Soil scientists divide soil into layers they call horizons. The A horizon is the uppermost several inches and consists mostly of what we know as topsoil. It is often dark in color and rich in organic matter, and it usually provides a favorable environment for plant growth. The next two layers, the B and C horizons, are lighter in color, lower in organic matter and relatively infertile. We call the B and C horizons subsoil. Plant roots generally extend through the A horizon and well into the B horizon. However, the C horizon, which may be well below the surface, is comparatively inhospitable for root growth.
In landscape situations, this natural layering often is absent due to soil movement during construction. All too often, this means that no topsoil layer is present, forcing the landscape installer to modify the existing subsoil to make it more favorable to plant growth.
Aside from horizons, which describe the position of the soil layer, soil scientists also refer to soil fractions. Fractions refer to organic or inorganic (mineral) substances. Thus, most soils are composed partly of a mineral fraction and partly of an organic fraction. A few soils are almost completely organic, and others are mostly mineral.
• The mineral fraction of soil, consisting of particles that ultimately originated from rock, comprises the largest percentage of most soils. The type of rock from which the mineral particles originated has some bearing on the chemistry of a soil. However, the mix of particle sizes has a greater impact on soil quality and how you must manage it. The age of the soil and how much weathering it has undergone determine particle size: Older, more weathered soils consist of smaller particles.
The smallest particles are clay. Larger (but still quite small) particles are silt, and the largest particles (that still qualify as soil) are sand (see table, “Sizes of soil particles,” above right). Soils rarely, if ever, consist of solely one size of particle. Thus, soils are classified according to the proportion of each particle size they contain—we commonly refer to this as soil texture (see Figure 1, at right).
Texture, in the broadest sense, is stated as coarse (sandy soils), medium (silts or loamy soils) or fine (clayey soils). Loamy soils are intermediate in nature and not totally dominated by the characteristics of any particular particle size, though they proportionately contain more silt than sandy or clayey soils. Thus, there is no such thing as a loam particle, only loam soils. Loamy soils generally have the best overall characteristics for plant growth.
To be even more specific, we combine these terms. For example, sandy clay has significant amounts of sand but is dominated by clay particles and clay characteristics. A sandy loam is a mix of particle sizes not totally dominated by characteristics of any particle size, but—due to a somewhat higher relative sand content—its qualities tend toward those of sand. Other terms you’ll often encounter to describe texture include light and heavy, referring to sandy and clayey soils, respectively. As we’ll see, texture, more than any other single aspect, determines the manageability of soils (see sidebar, “Testing for texture,”).
• Organic matter (OM), the other soil fraction, is present in most soils, but content varies widely. Soils low in organic matter may have less than 1 percent OM content, whereas highly organic soils range far higher. Most soils contain less than 10 percent, and many—especially in arid climates—hold only 1 or 2 percent OM.
OM results from decaying plant material. This decay is brought about mainly by bacteria and fungi that consume plant matter as food. The resulting residues are a rich mix of organic materials that usually have a positive effect on soil quality. As complex organic molecules break down into simpler forms, the organic matter eventually arrives at a semi-stable form we call humus—the dark-colored substance we commonly associate with “rich” soil.
Humus contains a variety of carbohydrates, proteins, lignin, cellulose and other materials, but its main benefit does not lie in its nutritional content (most of which is unavailable to plants). Humus improves the physical structure and chemistry of soils so that they have better water- and nutrient-holding capacities and greater permeability. Notably, humic acid causes clay particles to aggregate into larger particles that act more like sand than clay. This improves drainage and aeration and so is especially valuable in clay soils. Before plant material undergoes extensive decomposition—that is, before it becomes humus—it still is beneficial to soil because it improves physical structure.
As stated above, most of the nutrients in humus are unavailable to plants. Eventually, however, even humus can break down into inorganic compounds by the process of mineralization. At this point, nutrients become available to plants again, and the cycle is completed. The reverse of this process is immobilization, wherein microorganisms assimilate inorganic substances into organic compounds. Both of these processes are ongoing in soil, but the overall trend—not counting plant uptake of nutrients—is always toward mineralization.
