Understanding a Soil Test Analysis Report
Understanding a soil analysis report can be difficult. The testing labs use industry terminology to describe the results often without any explanation as to what the terms mean.
Understanding what is on the soil test sheet is crucial to knowing what your clients’ soils need.
What’s on the soil test report?
Cation Exchange Capacity (CEC). The first result that appears on many soil analysis reports is the Cation Exchange Capacity (CEC). It can be complicated and difficult to understand, but is an important part of soil science. It is included in the soil analysis because it is necessary for professional turf managers.
The CEC is a measure of your soil’s capacity to hold nutrients; specifically, positively charged ions, such as K, Ca and Mg. Clay and organic matter are negatively charged and contribute to the cation exchange capacity. The negative and positive charges attract each other like magnets, so soils with high CEC will retain nutrients better than low CEC soils. The results are then used to judge the potential fertility of a site. In short, the higher the CEC, the better the nutrient holding capability.
Soil analysis results denoting low CEC soils, such as sandy soil, will need the fertilization adjusted to deliver less fertilizer per application, but performed more times per year. Sandy soils don’t have as many negative charges to hold nutrients, so they will need to be approached a little differently.
Your local university extension office can provide specific information about conditions in your area and help further explain the soil analysis report results.
Active pH and buffer pH. Soil pH is a very important result to understand in the soil analysis report. The soil pH scale is measured from 0 to 14. A reading of 7.0 is the middle of the scale and is considered to be neutral. Anything below 7.0 is acidic and anything above 7.0 is basic or alkaline.
Each movement of one whole number on the pH scale represents a change by a factor of 10. For example, several months after liming the soil you may notice the pH moves from 6 to 7 on the pH scale. The move from 6 to 7 means the soil is now 10 times less acidic than it was at 6. A movement from 5 to 7 means it is now 100 times less acidic than it was at 5.
The letters “pH” mean “potential Hydrogen.” The soil analysis will record two different pH tests. They include the active pH test and the buffer pH test. Active pH is the measure of hydrogen ions in the soil at the root zone. The presence of less hydrogen translates to less acidity and a higher pH number.
Active pH is measured with a home soil test kit. Not all kits are the same, so be sure the soil test kit you choose is able to test for all the things you need. The soil analysis of a home and garden soil test kit is generally not as accurate as a lab test.
Buffer pH is performed in a lab and is the measure of the soil’s resistance to change. Simplified, soils high in organic matter or clay require more lime to correct acidity problems than sandy soils with the same pH reading. To explain further, a reading of 5 on the pH scale is the same for all types of soil. However, to correct the pH and bring it back to 7 would require differing amounts of lime depending on what elements are in the soil. That is what the buffer pH determines. A home soil test kit doesn’t test for buffer pH.
Before changing the pH of the soil, know the best growing range for your plants.
Correcting pH problems
Acidic soil (low pH) correction. Agricultural limestone is used to make soils less acidic and bring the pH number up. When applying lime, follow the soil analysis recommendations.
The best method is to rototill the lime into the soil before planting seed. For established lawns, apply lime and distribute it with a fertilizer spreader over the surface of the grass. It will, however, take much longer to affect the soil pH when spread over the surface. The lime must work itself into the soil. Finely ground lime is messy to apply, but it will get into the soil faster than coarsely ground material.
Some types of pelletized limestone are formulated to dissolve as soon as they receive water. The purpose of this is to allow the limestone to work into the soil faster. Try to look for those types of limestone first.
Turfgrass, like all living plants, is most vigorous when growing in a soil with the adequate soil moisture, the proper pH and the optimum blend of macro- and micronutrients. Photo: THINKSTO CK/WAVEBREAKMEDIA
Hydrated lime is used in cement and mortar. It is sometimes used on lawns, but has a high burn potential and is not the best choice. Agricultural limestone is safer to use, but works a little slower. Certain grasses, especially centipedegrass, are very sensitive to lime. If applied on sensitive grass, apply lime in the coolest part of the year. Hot weather increases the burn potential.
