An ideal soil for the growth of turfgrass contains a topsoil layer that’s often referred to as the “A” horizon. This layer is rich with organic matter and nutrients and has ideal chemical and physical properties for the development of roots. This topsoil layer forms over many years as a result of the weathering of the parent rocky material and the interaction of plants, animals and microbes.
A standard practice in many parts of the country is to remove this layer of topsoil during home construction and place a layer of sod over the subsoil that remains. Subsoil (The “B” horizon) is weathered parent material, but lacks the organic matter and isn’t desirable for the growth of turfgrass for many reasons – it tends to have a finer texture due to the lack of organic matter. Because of this, compaction and drainage are more of an issue.
Chemically, subsoil tends to be more deficient in nutrients. Also, depending on the parent material, it’s often not within the ideal pH range for grass growth.
For all of these reasons, it’s pretty common to see sodded home lawns begin to have issues about three years after establishment. Unfortunately, some athletic fields are also established on subsoils and thus will face the same problems. Having said this, while topsoil tends to be the better environment for turfgrass growth, the unique pressures placed upon the turf when using it for athletic fields can result in native soil fields established on topsoil still having issues with drainage.
A soil amendment is anything added to the soil for the purpose of improving either its chemical (pH, nutrient status) or physical (water retention, permeability, drainage) properties. Generally speaking, it’s more difficult to modify a soil’s physical properties than its chemical properties. A variety of materials are used in the turfgrass industry as soil amendments. Organic amendments, such as composts, tend to improve both chemical and physical properties of the soil. Inorganic amendments, depending on the material used and the soil being amended, may improve either the chemical or physical properties of the soil, but rarely do both.
The key to success with soil amendments is to test the soil, so that you understand what the deficiencies are, and then to understand how the various soil amendments may improve that soil. With many amendments, especially those intended to improve soil physical properties, in order to be effective, they must be mixed with the soil. When materials are layered incorrectly, several problems can arise. Potential issues include decreased water, air movement and rooting.
In this article we’ll focus on those materials that are added and incorporated into the soil (not topdressed) at the time of field construction or renovation:
The traditional organic soil amendments are either a high-quality topsoil, a topsoil blended with compost or Spagnum peat moss, which has long been the standard source of organic matter for amending both native soil and sand-based fields. For sand-based fields, the amount added tends to be around 10 percent, and on native soil fields the amount added is based on soil test results. Its addition improves not only nutrient status, but also water holding capacity and drainage. Because of some concerns over the sustainability of peat production and/or because of the trend to use composted waste materials, compost and vermicompost are some alternative options.
There are many sources for the production of composts and vermicomposts and there are also different processing methods. Because of this, two sources of vermicompost or compost derived from the same waste materials might have very different chemical and physical properties. It’s important to choose a source from a vendor that processes their compost using strict quality controls. You should also have it tested by a lab to determine its chemical and physical properties.
Compost: When organic material is broken down through biological decomposition and then stabilized and sanitized, it can be added to soil in order to improve its chemical or physical properties. Composted materials can be utilized as soil amendments, but the way that they’re used depends on field type. For example, on fine-textured, soil-based fields, the addition of compost will improve water management by improving soil structure. This leads to increased soil porosity and therefore infiltration rates and improved drainage. If the soil is low in organic matter (3 percent or less), compost amendments can increase water holding capacity.
In addition to the physical benefit of compost addition, soil chemistry can also be improved. Most composted materials contain macronutrients, but also micronutrients in slowly available forms. In addition to supplying nutrients, composts improve the cation exchange capacity of the soil, which is the measure of the soil’s ability to hold nutrients.
On sand-based fields, it makes little sense to topdress with compost because of the potential for decreasing water permeability. Therefore, they’re usually incorporated during field construction and typically at low rates, usually around 1 percent by weight.
Vermicompost: Vermicomposting is the process of using earthworms to break down various
wastes, usually from the animal, food, or dairy industries into a useable, nutrient rich compost-like material. This has been the subject of much research over the past 20 years. Essentially, vermicompost is the manure produced after waste materials are fed to worms. It’s nutrient rich and has physical properties that lend itself for use as a soil amendment.
