Liming is the practice of applying an agent to reduce soil acidity (raise pH) and make soils more favorable for turfgrass growth. Raising soil pH requires a quantity of liming material that is determined by the degree of soil acidity as well as the quality and type of liming material. Soil acidity is determined by a soil test, however, not all soil tests provide accurate information on how much lime should be applied. Most university and commercial laboratories will provide sound recommendations of how much lime needs to be applied to turfgrass areas.
What is soil pH?
All soils can be classified as acid, neutral, or alkaline. The degree of acidity or alkalinity of a soil can be described by a pH value. pH values range from 0 to 14. Any value below 7.0 is considered acid, a value of 7.0 is neutral, and a pH above 7 is alkaline. Thus, a soil with a pH of 5.8 is acidic, whereas a soil with a pH of 7.9 is alkaline.
Soils become more acid as pH values below 7.0 decrease numerically (see Fig. 1). In fact, there is a ten-fold increase in acidity for every decrease by one whole pH unit. For example, a soil with a pH of 5.5 is ten times more acid than a soil with a pH 6.5, and a soil with pH of 4.5 is 100 times more acid than the soil with a pH of 6.5. Similarly, for every whole unit increase in pH above 7, there is a ten-fold increase in alkalinity. pH values are usually reported to the nearest 1/10th of a whole unit on a soil test report (ie. 6.2, 7.0, 8.5).
Many plant species show a preference in regard to soil pH. Whereas rhododendrons and azaleas prefer soils in the range of pH 5.2 to 5.5, cool-season turfgrasses usually grow best in soils ranging from 6.0 to 7.2. Kentucky bluegrass, the most widely used cool-season turfgrass in Pennsylvania, grows best when soil pH is between 6.5 and 7.2. Fine fescues, bentgrasses, turf-type perennial ryegrasses, and turf-type tall fescues are somewhat more tolerant of slightly acid soils (6.0 to 6.5) than Kentucky bluegrass.
Figure. 1 The pH scale runs from 0 (most acid) to 14.0 (most alkaline). The pH values for a number of common substances are shown. Note that soil pH range extends from approximately 3.5 to 10.5.
Why do soils become acid?
Soils become acid through natural processes and human activities. The pH of most soils is controlled by the amount of rainfall. In humid areas, such as the northeastern United States, rainfall percolates through the soil, leaching ions such as calcium and magnesium which prevent the soil from becoming more acid and replacing them with acidic ions such as hydrogen and aluminum. Other natural processes that increase soil acidity include root growth and decay of organic matter by soil microorganisms.
Human activities that increase soil acidity include fertilization with ammonium-containing fertilizers and pollution of industrial by-products such as sulfur dioxide and nitric acid which enter the soil from acid rain. In most cases changes in soil pH, whether they are caused by natural processes or human activities, occur very slowly. This is the result of the tremendous buffering capacity (resistance to change in pH) of most mineral soils.
Why is liming important?
Soil pH affects turfgrass health by influencing availability of plant nutrients and other elements, thatch decomposition, some turfgrass pests, and pesticide activity.
Strongly acid soils (pH less than or equal to 5.5) may lead to deficiencies in calcium, magnesium, or phosphorus and increase availability of aluminum and manganese in amounts that may be toxic to turfgrasses. Liming improves plant nutrient availability and reduces toxicity problems in acid soils.
In strongly alkaline soils (pH greater than or equal to 8.5), the formation of insoluble tricalcium phosphate makes the phosphorus unavailable to the plant. Iron chlorosis, an indication of iron deficiency, may be seen on some plants growing in soils high in pH. Since most soils in Pennsylvania are not strongly alkaline, these types of deficiencies are not often encountered. Exceptions may occur when too much lime is applied to established turf or to the soil prior to planting.
Many beneficial soil microorganisms do not thrive in strongly acid soils. Some of these microorganisms break-down certain nitrogen fertilizers, thereby releasing the nitrogen for use by the turfgrass. Fertilizers containing nitrogen from ureaform, sulfur-coated urea, or natural organic sources are not effective unless certain microorganisms are present in sufficient quantities.
Soil microorganisms also aid in the decomposition of thatch and grass clippings. Thatch is the dense accumulation of organic material on the soil surface beneath the grass. A thatch layer restricts movement of air, water, nutrients, and pesticides into the soil. Soil pH in the range of 6.0 to 7.0 increases microbial activity and helps reduce thatch.
