds

mmhos/cm – millimhos per centimeter; a unit of electrical conductivity that is used to measure the relative concentration of soluble salts in the soil solution

N – the chemical symbol for nitrogen

Nitrate, NO3-N – a plant-available form of nitrogen that occurs in the soil; nitrate is also readily leached through the soil

Olsen Phosphorus – a soil testing procedure that is used to determine the level of plant-available phosphorus in soils with a pH greater than 7.4

P – the chemical symbol for phosphorus

Phosphate, P2O5 – the terminology and chemical formula used to express the amount of phosphorus required in a fertilizer recommendation and the amount of phosphorus in a bag of fertilizer

pH – a measurement of the acidity or alkalinity of the soil solution

Potash, K2O – the terminology and chemical formula used to express the amount of potassium required in a fertilizer recommendation and the amount of potassium in a bag of fertilizer

ppm – parts per million

SO4-S – sulfate, a plant-available form of sulfur that occurs in the soil

Soil Texture – the relative proportions of sand, silt, and clay particles in a soil, which determine the soil type; Coarse = sand, loamy sand, and sandy loam; Medium = loam and silt loam; Fine = clay loam, silty clay loam, and silty clay

Soluble Salts – ions (charged atoms or molecules) that are dissolved in the soil solution

SQ. FT. – abbreviation for square feet

Example Soil Test Report

The Example Soil Test Reports are in exactly the same format as the one you received in the mail from the University of Minnesota Soil Testing Laboratory. One example contains recommendations for a Home Lawn and the other example contains recommendations for a Vegetable Garden. The reports cover two very different situations and our discussion of them should be useful to you in understanding your own report no matter what type of plants you are growing. You will probably find it helpful to open the Example Reports now and print copies, so that you can refer to them as we go through their various parts. Example Soil Test Report – Home Lawn (.pdf)Example Soil Test Report – Vegetable Garden (.pdf)

The University of Minnesota Soil Test Report contains five sections that with one exception we will go through in the same order as they appear in the report:

  1. Header
  2. SOIL TEST RESULTS
  3. INTERPRETATION OF SOIL TEST RESULTS
  4. RECOMMENDATIONS
  5. Explanation of Soil Test Report

The Header, SOIL TEST RESULTS, INTERPRETATION OF SOIL TEST RESULTS, and RECOMMENDATIONS sections are on the first page of the Soil Test Report. The “Explanation of Soil Test Report” section is on a separate page and is included with the Example Reports. We will not go through the information on the Explanation page as a separate section, but will refer to different parts of it as we go through related information in the other sections.

Header

You will not see a “Header” title, but what we are calling the Header section is the general information at the top of the first page. This information is self explanatory, although we’ll point out that on the lower right side is a column of five items that contains two numbers you will need to refer to if you find it necessary to call the Soil Testing Laboratory to discuss some aspect of your report. These are the “Report No.”, which is 1 in the Example Home Lawn Report and 6 in the Example Vegetable Garden Report, and the “Laboratory No.”, which is 122 in the Example Home Lawn Report and 114 in the Example Vegetable Garden Report.

SOIL TEST RESULTS

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, Secondary Macronutrients, and Micronutrients sections in the University of Minnesota Extension bulletin Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants 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 and Lead in the Home Garden and Urban Soil Environment.

INTERPRETATION OF SOIL TEST RESULTS

The INTERPRETATION section focuses on four of the important Soil Test Results (Phosphorus, Potassium, pH, and Soluble Salts) and gives an explanation of them in graphical format. A short explanation of how to read the graphs is 2/3 of the way down the left-hand column of the Explanation of Soil Test Report page. Basically they are horizontal bar graphs that use letters and symbols for the bar and the scale along the bottom provides the interpretation.

For the Example Home Lawn Report, the phosphorus bar is filled with P’s showing that this soil tested very high for plant-available phosphorus using the Olsen P method. The bar of K’s for potassium extends out to the medium level and the asterisks for pH show that it is above the optimum level for a lawn and is slightly alkaline. Soluble salts were not tested, so that graph is blank.

For the Example Vegetable Garden Report, the phosphorus bar is about half-filled with P’s showing that this soil tested medium to high for phosphorus. The bar of K’s indicates that potassium is at the medium level, the asterisks for pH show that it is moderately acid and a little below optimum for many vegetables, and the asterisks for “Soluble Salts” show that salts are low and in the satisfactory range.

One of the Soil Test Results that is not shown in the Interpretation section is Sulfur for the Home Lawn sample, so we will have to wait for the RECOMMENDATIONS secton to see its impact. Another result not shown is Organic Matter, but an interpretation of organic matter levels and a short discussion of the importance of organic matter is provided on the Explanation of Soil Test Report page. The Home Lawn, with 2.8% organic matter, is classified as Low. The Vegetable Garden, with 3.5% organic matter, is classified as Medium. The amount of soil organic matter is one of the main factors determining nitrogen fertilizer recommendations for many plants. The higher the organic matter, the lower the nitrogen recommendation. This is because decomposition of organic matter and subsequent release of plant-available nitrogen is a significant source of this nutrient for plants. Examples showing the effect of soil organic matter level can be seen in the nitrogen recommendation tables for different types of plants in Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants.

The Explanation of Soil Test Report page also has brief discussions of soil pH, buffer index, soluble salts, lead, and other special tests. For more information that will help you interpret your Soil Test Results see Soil pH Modification, Overview of Essential Nutrients, which includes information about Nitrogen, Phosphorus, and Potassium results.

RECOMMENDATIONS

The RECOMMENDATIONS section gives you the bottom line: how much fertilizer or lime should you apply for optimum growth of your lawn, garden, or landscape plants? For some people, this may be the only part of the Soil Test Report that they are concerned about.