• Water is present in all soils. Texture has the greatest effect on how much water soil can hold: Finely textured soils hold more water than coarse soils. This is because of how soil particles hold onto water molecules. Water molecules “stick” to soil-particle surfaces by a force called adhesion because they possess positive electrical charges that are attracted to negative electrical charges on the soil particles. Thus, a layer of water surrounds soil particles. Even soils that may seem dry have very small layers of water around each particle (though this water may be unavailable to plants). Sandy soils hold the least amount of water due to low soil-particle surface area. A given volume of clay soil, because of the greater number of particles present, contains a far greater surface area onto which water molecules can cling and so has excellent water retention.
• Air is present in the pore spaces between soil particles. Because water is the other substance that can occupy significant amounts of pore space, air content is determined to a large extent by how wet soil is. The presence of air—particularly oxygen—in pore spaces is as important to most plants as water. Thus, good aeration is an important physical property of soil. Soils that hold a great deal of water are low or lacking in oxygen. That is why plants languish in saturated soils—their roots starve for oxygen.
• Living organisms are prevalent in nearly all soils. Bacteria, fungi, protozoans, nematodes and larger creatures such as earthworms inhabit soils, where they live on decaying plant matter and each other. From a soil-management standpoint, the main benefit of soil organisms is their role in decomposing organic matter, which we discussed above. Warm, moist conditions favor the activity of these organisms, so these types of climates favor rapid decomposition of organic matter. However, warm moist climates also favor rapid plant growth, which adds more raw material for the decay process. Thus, the cycling occurs more rapidly and on a larger scale.
HOW SOIL TYPE AFFECTS MANAGEMENT
• Water movement. Because soil particles are solid, water obviously cannot move through them. Instead, it must move around them. Water’s movement in and through soil depends on the arrangement and size of the soil’s pore spaces—the spaces between soil particles. Due to the random way soil particles pack together, pore spaces vary in size. Some are large and some are small. A “typical” soil may be about 50 percent pore space—25 percent small pore space and 25 percent large pore space. The proportion of soil occupied by pore space is its porosity and varies a great deal among soil types.
When water drains through a soil, most of its movement is through large pores. Coarse-textured soils have more large pore spaces than finely textured soils, and air normally fills these large pores. Larger pores are much better at conducting both air and water through the soil, and that’s why sandy soils have excellent drainage and aeration. The rate at which water can flow through a soil is called hydraulic conductivity. Coarser, sandy soils, with their larger pore sizes, have higher hydraulic conductivity than fine, clay soils, which tend to be lower in oxygen and retain more water.
Small pores more often contain water, rather than air. Because clay soils have more small pores and greater water retention, they also are more prone to saturation due to heavy rainfall or poorly drained conditions. When water completely occupies all the pore space in soil, the soil is saturated. Saturated soils, as we mentioned, lack oxygen and therefore make poor environments for root growth.
However, clay soils have their benefits as well. For example, they hold more available water and nutrients, so plants can last longer between irrigations and fertilizer applications. Sandy soils hold much less water and nutrients, so plants growing in them are more prone to drought and nutrient deficiency. One reason loamy soils are valuable is that they hold more water than sand, but they do not have the drainage problems of clays.
Infiltration rate describes how fast water enters the soil surface. Infiltration is similar to hydraulic conductivity and largely dependent on it. Whenever you apply water to the soil surface at a higher rate than the infiltration rate, you will have puddling or runoff.
Because clay soils often require short, light doses of water to avoid runoff or puddling (and possess low hydraulic conductivity), they are more susceptible to salt buildup. This happens because each time you apply water, you also apply a small amount of dissolved salt along with it. In well-drained soils, you easily can apply enough water so that some of it drains, or leaches, completely through the root zone. This water takes some of the dissolved salt with it, thus reducing the amount to which plant roots are exposed. However, when infiltration rates limit you to small doses of water, you cannot apply enough to leach any out of the root zone. Thus, while water leaves the soil by evapotranspiration, the salt stays behind and slowly accumulates to toxic levels as additional irrigation water brings more. This also illustrates why water quality is an important issue.
• Compaction and density. An aspect we have not touched on yet is soil-particle shape. Particles smaller than sand tend to be flattened and plate-like. This tendency is very strong with clay particles, and this has important implications. Clay particles, being flat, can stack tightly together, virtually eliminating any pore spaces between particles. In other words, porosity decreases. This is true with silts as well and is what happens when soils compact and why compacted soils conduct little water or air. Further, root growth is reduced because pore spaces through which small roots grow do not exist in compacted soil. Moist soil is more prone to compaction because when ample water is present in the soil, the particles can slip and slide past one another, making repositioning into a more compact state easier.
