Many green-industry textbooks describe an ideal growing media as having the following volumetric proportions: 50 percent solids, 25 percent air-filled porosity and 25 percent water (see the Figure on page 39). This assumes that the soil is at “field capacity,” meaning it was once fully saturated and gravitational drainage has stopped.
WHICH AMENDMENT SHOULD YOU USE?
The purpose of the solid component in the equation is obvious: plant structural support, etc., and the 25 percent water retention and 25 percent air-space provide a moisture reservoir, which minimizes drought stress, and the air space also contains oxygen for healthy root respiration. These physical properties can be found naturally in many loamy garden soils or soils that possess a granular structure. While these physical properties are probably enough to successfully culture many field, greenhouse and houseplants, turfgrass soils require additional considerations.
The main difference between soils for crops and those where turf is grown is that turfgrass soils require a functional consideration because they are often subjected to intense traffic. Therefore, to maintain the aforementioned physical properties, which ensure proper water infiltration, drainage, and a resistance to compaction, the actual soil particles and their arrangement is important. These properties are in addition to the nutrient retention and an active microbial population, so that the turf does not have to be fertilized often and the biological health of the soil is maintained.
Compounding the problem of creating the perfect environment for turf is the fact that turfgrasses are frequently established on infertile, poorly structured soils or areas that have been deemed unsuitable for building construction due to poor soil strength, flooding, etc. Thus, given these challenging site conditions, turf managers often consider improving soils with soil amendments to ensure successful establishment and maintenance.
There are many commercially available soil amendments, each with unique qualities that make it appropriate for a variety of situations. Therefore, it is important to understand the differences between soil amendments to be an educated consumer. First and foremost, it is important to remember that one size does not fit all. Before purchasing soil amendments, you need to keep in mind what your overall objective is going to be. Typically, improved soil structure, compaction resistance, better drainage and increased nutrient or water retention are the biggest benefits to modifying a soil.
TYPES OF AMENDMENTS
As you evaluate a specific amendment to determine whether it fits your use criteria, consider the following list of questions.
What is the current texture of the soil you are trying to amend?
Is it dominated primarily by sand, silt and clay, or does it contain a mixture of these particles (e.g. a loam)? Are you amending an established, heavily trafficked area, a poorly structured native soil, or an expensive synthetically prepared sand-based root zone?
How will the addition of this product affect soil texture, the physical properties, soil chemistry and the microbiology?
Is turf going to be grown in the area? Baseball and softball infields have “skinned areas” that do not need to support plant life but need a consistent moisture content and provide specific functional use characteristics. By contrast, the goal mouths on soccer fields or other sports fields receive significant traffic and the presence of turf in those areas is essential for player safety and proper use of that area.
What is the long-term stability of this product? Many soil amendments are byproducts from other industries (such as crumb rubber and expanded shales) and the quality control of some products might not be very high. How much will be required to achieve effective results? Is the product available in your region? If not, how much is it going to cost to ship this material? Some of the best mineral amendments in the world may be located somewhere outside of the United States. It might not make economical sense to ship them to the far reaches of the interior United States.
Realistically, you are probably trying to identify an amendment that achieves one or more of the aforementioned goals. Most importantly, you need to select an amendment that is not going to cause long-term soil-related problems, which might require expensive soil removal and turfgrass re-establishment. For example, the incorporation of an extremely fine-textured amendment to a sand rootzone with drainage problems would not be appropriate because it could further decrease percolation. Additionally, if you use a poorly cured compost product, an unfavorable soil carbon-to-nitrogen ratio will occur, which affects nitrogen availability.
There are two major classes of soil amendments: the organics, which are derived from plant and animal materials, and the inorganics, which are mineral based and found in naturally occurring deposits throughout the world.
Organic soil amendments have numerous benefits, but their major advantages are in moisture and nutrient retention. Large organic matter additions to clay soils help improve soil structure through increased aggregation, which, over-time, improves soil aeration, increases drainage and ultimately improves turfgrass root health. Organic materials also promote microbial activity, which is important for nutrient cycling and many other soil processes. In drought-prone sandy soils, adding certain organic materials like peat moss can dramatically increase moisture retention. They also improve soil resiliency and enhance the ability of soils to withstand traffic and other physical stress in these soils.