Some turfgrass diseases are influenced by soil pH. Although the reasons for this are not well understood, there is some evidence to suggest that in very acid soils the populations of microorganisms that suppress pathogenic fungi are reduced. In addition, plants growing in acid soils may be more susceptible to disease because they are suffering from nutrient deficiencies or aluminum toxicity. Conversely, there are at least two turfgrass diseases (take-all patch and Fusarium patch) that are suppressed in acid soils. Fortunately, these diseases rarely cause problems in home lawns. Optimum pH (6.0 to 7.0) does not prevent turfgrass disease, but it can reduce the severity of infestation.
Acidic soils create conditions that favor growth of certain weed species. One of the most common and difficult-to-control weeds, moss, in more prevalent in moderately to strongly acidic soils than in neutral soils or slightly acid soils. Shepherds purse is a lawn weed that is a good indicator of moderately to strongly acidic soils. Although weeds cannot be controlled with lime applications, applying lime before soils become too acid is one means of preventing severe weed infestation.
Research has shown that the activity of some pesticides is influenced by soil pH. Strongly acid soils can reduce the effectiveness of some turfgrass herbicides and insecticides.
Is Lime Needed?
Liming is needed if the pH of your soil is too low for optimum growth of the turf species you want to maintain. Soil pH can be determined only by a soil test. Although home test kits can provide a fairly good indication of soil pH, they cannot provide meaningful liming recommendations. Kits for collecting and mailing soil samples to the Agricultural Analytical Services Laboratory at Penn State are available from all county Penn State Extension offices. Most commercial soil test laboratories also can determine soil pH and provide lime recommendations for turf. Test results showing pH and nutrient status of your soil sample and recommendations for limestone and fertilizer applications (if needed) will be mailed to you. Apply limestone only if your soil test results show a need for it, and never guess at the amount of limestone needed.
What liming material should be used?
Agricultural ground limestone is the most widely used liming material for turfgrass areas. Manufactured by grinding rock containing high concentrations of calcium carbonate into fine particles, it can be purchased at most garden supply stores, hardware stores, farm supply stores, and many supermarkets. Dolomitic limestone is manufactured by grinding rock containing calcium carbonate and magnesium carbonate and is recommended where a soil test shows low pH and deficient levels of soil magnesium.
Pelletized limestone is ground agricultural limestone, calcitic or dolomitic, that has been aggregated into larger particles to facilitate spreading and to reduce dust. The aggregates are bound together by a water-soluble substance that dissolves quickly when wet.
Other liming materials, such as hydrated lime (slaked lime), burned lime (quicklime), marl, shells, and blast-furnace slag are not recommended for turfgrass areas. Gypsum is not a liming material.
How much limestone should be applied?
A lime requirement test is conducted on all turf soil samples sent to Penn State's Agricultural and Analytical Services Laboratory. Results of this test will indicate the total amount of limestone needed to raise the soil pH to the optimum level for your turf.
Established lawns, athletic fields, cemeteries, golf course fairways, and other general turfgrass areas should not receive more than 100 pounds of limestone per 1,000 square feet in any single application. Golf course greens should receive no more than 25 pounds per 1,000 square feet per application. If a soil requires more limestone than can be applied in a single application, use semiannual applications until the requirement is met.
When establishing new turfgrass, the total limestone requirement may be applied in a single application if thoroughly worked into the first four to six inches of soil prior to seeding.
As you might expect, sources of limestone vary in quality and hence, effectiveness. The quality of a liming material is governed by two main factors; purity and fineness.
The lime recommendation on the Penn State Agricultural Analytical Services Laboratory soil test report is based on the use of a liming material that has the same neutralizing potential as pure calcium carbonate. To put it another way, if your Penn State soil test report recommends that you apply 50 lb limestone/1000 sq ft, it assumes that you will use a lime source that, at 50 lb/1000 sq ft, will raise the soil pH to the same extent as 50 lb of pure calcium carbonate/1000 sq ft. A liming material that has the same neutralizing potential as pure calcium carbonate has what agronomists call a 'calcium carbonate equivalent' (or CCE) of 100 percent.
The recommended amount of any liming material that is not equivalent to pure calcium carbonate in neutralizing potential (100 percent CCE) should be adjusted so that you apply enough liming material to raise your soil pH to the desired level. All agricultural liming materials sold in Pennsylvania are required by law to be labeled with their calcium carbonate equivalent (CCE). Using the CCE of your liming material, the actual amount required to neutralize the acidity in your soil can be calculated as shown below or read directly from the liming material conversion table.