Clay particles also can seal, for the same reason. The flattened particles all can be oriented in the same positions— flat—and form a barrier through which water and air cannot penetrate. That’s why it’s important to score glazed surfaces—such as those created by tree-spades in planting holes—to disrupt this barrier and allow water and air penetration.
Sand tends not to compact because, unlike clays, sand particles are not flat. They cannot “stack” in a way that reduces pore space. That is why sand is the preferred medium for high-traffic turf such as golf greens and athletic fields—turf growing on sand is not as prone to the damage that compaction causes.
At this point, we should mention pans. Pans are impermeable layers present below the surface of some soils at varying depth. Hard pans are rock-like while clay pans are softer. Most pans occur naturally, but some cultural practices can create them. For example, repeated core aeration at the same depth can create a pan layer of highly compacted soil just below the depth of tine penetration.
Pans all cause serious drainage problems in landscapes. They prevent water from draining and so create perched water tables. This not only saturates soil, it also causes salt buildup because salt cannot leach out of the soil. Even if you’re able to manage irrigation well enough to prevent these problems, pans still effectively create a “bottom” to the soil, which may be quite shallow. This can restrict the rooting depth of trees and shrubs.
Soil chemistry is the interaction of various chemical constituents that takes place among soil particles and in the soil solution—the water retained by soil. The chemical interactions that occur in soil are highly complex, but understanding certain basic concepts will better help you manage turf and ornamentals.
• Nutrition. Having discussed water relations, it now is a bit simpler to discuss nutrient-holding capacity. Soils hold onto nutritional elements in a way similar to how they retain water: Positively charged nutrient molecules, cations, are attracted to the negative charges on the soil particles. This is called adsorption. The sites where cations attach to particles are cation-exchange sites (see Figure 2, left). Thus, clay retains more nutrients than coarser soils, just as it holds more water, because of the greater surface area (greater number of cation-exchange sites) to which nutrients can adsorb. The ability to hold cation nutrients is called the cation-exchange capacity (CEC) and is an important characteristic of soils in that it relates to a soil’s ability to retain nutrients and prevent nutrient leaching. Coarse soils have low CECs, while clays and highly organic soils have high CECs. A sand may have a CEC of under 10—a very low figure. Any CEC above 50 is high, and such soils should be able to hold ample nutrients.
• Salinity. Some soils, particularly in arid regions, hold high levels of salt. We discussed earlier how clay soils are more prone to salt buildup, and the same principle applies to arid-region soils. Low rainfall prevents leaching of salts, so they build up in soils. Pan layers, common in arid regions, further inhibit drainage and leaching. Some fertilizers and amendments also can increase salinity.
• Soil pH. This is perhaps the single most important aspect of soil chemistry. Strictly speaking, soil pH, or reaction, is a measure of the number of hydrogen ions (H+) present in a solution. In more common terms, it is a measure of alkalinity and acidity. The pH scale runs from 0 to 14. Seven is neutral, 0 is the most highly acidic value possible, and 14 is the most alkaline, or basic, value. Most plants grow best in the range of 6.5 to 7.0, which is acidic, but only slightly. The so-called acid-loving plants prefer lower pH, in the range of 4.0 to 6.0. Under 4.0, few plants are able to survive. Slightly alkaline soil is not harmful to most plants (except acid lovers). In strongly alkaline soils, however, nutrient-availability problems related to pH result.
The parent material of soils initially influences soil pH. For example, granite-based soils are acidic and limestone-based soils are alkaline. However, soil pH can change over time. Soils become acidic through natural processes as well as human activities. Rainfall and irrigation control the pH of most soils. In humid climates, such as the Northeastern United States, heavy rainfall percolates through the soil. When it does, it leaches basic ions such as calcium and magnesium and replaces them with acidic ions such as hydrogen and aluminum. In arid regions of the country (less than 20 inches of rain per year), soils tend to become alkaline. Rainfall is not heavy enough to leach basic ions from soils in these areas.