The initial line of the RECOMMENDATIONS section tells you what type of plants the recommendation is for. In our case, of course, we have one for a Home Lawn and one for a Vegetable Garden. Immediately below is an area between two solid lines that gives the actual amounts of lime and fertilizer nutrients to apply. Below that at the bottom of the page are additional instructions on how to select a fertilizer, application timing, and methods of application.

In the Example Home Lawn Report, on the left side of the first line of the RECOMMENDATIONS it tells you that the “LIME RECOMMENDATION” is 0 LBS/1,000 SQ. FT. (0 pounds of lime per 1,000 square feet), so liming is not required. On the right side of the first line it says “Grass watered Clippings removed” This is information you supplied on the Soil Sample Information Sheet when you sent in your sample. It is important because nitrogen recommendations for lawns vary depending on the way they are managed. You can find more information on this at Established Lawns and Turf, Nitrogen Recommendations. The next line says “TOTAL AMOUNT OF EACH NUTRIENT TO APPLY PER YEAR*” and below that are the recommendations for the three major nutrients: NITROGEN – 4 LBS/1,000 SQ. FT., PHOSPHATE – 0 LBS/1,000 SQ. FT., and POTASH – 3 LBS/1,000 SQ. FT. The asterisk at the end of the “TOTAL AMOUNT” line refers you to a “CAUTION!” statement below that discusses split nitrogen applications and slow release fertilizers to avoid burning the grass. This is followed by a schedule for split applications.

The terms phosphate and potash are used to express the amounts of phosphorus and potassium in fertilizer recommendations, as well as their percentages in bags of fertilizer. They are also frequently referred to by their chemical formulas, which are P2O5 for phosphate and K2O for potash, although these chemical formulas are not used in the University of Minnesota Soil Test Report.

The next line below the total amount of nutrients to apply is the approximate NPK (Nitrogen:Phosphate:Potash) ratio of the fertilizer recommendation, which is 20-0-15. This ratio helps you choose a fertilizer that will be as close as possible to matching the recommendation. This is discussed in the first paragraph of the information section immediately below and additional details on selecting a fertilizer and determining application rates are in the Explanation of Soil Test Report page. You can find a more complete discussion of fertilizers, rate calculations, and application methods at Inorganic and Organic Fertilizers, Slow Release Fertilizers, Fertilizer Grades, Calculating Fertilizer Rates from Soil Test Recommendations, Methods of Applying Fertilizers, and Yard Waste Compost Application.

Application of phosphorus fertilizer to established lawns in Minnesota is regulated by state law to protect water quality. Phosphorus containing fertilizer cannot be applied unless a soil test shows a need for phosphorus to maintain adequate fertility. Because this soil test does not demonstrate a phosphorus requirement, it will be extremely important to select a fertilizer that contains no phosphorus (the middle number on the fertilizer bag is zero). For more information see Preventing Pollution Problems from Lawn and Garden Fertilizers.

The final recommendation on the Home Lawn Report is for Sulfur: 0.5 LBS/1,000 SQ. FT. Most soils in Minnesota will not require sulfur fertilization for adequate grass growth, but the coarse-textured soil with low organic matter in this location is a situation where sulfur can be deficient.

 

In the Vegetable Garden Report, the “LIME RECOMMENDATION” is 20 LBS/100 SQ. FT. The first thing to recognize is that lime and fertilizer recommendations for vegetable and flower gardens, as well as landscape trees and shrubs, are given for an area of 100 SQ. FT. This is different than the recommendations above for the Home Lawn, which were for an area of 1,000 SQ. FT. If you calculate your fertilizer rate using the wrong unit of area, you will over- or under-fertilize by a factor of 10.

Plants differ in their optimum pH requirements and liming rates differ accordingly. For a Vegetable Garden, the lime requirement is designed to raise soil pH to 6.5. See Raising Soil pH for more information on liming soils. Some plants, such as azaleas and blueberries, require acid soil and you may have to lower soil pH for them to grow well. See Soil Acidification for more information if your Soil Test Report is for plants where you need to lower soil pH. Information on the pH requirements of some common landscape plants can be found at Soil pH Preferences for Selected Landscape Plants.

The “TOTAL AMOUNT OF EACH NUTRIENT TO APPLY PER YEAR*” for the Vegetable Garden are: NITROGEN – 0.15 LBS/100 SQ. FT., PHOSPHATE – 0.2 LBS/100 SQ. FT., and POTASH – 0.3 LBS/100 SQ. FT.. At the bottom of the page is a recommendation for a midseason application of additional nitrogen for certain vegetable crops. The approximate NPK ratio of this fertilizer recommendation is 15-20-30. The information section below the recommendations, the Explanation of Soil Test Report page, and the information links listed above in the Home Lawn discussion provide details on selecting a fertilizer, determining application rates, and methods of application.

If your Soil Test Report is for something different than an established lawn or a vegetable garden, such as a flower garden, new lawn, trees, shrubs, or a fruit planting, it will follow the same format as the examples we have gone through and the information presented here should be helpful. For additional information on fertilizing other types of plants, refer to: Trees, Shrubs, and Fruits, Vegetable and Flower Gardens, New Lawns and Turf, and Tree Fertilization: A Guide for Fertilizing New and Established Trees in the Landscape.

Still Have Questions?

Hopefully this discussion has helped you understand the information contained in your Soil Test Report. If you have additional questions, contact:

Yard & Garden Desk
952-443-1426
www.extension.umn.edu/Garden
or Carl Rosen
612-625-8114
crosen@umn.edu