HEAT TREATED CLAYS
Some examples of commonly used organic amendments are composts, manures, sphagnum or reed sedge peat moss, granular sea kelp and many other humic substances. When selecting organic materials for soil modification, it is important to use properly composted or well-decomposed sources because they are less likely to cause problems. Some products (e.g. peat moss) hold several times their weight in water, making them excellent for water retention. Once dry, however, they can be extremely difficult to re-wet. Some of the drawbacks to the organics are that their influence may be short lived because soil micro-organisms will eventually consume them and many materials are not sized for easy, uniform applications with traditional spreading equipment. They also tend to be regional in nature due to their link to local municipal or agricultural operations. The organics are relatively inexpensive compared to many of the inorganic products.
Inorganic soil amendments include a wide variety of materials, which range in cost depending on the specific material, its proximity to where it is going to be used and, of course, marketing expenses.
Robert Carrow, professor of turfgrass breeding and stress physiology in the University of Georgia's department of crop and soil sciences at the University's Griffin campus, found that an effective inorganic soil amendment will have the following qualities:
CALCINED VS. VITRIFIED CLAYS
Retains water in pores within the particles and most of the water is released to the turf plant.
Will not accumulate salt in the pores.
CRUMB RUBBER USAGE
Retains its physical structure and does not deteriorate.
Is cost effective compared to organic amendments at application rates to achieve comparable water retention.
With these guidelines in mind, there are a wide variety of inorganic soil amendments that turfgrass managers can choose from. Some examples of common inorganic amendments often used for turfgrass soil modification include sand, diatomaceous earth, porous ceramics, zeolites and also crumb rubber.
Sand is an obvious choice for improving heavily trafficked turfgrass soils because sand particles are strong and extremely stable, which is why they are frequently used in road building. Furthermore, sand is relatively inexpensive and widely available most anywhere. However, not all sands have the same physical properties. Sands suitable for road building and brick laying are generally not appropriate for use in turfgrass situations. It is important to carefully consider the sand source and its particle size distribution before using it. Repeated applications of coarse-textured sands over a native soil can make an area droughty, and frequent applications of very fine sands can actually hold substantial moisture. Round sands move and tend to be rather instable, whereas extremely angular sands may pack tightly affecting proper drainage and aeration. By their nature, sand particles have virtually no internal porosity and no ability to hold moisture inside the individual grains. Regardless of which sand you use, in order to effectively improve the structure of a native soil, it will take at least 60 percent sand by volume, which must be evenly distributed throughout the soil with proper mixing. This process normally uses much more sand than most turf managers have the capacity to apply or mix in one single application. A simple garden tiller will not properly accomplish this task. Lastly, sand has very poor nutrient retention and frequent applications during stress times like drought or heat can be very abrasive and inflict damage to sensitive turfgrass plants.
MIXING IT UP
Diatomaceous Earth is a product that is mined from deposits of diatom shells. Diatoms are single-celled aquatic organisms having rigid cell walls composed of silica. The skeletons of these diatoms have a high degree of internal pore structure offer the opportunity for water-holding capacities and some nutrient retention. These products can be produced with and without small amounts (5 percent or less) of clay binders, and most are subjected to a ceramic heating process. Several products have been shown to increase water retention in sand-based rootzones. Diatomaceous earth products also confer a very small degree of nutrient retention and can be used for filtering. Diatomaceous earth is often used in swimming pool filters. The long-term stability of some of these materials is not yet fully understood, but materials that are heat-treated probably have greater stability than those that are not.
A small to moderate quantity of clay can be beneficial to a sandy soil. Clays hold substantial quantities of water and serve as a repository for soil nutrients. Many of the clay-based amendments are derived from illites, montmorillonite and attapulgite minerals. Clays with large surface areas like attapulgite are favored by the industries that produce amendments because they have the capacity to absorb large quantities of water or other fluid. The clays are usually heat-treated, a process sometimes called “calcining.” The intent of this process is to improve particle strength. In addition to its use as a soil amendment, heat-treated attapulgite is often used as pet litter and oil absorbent.
Another term that has been often used to describe the clay products used for the turfgrass soil market is porous ceramic. A porous ceramic is merely a mineral material that is heat-treated at high temperatures and possesses intrinsic porosity while retaining its chemical properties. Depending on the temperature and duration of firing, the heat hardens the amendment, which increases particle stability. Other benefits associated with the high temperature process are that the materials become sterilized, killing any weed seeds, fungal pathogens, spores and nematodes.
Expanded shales are another heat-treated product that have been successfully used for modification of heavy soils with compaction problems. These products have excellent particle strength and are moderately porous, which enables them to retain some moisture. Nutrient retention is slightly more than that provided by sand but less than clay-based products. Like sand additions, their biggest advantage is improved soil structure.