Other natural processes that increase soil acidity include root growth and decay of organic matter by soil microorganisms. Whereas the decay of organic matter gradually will increase acidity, adding sources of organic matter with high pH values (such as some manures and composts) can raise soil pH.
Human activities that increase soil acidity include fertilization with ammonium-containing fertilizers and production of industrial by-products such as sulfur dioxide and nitric acid, which ultimately enter the soil via rainfall. Irrigating with water high in bicarbonates gradually increases soil pH and can lead to alkaline conditions.
In most cases, changes in soil pH—whether natural processes or human activities cause them—occur slowly. This is due to the tremendous buffering capacity (resistance to change in pH) of most mineral soils. An exception to this is high-sand-content soils, where buffering tends to be low, as we’ll discuss below.
Nutrient availability varies markedly according to pH. This, in fact, is the main reason why pH is so critical. The best pH for overall nutrient availability is around 6.5, which is one reason why this is an optimal pH for most plants.
Calcium, magnesium and potassium are cation nutrients, meaning they are available to plants in a form with a positive charge. As we discussed earlier, these nutrients adsorb to soil particles, especially clay particles. Soils high in clay or organic matter have high CECs. Thus, these soils act as reservoirs for these nutrients and plants growing in them seldom are deficient in the cation nutrients.
Cations do not adsorb permanently to particles. Other compounds that are more strongly attracted to the cation-exchange sites can replace them. This is one way that pH affects nutrient availability. Low-pH soils, by definition, have many of their cation-exchange sites occupied by H+ ions. By default, exchange sites holding H+ ions cannot hold other cations. Therefore, low-pH soils are more likely to be deficient in nutrients such as magnesium, calcium or potassium. If cations are not held by particles, they can leach out of the soil.
Soil-solution pH also affects the solubility of other nutrients in the soil. In fact, pH affects the availability of all nutrients one way or another (see Figure 3, above). Therefore, maintaining pH close to the ideal level—6.0 to 7.0 for most plants—is important.
Buffering capacity is the ability of soil to resist changes in pH. Soils with a high buffering capacity require a great deal of amendment to alter pH. This is good if the soil already has a desirable pH, but it can be a problem if the soil needs pH modification. Normally, soils high in clay or organic matter (those that have high CECs) have high buffering capacities. Calcareous soils often have high buffering capacities because lime effectively neutralizes acid—a great deal of acidification may be necessary to eliminate the lime before you can realize a significant drop in pH. Conversely, in lime-free soils, acid treatment can drop pH significantly. Soils also can resist upward changes in pH, depending on their composition. Because buffering capacity determines how much amendment it will take to change pH, this is an important characteristic. Soil labs determine buffering capacity and adjust their recommendations according to the buffer pH.
Landscape managers commonly manage soils to improve their physical structure. Doing so entails cultivation and, often, the addition of some organic or inorganic amendment.
One of the main reasons we amend and cultivate soil is to alleviate compaction (see “Testing for compaction— bulk density,” below). Thus, it’s appropriate that this discussion should address preventing compaction as the first step in improving soil structure.
Trees commonly suffer from construction activity, which compacts soil to an extent that often can kill the plant. On construction sites, create a zone around trees in which equipment is prohibited. In areas with high foot traffic, take steps to route people along paths that will not affect the root zones of existing ornamentals. The same thing applies to vehicular traffic. Other practices also help reduce or prevent compaction:
Do not cultivate when the soil is wet. This can be a very frustrating situation during wet periods because it seemingly takes forever for soil—especially clay—to dry. However, cultivating soil when it’s wet will only destroy soil structure and cause the formation of blocky, hard clods impossible to break up.
Keep traffic, including foot traffic, off of wet soil—soil compacts more easily when it’s wet.
Improve drainage to speed soil drying and reduce saturation during wet periods.
Apply mulch around trees, as far as the drip line if possible. This will lessen compaction effects on the root zone and improve the soil environment for root growth.
• Physical cultivation. Cultivation can take place in a variety of situations and by several means. The easiest and best time to perform cultivation is before the installation of the landscape or turf.
If pan layers exist in your soil, now is the time to break them up, because it is nearly impossible to do so after the landscape is established. This may require some heavy-duty equipment but is well worth the trouble because pans can cause you no end of problems. Breaking a pan layer may require the use of a deep-ripper implement. If you cannot do this over the entire landscape, at least use augers or some other method of punching through the pan layer in your tree- and shrub-planting holes. Otherwise, the plants will sit in a “bathtub.” If you must dig the planting hole deeper than you normally would to accomplish this, do so. Just be sure to compact the backfill below the root ball to prevent too much settling.