Compost teas: Compost teas are, much in the same way you brew tea, produced by steeping finished compost in water. The extract contains beneficial microorganisms and plant growth compounds. Some research shows that they can improve plant quality and suppress disease. With most soil amendments, in order to realize maximum benefit, the amendment requires thorough incorporation into the soil. With compost teas, the chemical and biological benefits of adding compost are realized, but not the physical benefit of improved soil structure. But, compost teas may offer a way to realize some of the benefits of compost, but without the need for soil incorporation, thus they could be applied while the field is in active use.
The key to good compost tea is that the right source of compost is used and that the tea is steeped under aeration, which is important for the production of the beneficial microorganisms. When trying it for the first time, leave a check plot so that you can determine if the application caused a benefit. (Some research conducted on the benefits of compost tea when applied to athletic fields has found no increase in turfgrass color, quality or cover.)
Some inorganic soil amendments are added principally to improve chemical properties of soil, while others are added to improve physical properties. In rare instances, some may accomplish both.
Sand: Not all sands are equal, so when sourcing it, you’ll want to have it tested. Also, avoid sand that has high amounts of gravel or other undesirable properties. Recognize that in order for sand to work to improve drainage of clay-based soil, a very large amount of it must be added, otherwise when the sand is mixed with the clay it will make drainage and compaction worse rather than better. The amount of sand added should be based on the particle size distribution of the soil being modified. In other words, if there are less sand-size particles in the native soil, then more sand will be required. You should expect to have to add as much as 75 percent, or about 12 inches of sand, to obtain a modified root zone that’s 16 inches deep.
Lime: This is a carbonate of calcium or magnesium, and its principle use is to raise soil pH. Soil that is too acidic can be deleterious to the growth of turfgrass. In soil that is too acidic, not only are nutrients such as calcium, magnesium and phosphorus likely to be deficient, but aluminum and manganese may be at levels that are toxic to grass. In addition to the effects on nutrient availability, turf grown in low pH soil may have more thatch. There may also be more issues with weeds and disease.
The recommended pH for grass growth varies by species, but generally falls in the 6.0 to 7.0 range. A pH of 6 is actually 10 times more acidic than a pH of 7, which is neutral. The addition of lime as an amendment will not do much to improve the field if the soil has a pH above 7, and there are no deficiencies in either calcium or magnesium. (A soil test will determine whether adding a liming material will be beneficial and in what amount.)
Gypsum: One of the issues with certain amendments is that they (in some circumstances) can improve soil conditions, but in others they don’t do much.
Gypsum is certainly an example of this. Chemically, gypsum is called calcium sulfate dehydrate (CaSO4 . 2H2O) and it’s a moderately water-soluble source of the nutrients calcium and sulfur. The classic use of gypsum is for the reclamation of sodic soils, which occur when there’s a high concentration of sodium in the soil. Sodium is a univalent (-1 charge) cation. But when present in high concentrations, it displaces divalent (-2 charge) cations such as calcium and magnesium off the soil particle. Because of the way clay chemistry works, this results in dispersion of the soil (the soil particles break apart into smaller particles) and this greatly disrupts drainage.
Addition of gypsum to a saline or sodic soil adds calcium, which displaces the sodium from the soil particles, and the sodium can then be leached. This results in flocculation of the soil particles (the smaller soil particles come together to form aggregates) thus leading to improved soil structure and better drainage. But this benefit of gypsum only applies when saline or sodic soil conditions exist. In other words, adding gypsum to non-sodic soil isn’t going to improve your drainage because it will not, under non-sodic conditions, cause the soil to flocculate.
There are other potential benefits of gypsum, but they aren’t realized under all soil conditions. For example, if gypsum is added to acidic soil (below pH 4.5), some of the aluminum in the soil is displaced and leached away, allowing the soil pH to rise.
Amending the soil with porous inorganic materials – such as calcined clay and diatomaceous earth – can be beneficial, but these are used more to amend sand-based fields than native soil fields. These materials can retain and release water and their addition may reduce compaction, increase water infiltration and water holding capacity.
Most soil amendments are more effective if thoroughly mixed with the soil. Though it happens less often, it’s easier to amend your soil during construction or renovation. If planning a construction or renovation project, test your soil to determine its deficiencies and then plan to add the right amendment in the appropriate amount in order to correct it.