Alkaline soil (high pH) correction. If the soil report indicates the soil is alkaline (pH above 7), using sulfur or aluminum sulfate will reduce soil alkalinity. Sulfur is an age-old product, but may take months to be effective. Sulfur requires soil microbes to break it down first. Aluminum sulfate works immediately and is what most professionals use.
All of these products can burn the grass, so water them in well after applying them.
Results of macronutrients
Macronutrients are the nutrients needed by grasses in the greatest amounts. They are usually listed first on a soil analysis. The big three are nitrogen, phosphorus and potassium and represent the three numbers on a fertilizer bag. Due to the importance of these nutrients, a soil analysis report will provide amounts to apply for any soil deficiencies.
Nitrogen (N). Nitrogen is the element used in the largest amount by turfgrass. When spreading a bag of fertilizer, it is the amount of N per 1,000 square feet that is the most important to note because fertilizer application rates are based on this number.
Nitrogen is occasionally, but not always, tested in labs. When it is tested, the soil analysis results will be shown on a scale from very low to excessively high.
The reason it is not often tested is because nitrogen is so unstable in the soil. A hard rain, hot weather, even mowing the lawn and leaving the clippings on the lawn will affect N levels. Levels of N fluctuate rapidly, and the soil analysis would not likely be accurate by the time you receive the results.
Nitrogen is important for grass growth and chlorophyll production. A properly planned fertilization program will not cause excessive growth. Excessive growth is usually a problem when using the wrong fertilizer; too much immediately available nitrogen; or by applying more fertilizer than needed.
Phosphorus (P). Phosphorus represents the second number on a bag of fertilizer. A bag of 14-3-8 contains 3 percent P by weight. Established lawn grasses don’t require a lot of added phosphorus unless a soil test says the soil requires it. Mature grasses are very good at removing it from the soil.
Plants cannot grow and develop properly without phosphorus. Its use in root development has been known and well-documented for many years. Phosphorus also increases the plant’s disease resistance and improves heat and drought tolerance.
What is very important to know about phosphorus is that it is relatively immobile in the soil and almost never leaches after it is applied. Due to its immobile state, starter fertilizers high in P are often used on newly seeded lawns to ensure enough P is directly within reach of the very short root systems of young turf seedlings.
Potassium (K). Potassium represents the third number on a bag of fertilizer. A bag of 14-3-8 would contain 8 percent K by weight. The understanding and the use of K have changed in recent years, as scientists better understand how plants use it. Now we know turfgrass uses far more potassium than we realized. One of your lawn’s biggest enemies is stress. Potassium decreases stress and helps turfgrass endure tough conditions. Additional applications of K have been a lifesaver for many sports fields by increasing the turf’s ability to withstand the traffic, heat and drought problems of summer.
Soil analysis results of micronutrients
Micronutrients are nutrients used by grasses in small (trace) amounts. However, they are still very important.
Boron (B). Grass needs only very small quantities of B for healthy growth. As a result, B is rarely added to any fertilizer mixture and is rarely deficient in soils.
Copper (Cu). Cu is used only in very small (trace) quantities. It is rarely deficient in soils and is rarely added to fertilizer. Organic fertilizers may contain traces of Cu and other elements. How grasses use Cu is still not completely understood. When too much copper is used it causes lesions that look similar to a disease.
Iron (Fe). Iron is the most common micronutrient to be deficient in soils. Deficiency problems, in most cases, occur when soil pH is high (above 7). In alkaline soils, the iron becomes bound up in a form that the roots can’t take up. It is also the most common micronutrient to be included in fertilizer.
Iron chlorosis is the term used to describe iron deficiency. The symptom is a pale green color. Iron is not a part of chlorophyll, but without iron, grass will not produce any.
If the pH is high, the additions of granular iron such as “ironite” or fertilizer containing iron will probably not help. Instead, foliar applications of liquid iron are a better choice and will provide temporary relief. This way, the iron is absorbed through plant tissues and not taken up by the roots.