In established landscapes, cultivating soil is a more complex matter. To treat compaction problems around trees, several options exist. Air injection and vertical mulching are techniques finding some use, but they have their drawbacks. A treatment gaining in popularity that provides excellent results for trees growing in compacted soil is soil replacement with radial trenching. This involves digging a trench starting near the trunk and extending it outward to near the drip line. A recent study of this method used trenches that started 10 feet from the trunk of white oaks and radiated outward. The trenches were 10 feet long, 2 feet deep, 14 inches wide and held about 1 cubic yard. The trenches were refilled with amended soil rich in organic matter. These trenches reversed the decline of trees suffering from highly compacted urban soils by providing a favorable soil environment for the tree roots. Such trenches are easy to dig with a variety of equipment (or even by hand) and so represent a viable method of alleviating compaction around existing trees. Any digging around trees should avoid damaging major roots.
Surface mulching around trees also is an effective method of improving soil conditions if the mulch covers a large enough area. Mulch should extend to the drip line if possible. This produces results more slowly but is perhaps the best long-term strategy for alleviating compaction around trees.
Turf-soil amendment is a different matter. The most common method of cultivating turf soil is through core aeration. This method uses hollow tines that pull soil cores from the turf and deposit them on the surface. The resulting holes, though they soon fill in with material, increase air and water penetration to the root zone. In many instances (low- to medium-traffic sites), doing this once or twice a year provides adequate relief from compaction. In high-traffic situations, such as golf courses and athletic fields, turf managers may core-aerate several times a year.
Repeated coring at the same depth gradually can create a compacted soil layer. Deep-tine aeration, using much longer tines, reduces this problem. Drills or water jets also are aeration options that avoid the problem of compacted layers. Many golf-course superintendents use a combination of these aeration techniques.
• Amending soil. Cultivation techniques such as aeration help alleviate compaction created by traffic. Often, however, soil has innate properties that make it difficult to manage. You can improve these soils with amendments that impart more desirable qualities to the soil.
►Organic amendments benefit soils in several ways. They increase nutrient- and water-holding capacities and improve drainage and aeration. In different ways, organic amendments benefit both coarse and fine soils. Because OM increases nutrient and water-holding capacity, it helps counter the drawbacks of sand-based soils. In clay soils, water and nutrient-holding capacities are not usually a concern. However, tilth (the quality that allows you easily to work a soil into a loose state), infiltration and drainage often are poor in clay soils. These, too, benefit from organic matter, as already discussed.
Organic amendments are available in many forms (see table, “Organic amendments,” above right), often as processed wood products. These amendments take some time to decompose to the point where they create actual humus, but they still provide infiltration, drainage and tilth in the meantime. Other common amendments include manure and peat.
Wood-based amendments are infamous for their ability to tie up soil nitrogen. Obviously, this can be a problem and may require the addition of supplemental nitrogen to offset this loss. Manure can contain high salt levels, another problem that may be of concern in your situation. See Chapter 10 for more information on the effects of amendments on soil fertility.
You will do no harm by adding large amounts of organic amendments to soil. Thus, there is little danger of overdoing it. A more common problem is adding too little. Often, amounts greater than 50 percent by volume are necessary to achieve significant modification. If you feel you need a more precise idea of how much to add to achieve the desired changes, have a laboratory test your soil.
►Inorganic amendments can be quite useful for improving soil quality. The main reason to amend soil with inorganic amendments is to improve porosity and thus increase water and air permeability of the soil. Therefore, this discussion pertains mainly to clay soils. The best way to improve porosity with inorganic amendments is with coarse amendments. These consist of particles that range in size from sand to fine gravel. Smaller particles do not increase porosity enough to be useful as amendments. Coarse amendments should be of uniform particle size: amendments with a wide mix of particle sizes tend to pack tightly and reduce porosity rather than increase it.
For amendments to be effective, the amendment particles must bridge. That is, they must touch each other so that they create large pore spaces in between. This can require between 50 and 80 percent amendment by volume. Small amounts of amendment are not very effective because they are too sparse to bridge with one another.