Actual liming material required = (soil test recommendation/CCE of liming material) x 100
Soil test recommendation: Apply 75 lb limestone/1000 sq ft
Liming material label: CCE = 80 percent
Actual liming material required: (75/80) x 100 = 94 lb liming material/1000 sq ft
As you can see by the above example, you need approximately 20 additional pounds of a liming material with a CCE of 80% to raise the soil pH to the same extent as a material with a CCE of 100%.
A liming material must be finely ground to be effective. This is important because (1) limestone's solubility increases as it is ground finer, and (2) limestone affects only a very small volume of soil around each particle. Therefore, the finer limestone is ground, the more particles it has, and if there is adequate mixing, more of the soil comes in close contact with limestone and thus is neutralized.
Pennsylvania law requires that limestone used to make pelletized lime, agricultural ground limestone, and industrial by-products meet the following minimum standards:
95% through a 20-mesh per inch screen
60% through a 60 mesh per inch screen
50% through a 100 mesh per inch screen
Granular limestone (lawn and garden limestone) must meet the following minimum standards:
95% through a 20-mesh per inch screen
40% through a 60 mesh per inch screen
30% through a 100 mesh per inch screen
A liming material meeting these minimums is considered adequate in most situations. The actual range of particle sizes must be printed on the label. The calculations and table for adjusting your recommendations for the CCE of your liming material assume that the material meets at least these minimum fineness standards. In selecting a liming material, there is generally little advantage in using material much finer than the minimum standards.
How should limestone be applied?
Ground agricultural limestone is sometimes difficult to spread with a conventional drop-type fertilizer spreader because the finely ground material tends to bridge over the spreader outlets. Spinner-type fertilizer spreaders can, with frequent stirring in the hopper, be used to apply limestone. On small areas, limestone can be applied by hand. For very large areas, commercial spreader trucks are available for custom spreading.
Pelletized limestone is easily spread with conventional drop or spinner spreaders.
When should limestone be applied?
Ground limestone may be applied anytime of the year, but is most effective when applied in the fall. Rain, snow, and heaving of the soil during winter help work the limestone into the soil. Movement of limestone into the soil is slow, even under the best of conditions.
When the total amount of limestone needed for turfgrass maintenance exceeds the amount suggested for a maximum single application (25 lb/1,000 sq ft) on greens; 100 lb/1,000 sq ft on all other established turf areas), spring and fall applications at the maximum rate are suggested until the limestone need is met.
When putting in new seedings, all limestone required may be applied before seeding, but it must be mixed thoroughly throughout the top four to six inches of soil.
- Do not use hydrated lime or burned lime on established turfgrass or turfgrass seedbeds at any time. Both can cause turfgrass burning; they also stick to shoes and can be tracked into homes or other buildings.
- Do not lime unless a lime requirement test shows that limestone is needed.
- Do not expect maximum fertilizer response if the soil is strongly acid.
- Apply the total amount of limestone recommended by the lime requirement test, and take care not to exceed the maximum recommended amount per application.
- Apply ground agricultural limestone when the air is calm. Blowing dust may reach places where it is not wanted, and limestone that blows away is wasted.
- Keep limestone away from plants such as azaleas, rhododendrons, and blueberries that flourish in acid soil.
Liming material conversion table
Using the conversion table: Find your soil test limestone recommendation value in the left hand column of the table and then read across that line until you come to the column headed by the % CCE nearest to that of your liming material. The number at that point is the pounds of liming material required to meet the limestone recommendation on your soil test.
For example: If it is suggested that you apply 100 pounds CCE per 1,000 sq. ft. and the liming material you plan to use contains 85% CCE you would need 118 pounds of the material you chose to obtain sufficient neutralizing power in your soil.
Because there is no advantage gained by applying more than 100 pounds of CCE per 1,000 sq. ft. in any one application, the table is divided into three sections suggesting how the total liming material required may be split for most efficient use. (See the right hand column).
|Pounds / 1,000 ft² of CCE recommended on your soil test.||Percentage calcium carbonate equivalent (%CCE) of your liming material||Divide total CCE by the following number of applications|
This publication replaces Special Circular 167 "Liming Turfgrass Areas".
Revised by Peter J. Landschoot, Associate Professor of Turfgrass Science, from Special Circular 167 "Liming Turfgrass Areas" by John C. Harper II, professor emeritus of agronomy.