Organic matter within a soil is the food source for the microorganisms that are necessary to make available the elements that plants can use. Photo: THINKSTOCK/FUSE
Iron sulfate or iron chelates are the best sources for liquid iron. Once it is sprayed on the grass, do not wash it off. If that happens, as soon as the iron touches the high pH soil, it will change into a form that the roots cannot take up. A long-term solution would be to fix the pH problem.
Iron causes rust stains on concrete, so make sure you rinse off sidewalks and driveways after spreading.
Magnesium (Mg). Mg is an essential element because it is part of the chlorophyll molecule. Chlorophyll is what gives the grass its green color. Without sufficient Mg, grass will appear to suffer from chlorosis. Sandy soils are most likely to experience Mg deficiencies.
Another problem that can produce an Mg deficiency is low pH soil. Soils that have a pH lower than 7 are likely to develop Mg problems. The lower the pH number, the more deficient the Mg will be.
If the soil test shows a deficiency in Mg, then applications of Dolomitic limestone will help. Dolomitic limestone contains both lime and Mg. It will raise the pH while delivering needed Mg.
Manganese (Mn). This element is also needed in small amounts and is rarely deficient. Problems arise when they become less available in high pH soils. In high pH soils, the element becomes bound up making it unavailable to the roots.
Mn is an element necessary for the production of chlorophyll. Since it is needed in small amounts, fertilizers are available with Mn included to correct this problem. The soil test should indicate if levels are low.
Sulfur (S). Researchers tell us that sulfur is rarely deficient because of acid rain. Acid rain is the term used when smoke from coal is burned and brought back to earth in rain. The amount of sulfur received each year in rain exceeds the annual turf requirement for the nutrient. Researchers tell us that 90 percent of the grasses in the U.S. get sulfur in this manner.
However, acid rain may not be enough to correct high pH soils. When there is a sulfur problem, the symptom will be a yellowing of the grass—a condition that won’t usually change even with an application of nitrogen. Sulfur can then be directly applied using a fertilizer spreader.
Organic matter’s importance
The quality and amount of organic matter is more important than most people might think. Most lawns will contain 2 to 3 percent organic matter. It may seem like a small amount, but consider that a lawn with 6 percent is viewed as high in organic matter.
Organic matter is necessary for healthy soil and healthy lawns. Something you may not be familiar with is that organic matter is the primary food source for microorganisms. Soil microbes exist in tremendous numbers and break down soil elements into a form that can be taken up by plant roots. They are so important, that without microorganisms plant life would cease to exist.
Unfortunately, the results shown on the soil analysis report give the percent of organic matter, but not the percent of usable organic matter. Not all of the organic matter in the soil has nutritional value for soil microbes. Soil microbes are the natural enemy to many disease pathogens that live in the soil. Soils that are high in fresh organic matter have the least problems with grass diseases. Unfortunately, soil microbe information does not appear on the soil analysis report. Healthy soil is the best assurance for healthy microbe populations.
Of the different types of soil, heavy clay is probably the least favorable for optimum plant growth. It compacts easily, leaving little room for oxygen.
If you haven’t planted any turfgrass, then rototill topsoil or compost into the clay soil for best results. To add organic matter to established turf, you can spread a layer of compost approximately 1 inch thick over the turf surface.
To step it up a notch, core aerate the soil. Core aeration pulls out a plug of soil and relieves compaction. The plugs are about 3/4 inch wide and 2.5 to 3 inches deep. You can either remove the cores or leave the cores on the grass to break down naturally. Core aerate heavier than usual, then cover with a 1-inch layer of organic matter. The organic matter will cover the surface and fill in the holes.
Russ James’ career in the green industry began in 1978 at the Lewisville Country Club in Lewisville, Texas. Then, he owned a commercial turf business in southern Missouri. Now semi-retired, he shares turf maintenance tips on his website, www.lawncareacademy.com.