Sand is the most commonly used inorganic amendment due to its low cost and effectiveness. Calcined clay, perhaps most recognized as cat litter, is another effective coarse amendment that also increases CEC. Other amendments that grounds-care professionals occasionally use include diatomite, zeolite, expanded shale, pumice, blast-furnace slag and sintered fly ash. The latter two materials are by-products that are available on a regional basis. Perlite and vermiculite are materials used primarily in greenhouse and container culture but have disadvantages in landscape use due to their inability to remain intact under traffic.
Gypsum (calcium sulfate) is an amendment professionals often use to increase infiltration in some types of saline soils. Sodium in saline soils destroys good soil structure by causing clay particles to disperse. This dispersion effectively seals soil to water infiltration and percolation. Gypsum (and lime) displaces sodium, causing clay particles to aggregate (clump together) and create large pore spaces through which water can flow. The displaced sodium is then free to leach through the root zone (with enough water).
Incorporating amendments—organic or inorganic—is simply a matter of tilling the material into the soil after you’ve spread it on the surface. Don’t confuse the term amendment with mulch. Mulch refers to material that remains on the soil surface. Mulches can improve soil by reducing compaction, conserving moisture and decomposing to increase OM in the surface layer of soil. However, by definition, they are not amendments.
You can amend soil in existing turf by core aeration followed by topdressing that you drag into coring holes. This type of soil replacement is not difficult but requires some time—perhaps a year or two depending on frequency of aeration—to achieve significant replacement of soil.
►Topsoil. Many times, it is simply more efficient to bring in high-quality soil than to modify the poor soils already present on a site. Though this use of topsoil does not, strictly speaking, make it an amendment, the idea is the same: Provide a good soil environment for plant growth. Topsoil for sale often is actually loam. It may be of excellent quality, but it is a misnomer to call it topsoil. Of course, it is wise to inspect topsoil before purchasing it to ensure it’s of the quality you’re looking for. Ideally, the soil should be reasonably weed-free and should not contain too many large clods.
If the difference between the topsoil and the site soil is great—as it usually is—till a shallow layer of the topsoil into the top few inches of site soil. This will create a transition zone that will aid water movement and root growth between the two soils.
After improving porosity, changing pH is the most common reason for altering soils. Raising and lowering pH both are necessary at times, depending, of course, on the pH with which you’re starting.
• Reducing acidity. Liming is the practice of applying an agent to reduce soil acidity (raise pH) and make soils more favorable for plant growth. The amount of lime you must add depends on the degree of soil acidity, the buffering capacity of the soil, the desired pH, and the quality and type of lime you use.
►Liming materials. The most widely used liming materials for turfgrass areas consist of carbonates of calcium or magnesium. These include ground, pelletized and flowable limestone. Of these three, ground limestone is the type used most widely. Crystalline calcium carbonate (CaCO3), one type of ground limestone, is termed calcitic limestone. Dolomitic limestone, another ground-limestone product, comes from ground rock containing calcium-magnesium carbonate (CaMg[CO3]2) and has a higher neutralizing value than calcitic limestone. Dolomitic limestone not only lowers pH but also can supply magnesium in soils that are deficient. Although ground limestone is the most inexpensive source, it is dusty and not as easy to spread as the pelletized form.
Pelletized limestone is ground limestone (either calcitic or dolomitic) that has been aggregated into larger particles to facilitate spreading and reduce dust. The pellets quickly disintegrate when wet.
Flowable limestone is available for use on turf when you need to use a liquid application. Although liquid applications are dust-free and uniform, you only can apply relatively small amounts at one time, and lime-spray suspensions may be abrasive to sprayer parts.
Hydrated (slaked) lime [calcium hydroxide, Ca(OH)2] and burned lime (quicklime—calcium oxide, CaO) provide a rapid pH change but can be phytotoxic. These products are corrosive and difficult to handle.
As you might expect, sources of limestone vary in quality and effectiveness. Even two pelletized limestones made by different companies may vary in their ability to neutralize soil. To get the best bargain when purchasing lime products, look for quality, not just the lowest price. Two main factors govern the quality of a liming material: purity and fineness.