Both calcined clays and vitrified clays could be categorized as porous ceramics. At baseball or softball fields, a combination of both materials is used as infield conditioner. Used together, these materials allow improved water relations, decrease the soil strength and allow the “skinned” areas to be worked easier. The soil can be repeatedly wetted and dried without becoming bricklike. Both vitrified and calcined products often start with similar base minerals. The difference is in the firing process. Both calcining and vitrifying helps the granules maintain their physical hardness. In some ways, vitrified products are similar to large sand grains, which also do not retain moisture very well. In terms of appearance, vitrified clays have a slightly redder color due to the oxidation of iron during the firing process, than the tan colored calcined clays.
Zeolites are tremendous absorbers, and they have long been used for many industrial processes like the removal of environmental pollutants. Some zeolites have even been fed directly to livestock to improve gastrointestinal difficulties. The application of these minerals to fine turf has become very popular in the last decade. Zeolite, like the negatively charged clays, has a strong affinity for cations. Although zeolites retain moisture, they do not retain as much as the clay-based porous ceramics. The main reason a turf manager might consider a zeolite would probably be to increase nutrient retention on infertile soils. Several university studies have documented reductions in fertilizer needs in zeolite-amended sand rootzones. One minor concern with some of the zeolites is that they have exchange sites that are dominated by the sodium (Na
Millions of tires are discarded annually, many of them once ended up in landfills. However, it's now a common process to recycle them, where the tire is broken down and the steel removed. Finding a market for the crumb rubber particles has been difficult. One use, however, has been for improving turfgrass soils that are heavily trafficked. The theory is that introducing crumb rubber into the soil will increase wear tolerance, minimize soil compaction and enhance long-term performance. Researchers have found that when relatively small particles (¼ inch diameter) have been surface applied or incorporated (20 percent by weight) into the soil, the method has successfully improved the turf surface. This provides a very resilient soil. Additionally, compared to conventional sand topdressing, the repeated application of crumb rubber seems to be less abrasive to the turf plant because there are no sharp edges, which could tear and damage the turf leaves.
All of the inorganic amendments possess varying degrees of internal pore space, which give them the ability to absorb water. The number of pores and their size affect the ability of that amendment to release water. Thus, because of the variety of materials on the market and the variability in the heating processes, there is much debate about regarding much of the internal water is actually released. Throughout the years several research experiments have documented the water release of many of the amendments on the market. In a study at North Carolina State University, researchers characterized several inorganic soil amendments and compared them to three quartz sands and a sphagnum peat moss. It was documented that the inorganic amendments and sphagnum peat moss had very high total porosities as evidenced by their high saturation values (55 to 75 percent). Researchers also observed that the inorganic amendments released a substantial amount of their water rather quickly, similar to the sands tested. Most of the inorganic amendments, however, also retained a substantial quantity of water (20 to 40 percent compared to < 1 percent for sand) at relatively high tensions equivalent to wilting conditions. What this means is that there is still water contained inside the particles and that water might be beneficial and available if the plant can access it. The problem is that in many situations this water is located in extremely small pores and it might not be accessible to the plant roots. By contrast, the peat moss that was tested released most of its water at a much more gradual rate. This makes it a very desirable soil amendment. Certainly, as new materials are marketed and variations in the ceramic process continue to occur the debate will continue on specific amendments and their ability to retain and release water.
With any amendment addition, the success or failure has to do with the soil in which it is being incorporated and the quantity of amendment applied. In most situations, the overall pore geometry and architecture of the pores in the existing soil is probably going to have a much greater influence on the water release/retention than the intrinsic pores of the amendments. Remember, for most turfgrass areas we are trying to promote a granular structure with abundant macropores to enhance drainage and natural soil aeration
Ultimately, as with all business decisions, the final determination in amendment selection is probably going to come down to economics. Among the amendments there is a substantial difference in cost. In most cases, inorganic amendments cost considerably more than organic products such as sphagnum peat moss when used at the same incorporation rate. The difference may be five or 10 times as much. Comparatively, where improving soil structure is important and crumb rubber is being considered, crumb rubber costs much more than sand. However, 40 to 50 percent less crumb rubber could be used, which may make it more cost effective but still slightly more expensive than adding sand.
Remember not all soil amendments are suitable for every situation. Each amendment will react differently depending on the existing soil texture and structure and the quantity of the amendment incorporated. Depending on your final goal, it may be appropriate to use a mixture of several different amendments. For example, mixing sand, organic matter and perhaps zeolite together will create an amendment mixture that, when properly incorporated, produces a soil that resists compaction, encourages aggregation, drains well and increases nutrient retention than a soil that was deficient in these properties.
Cale A. Bigelow, Ph.D., is assistant professor of agronomy at Purdue University (West Lafayette, Ind.).
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