Managing soil pH in turf
Too often, adjusting soil pH takes a back seat to other maintenance tasks that show more immediate results, such as fertilization and overseeding. However, adjusting the pH of your soil is one of the most important maintenance practices in your turf-management program. If you ignore soil pH for too long, it may fall above or below the optimum range for turf growth. A soil pH that is too high or too low can result in inefficient use of fertilizer by turf, excess thatch build-up and increased pest problems.
Understanding soil pH A number ranging from 0 to 14 describes the soil pH. A value of 7.0 is neutral. Any pH below 7.0 is acidic (decreasing numeric values indicate greater acidity), and any pH above 7.0 is alkaline (with greater values indicating greater alkalinity).
The pH scale is logarithmic, which means acidity increases tenfold for every decrease of one whole pH unit. For example, a soil with a pH of 5.5 is 10 times more acidic than a soil with a pH of 6.5, and a soil with pH of 4.5 is 100 times more acidic than the soil with a pH of 6.5. Similarly, for every whole-unit increase in pH above 7.0, alkalinity increases tenfold. Soil tests usually report pH to the nearest tenth of a whole unit--for example, 7.2 or 6.5.
Different turfgrass species and varieties show preferences regarding soil pH. Kentucky bluegrass grows best when soil pH is between 6.0 and 7.0, whereas fine fescues grow best in the more acidic range of 5.5 to 6.0 (see table, opposite page). Textbooks, soil-test reports and product literature often vary when listing optimum pH ranges for commonly used turfgrass species. This partly is due to the limited research done on pH tolerances of turfgrass species and the variation in pH tolerances among varieties within species.
Why is pH important? Soil pH affects turf health in several ways: * Strongly acidic soils (below pH of 5.5) may result in calcium, magnesium and phosphorus deficiencies in some turfgrass species and increase the availability of aluminum and manganese to levels that may be toxic. Soils with a high pH (greater than 8.5) tend to make phosphorus less available to plants. Also, iron chlorosis, an indication of iron deficiency, occurs more frequently on turf growing in high-pH soils.
* Soil microorganisms aid in the breakdown of some nitrogen fertilizers and thatch. Soil pH in the range of 6.0 to 7.0 usually favors such microbial breakdown. However, most of these beneficial microbes do not thrive in strongly acid soils. Fertilizers that may be less effective or slower-acting in strongly acid soils include ureaform, sulfur-coated urea and natural organic sources.
* Strongly acid soils create conditions that favor the growth of certain weed species. One of the most common and difficult-to-control weeds--moss--is more prevalent in acidic soils than in neutral or alkaline soils. Shepherd's purse is another lawn weed that is a good indicator of acid soils.
* Soil pH influences some turfgrass diseases. Although the reasons for this are not well understood, one possibility is that strongly acidic soils reduce populations of microorganisms that suppress pathogenic fungi. Also, plants growing in strongly acidic soils may be more susceptible to disease because they suffer from nutrient deficiencies. Conversely, acidic soils suppress at least two turfgrass diseases (take-all patch and Fusarium patch). Growing turfgrasses within their optimum pH ranges does not prevent all turfgrass disease, but it can reduce the severity of some infestations.
* Research shows that certain pH ranges reduce the effectiveness of some herbicides and insecticides.
Why are some soils acid and some basic? 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 be 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. This can actually act as a substitute for lime, at least during the first few years of establishment.
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. Another instance of high-pH soil is in golf-course putting greens constructed with sand containing high amounts of calcium carbonate.
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. Thus, you can lower the pH of a high-sand-content putting green faster and with less acidifying material than a green of a silt-loam soil.
Reducing acidity by liming Liming is the practice of applying an agent to reduce soil acidity (raise pH) and make soils more favorable for turfgrass growth. The amount of lime you must add depends on the degree of soil acidity, the desired pH, and the quality and type of lime you use.
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 you should apply. Test kits using dyes, pH pens or pH paper determine pH rapidly in the field. Kits containing dyes provide a fairly accurate pH reading within a certain range, usually 5.0 to 7.2. Leeds and Northrup Instruments makes a pH "pen" for measuring soil pH. According to product literature, the pen has an operating range between 2.0 to 12.0 and is accurate to within 0.1 unit. 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.
Commercial and university test labs accurately determine pH values for many different soils over a range of pH values. They also provide meaningful lime recommendations for acid soils. They base their lime recommendation on a special "lime-requirement" test that tells you how much lime is necessary to bring the soil to an optimum pH for your turf.