► Purity. Most lime recommendations assume you will use liming materials that have the same neutralizing potential as pure calcium carbonate. In other words, if your soil-test report recommends that you apply 50 pounds of limestone per 1,000 square feet, it assumes you will use a lime source that will raise soil pH to the same extent as 50 pounds of pure calcium carbonate at the same rate. A liming material with the same neutralizing potential as pure calcium carbonate has a calcium carbonate equivalent (CCE) of 100 percent.
You should adjust the recommended rate of any liming material with a CCE of less than or greater than 100 percent (see “CCE of liming materials,” above) so that you apply the right amount of material to raise your soil pH to the target level (see “Calcium carbonate equivalent [CCE] and liming rates,” page 14). Generally, because of impurities such as clay, the neutralizing value of most agricultural limestones is 90 to 98 percent. Most states require that agricultural liming materials state their CCE on the label.
►Fineness. Any effective liming material should be finely ground. This is important because the rate at which limestone raises pH increases with the fineness of the particles. Plus, limestone affects only the small volume of soil surrounding each limestone particle. A given volume of limestone contains more particles if it is finely ground and thus affects more soil than coarser limestone. Many states govern the sizes of limestone particles in pelletized lime and agricultural ground limestone. Manufacturers usually print the actual range of particle sizes on the label. However, you will generally find little advantage in using material much finer than these minimum standards.
►How and when to apply limestone. Lime will neutralize soil acidity and benefit turf growth faster if you incorporate it directly into the soil. You can incorporate lime by spreading a layer on the soil surface following a rough grading, then mixing the lime 4 to 6 inches into the soil with rotary tilling equipment. Not only does this practice distribute the lime throughout the entire root zone, you can apply much more in a single application than with a surface application. Often, you can supply the entire lime requirement in a single application during establishment, whereas several surface applications may be necessary on established turf or landscape beds.
A means of incorporating lime in established turf is through core aeration. If your soil-test report indicates that an area about to undergo renovation requires liming, apply the recommended amount of lime (along with any needed phosphorus and potassium) after herbicide treatment and thatch removal, and just before or just after aeration. As you aerate and drag the area, some of the lime/soil mix will fall into the aeration holes and some will remain on the soil surface. The more vigorous the aeration treatment the better the lime will mix with the soil.
Established turfgrass areas should not receive more than 25 to 50 pounds of limestone per 1,000 square feet in any single surface application. If you use hydrated or burned lime, apply no more that 25 pounds per 1,000 square feet in a single application. The main reason for this is to ensure that a layer of excess residue does not remain on or near the surface after watering or, in the case of hydrated or burned lime, that plant injury does not occur. If a soil requires more limestone than you can apply at one time, use semiannual applications until you meet the requirement.
Ground limestone sometimes is difficult to spread with conventional spreaders. However, pelletized limestone spreads easily with conventional drop or broadcast spreaders. For large areas, commercial spreader trucks are available for custom spreading. You can apply ground limestone anytime during the year, but it is most effective in the fall or winter because rain, snow and frost heaving help work limestone into the soil.
• Lowering soil pH—acidification. Soils often need acidification in semiarid and arid regions or when you’ve applied excess lime. Plus, golf-course superintendents sometimes apply acidifying materials to their greens as a means of managing certain diseases. They accomplish this by applying ammonium-containing fertilizers such as ammonium sulfate or elemental sulfur, or by injecting sulfuric acid into their irrigation systems.
Ammonium-containing fertilizers are effective for lowering soil pH when you need only slight acidification over an extended period. In the Northeastern United States, some golf-course superintendents use ammonium sulfate to lower the pH of putting greens affected by take-all patch and summer patch diseases. While this practice is effective in some cases, take care to avoid foliar burn and over-stimulation of turf with nitrogen. To avoid burning, make the applications during cool weather (spring and fall) at low rates. When using this approach for disease management, you should monitor soil-pH levels frequently to avoid nutrition and thatch problems caused by low pH.
If you require greater and more rapid acidification, you can use high-sulfur-content products. When you apply sulfur to soil, soil-borne bacteria convert it to sulfuric acid, thereby lowering soil pH. Powdered elemental sulfur typically is yellow and fairly pure (greater than 90 percent sulfur). As with lime, sulfur is more effective in a finely ground state. Several sulfur products are available in powder form but, as such, are dusty and not easy to apply with spreaders. You also can obtain sulfur in pelletized form (90 percent powdered sulfur and 10 percent bentonite clay). This is easy to spread with conventional fertilizer spreaders and quickly breaks down into the powdered form when moist. If you want to apply sulfur as a liquid, flowable forms also are available.
The best time to apply sulfur is before establishment. By applying sulfur directly to the soil surface, and then tilling it into the soil, sulfur will be in direct contact with soil microbes and distributed throughout the entire root zone. Incorporating sulfur before planting also allows you to use greater amounts than possible with surface applications on established turf.
Generally, sandy soils require smaller amounts of sulfur to lower pH than mineral soils. For example, lowering the pH of a 6-inch-deep layer of sandy soil from 8.0 to 6.5 requires 27.5 pounds of sulfur per 1,000 square feet. However, a clay soil needs 45.9 pounds of sulfur per 1,000 square feet for the same adjustment.
Established turf generally requires frequent applications of sulfur at relatively low rates to lower pH. On putting greens, applications normally are around 0.5 pound sulfur per 1,000 square feet and should not exceed about 2.3 pounds per 1,000 square feet per year. You can double these rates on high-cut turf if you apply the product in cool weather. Remember, excessive sulfur can injure turf, especially in hot and humid weather.
To determine if your sulfur applications are having the desired effect on pH, monitor your soil with laboratory tests. Make sure that you test the surface soil (upper 0.5 to 1 inch) separately because most of the sulfur you apply to established turf will remain and react near the soil surface. This possibly can create highly acidic conditions in the top 0.5 to 1 inch of the soil.
In recent years, some golf courses in the Southwestern United States have used sulfuric-acid irrigation-system injections to acidify soil. At least one system uses pH electrodes and a computer to maintain water pH at a constant 6.5. If the pH falls outside the operating range, the system automatically shuts down. With innovations such as these, acidification of soils with acid injection undoubtedly will become more common in the near future.
You can determine soil pH with one of several types of soil tests. However, not all soil tests provide accurate information about how much lime or acidifier you should apply. Test kits using dyes, pH pens or pH paper determine pH rapidly in the field. The least accurate means of determining soil pH is with pH paper, but it can be useful in obtaining an approximate value. While each of these tests can provide a fair indication of soil pH and tell you if you need lime, they do not provide accurate information on how much lime you should apply. The table at left gives amounts of material needed to raise and lower pH. These figures are only approximate—consult a soil lab before undertaking pH modifications.
Commercial and university testing labs accurately determine pH values for soils over a range of pH values. They also provide meaningful lime recommendations for acid soils. They base their lime recommendation on a lime-requirement test that tells you how much lime is necessary to bring the soil to an optimum pH. The lime-requirement test takes the buffering capacity of a soil into account to provide buffer pH. Regarding pH amendments, buffer pH is more important than active pH.
Each lab bases its lime recommendations on what they consider to be optimal pH for the turf or ornamentals you’re growing. Before submitting your soil samples, realize that differences exist among labs regarding what they consider to be the optimum pH ranges for turfgrasses and ornamentals. This is why lime recommendations vary from one lab to another. The best way to deal with this problem is to choose a lab that provides recommendations that make sense to you and then stick with that lab for future testing to maintain consistency.
TESTING, SAMPLING AND SOIL LABORATORIES
Soil laboratories are necessary to provide accurate analysis and meaningful recommendations. Many kits and test methods—some of which we mentioned earlier—are available that allow you to conduct crude analyses for various nutrients, as well as pH, texture, density and other factors. However, you should consider these only rough indicators of soil quality. A laboratory analysis is necessary for you to get a good grasp of your soil’s condition.
For small landscapes, the cost of testing may not be justified unless serious problems are occurring. However, for larger landscapes and golf courses, the cost of testing is trivial compared to the benefits. The information labs provide allows you to take the appropriate management steps to maximize plant growth. Otherwise, you’re just guessing at how much and what type of material to apply if you wish to amend soils.
The results from any kit or lab are only as good as the sample taken. Therefore, ensure that you follow instructions on the soil-test form. Pay particular attention to the suggested number of subsamples per unit area, sampling pattern, sampling depth, mixing procedure and whether to include thatch as part of the sample. Take care not to contaminate the sample with fertilizer, lime or any other substance that may influence results.
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