Each lab bases its lime recommendations on what they consider to be optimal pH for each turfgrass species or mix of species. Before submitting your soil samples, realize that differences exist among labs regarding what they consider to be the optimum pH ranges for turfgrasses. This is why lime recommendations vary from one lab to another. Researchers at The Pennsylvania State University conducted a survey of seven soil-test laboratories in the United States. The survey revealed that, for identical soil samples, lime recommendations varied from 0 to 165 pounds of lime per 1,000 square feet. Although the procedures the labs used to calculate the lime requirement were similar, the pH values on which labs based their recommendations varied considerably. For example, one lab suggested liming the turf to 5.8, whereas another suggested 6.8--a tenfold difference in acidity. 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.
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 pH.
Which liming material is best? The most widely used liming materials for turfgrass areas consist of carbonates of calcium or magnesium. These include agricultural ground limestone, pelletized limestone and flowable limestone (see photo, page G 16). Of these three, agricultural ground limestone is the type turf managers use most widely. Dolomitic limestone, another ground-limestone product, comes from ground rock containing calcium carbonate and magnesium carbonate and is appropriate where a soil test shows low pH and deficient levels of magnesium. Although agricultural ground limestone is the most inexpensive source--about $3.50 for a 50-pound bag--it is dusty and not as easy to spread as the pelletized form.
Pelletized limestone is ground agricultural limestone (either calcitic or dolomitic) that has been aggregated into larger particles to facilitate spreading and reduce dust. A water-soluble substance that quickly dissolves when wet binds the aggregates together.
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.
Some turf managers occasionally use other liming materials, such as hydrated (slaked) lime and burned lime (quicklime), on turfgrass areas. These products are made from hydroxides and oxides of calcium and magnesium. Although they can raise pH more quickly than carbonate forms, they tend to be caustic and powdery and are not easy to handle. Therefore, applicators must be extremely careful when working with these products. Soluble calcium products and gypsum are not liming materials.
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 100 percent so that you apply enough material to raise your soil pH to the target level (see "Calculating CCE," at right). Most states require that agricultural liming materials state their CCE on the label.
* Fineness. Any effective liming material will 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. In Pennsylvania, for example, limestone used to make pelletized lime should conform to the following sieve sizes:
* 95 percent of the limestone must pass through a 20-mesh-per-inch screen. * 60 percent of the limestone must pass through a 60-mesh-per-inch screen. * 50 percent of the limestone must pass through a 100-mesh-per-inch screen.
A liming material meeting these standards is adequate in most situations. Manufacturers usually print the actual range of particle sizes on the label. The calculations for adjusting your liming recommendations assume that the material meets at least these minimum fineness standards. However, you will generally find little advantage in using material much finer than these minimum standards.
How and when should you apply limestone? Lime will neutralize soil acidity and benefit turf growth faster if you incorporate it directly into the soil. During establishment, 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.
Another means of incorporating lime 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 lawns, athletic fields, cemeteries, golf-course fairways and other turfgrass areas should not receive more than 100 pounds of limestone per 1,000 square feet in any single surface 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. Because of the density and short cutting height of golf-course greens, they should receive no more than 25 pounds per 1,000 square feet per application. If a soil requires more limestone than you can apply at one time, use semi-annual applications until you meet the requirement.
Ground agricultural limestone sometimes is difficult to spread with a conventional drop-type fertilizer spreader because the finely ground material tends to bridge over the spreader outlets, requiring you to stir the hopper. You also can use spinner-type fertilizer spreaders, but again, you must stir the hopper frequently. Pelletized limestone spreads easily with conventional drop or spinner 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 semi-arid 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 or 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 relatively 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-born 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 during 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 seeding or sodding 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. You can obtain more information on rates of sulfur necessary to lower pH from the Western Fertilizer Handbook (1990 Horticulture Edition, Interstate Publishers Inc., Danville, Ohio) and university extension publications.
Established turf generally requires frequent applications of sulfur at relatively low rates to lower pH. On putting greens, applications normally are in the range of 0.5 pound sulfur per 1,000 square feet and should not exceed 10 pounds per 1,000 square feet per year. You can use higher rates (2 to 5 pounds of sulfur per 1,000 square feet) 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, perform periodic soil 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. Like any other new technology, acid injection required some time to evolve. By now, however, manufacturers have addressed problems such as leaks, corrosion and explosions. For example, systems now are available with double-enclosed acid-handling components and non-pressurized systems (reducing the occurrence of ruptured fittings) to help reduce leaks. At least one system uses pH electrodes and a computer to maintain water pH at a constant 6.5. If the pH falls outside of 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.
Dr. Peter Landschoot is associate professor of turfgrass science at The Pennsylvania State University--University Park, Pa.
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