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H. L Mencken wrote, "All life is a struggle...against the hydrogen ion." Life does involve balance between acids and bases, with both acids and bases playing key roles in keeping living things alive. The hydrogen ion is also important in the soil. It enables plant uptake of nutrients, respiration of roots, and the decomposition of organic matter. However, it can limit nutrients on soil cation sites. We need some but not too much.

Sometimes a water molecule receives another proton. It then has a positive electrical charge. This is the hydronium ion (H3O+), which is how the hydrogen ion (H+) exists in water. When a water molecule loses a proton, it then has a negative charge. This is the hydroxyl ion (OH-). If a solution has more H+ than OH-, it is acidic (pH less than 7). If it has more OH- than H+, it is basic (pH more than 7). Most turfgrasses grow best when the soil solution is slightly acidic, but they tolerate a wide range of pH.

Preferred pH ranges of selected grasses (Robert Emmons and Frank Rossi, Turfgrass Science and Management, Cengage Learning, 2015)
Bermuda 5.7-7.0
Fine fescue 5.5-6.8
Kentucky Bluegrass 6.0-7.0
Perennial ryegrass 6.0-7.0
Tall fescue 5.5-7.0
Zoysia 5.5-7.5

Grass with a pH under 5.5 is subject to aluminum and manganese toxicity, which stunts roots. Toxicity can be a serious problem if the pH is less than 5.0. Some micronutrients are less available when pH is high: iron, manganese, copper, zinc. Boron is less available in moderately alkaline soil. Molybdenum is less available when pH is low. In alkaline soil phosphorus tends to bind with calcium, making phosphorus less available to the plant. It's there but in a less soluble form. In acidic soil phosphorus tends to bind with iron and aluminium. Phosphorus is most available between 6.0-7.0. All nutrients are most available when the pH is between 6.0 and 7.0.

Microorganisms in the soil function best in a neutral pH soil. Pesticides and herbicides are less effective in acidic soil. Some weeds favor acidic soil. Turfgrass diseases are affected by pH in various ways.

Where does acidity come from?
Aluminum. Most acidic soils have a lot of soluble aluminium. When aluminium hydrolyzes in soil water, H+ is released.
Rainfall. Rain is slightly acidic (around 5.6). Acid rain is even more acidic (<5.0). Rain also leaches nutrients.
Crop uptake. As plants take in nutrients, they release H+ at the root.
Fertilizer. As NH4+ (ammonium) or NH3 (ammonia) transforms to NO3- (nitrate), it releases H+.
Root respiration (including respiration of microbes, fungi, soil animals) and decay of plant residues.

H+ will cling to the negatively charged cation sites in the soil, preventing other cations like calcium, magnesium, potassium, and ammonium from finding a place. If cations don't find a site and they are not taken up by the plant, they will leach away. The clear advantage of high CEC (cation exchange capacity) soils is they can hold more nutrients, like having a full pantry available for the plants. But if those pantry shelves are filled with aluminium and hydrogen, plants can't get the nutrients they need.

Once excessive hydrogen and aluminium have been neutralized by lime, it not only allows the cation sites they occupied to be filled with nutrients, but in many soils it also opens up additional pH dependent cation sites, enabling the soil to hold onto nutrients better. Adding organic matter over many years is another way to increase cation sites. CEC is an inherent characteristic of the soil, not something we can do much about. Freeing up the pH dependent sites is an important benefit of liming acidic soil.

Addition of calcium and magnesium in the form of lime will raise pH and help fulfill nutrient deficits. Addition of calcium and magnesium in other forms like gypsum or epsom salts will help fulfill nutrient deficits but won't raise pH. Addition of potassium sulfate or potassium chloride will help fulfill a potassium deficit but won't raise pH. To raise pH you must use a liming agent. The hydrogen ion is neutralized by the carbonate action of calcium carbonate or magnesium carbonate, not the calcium or magnesium.

A soil test that includes a buffer test will give an estimate of the lime needed to raise pH to a target pH. If the report gives lb/ac, divide by 43.56 to get pounds per 1000 square feet (lb/k). Find out the calcium carbonate equivalent (CCE) that the lab assumes in making its recommendation. VA Tech's lab assumes a CCE of 100%. You can then adjust the amounts if the product you buy has more or less CCE than what the lab assumes in its recommendation. Divide the CCE assumed by the lab by the CCE of the product and multiply by the lab recommended rate.

Lime that is ground finely reacts faster. The higher the fineness number, the faster it will change the soil pH. Particles passing through a 100 mesh sieve will react within weeks, those passing through a 60 mesh will react in 1-2 years, those passing through a 20 mesh will react in 2-3 years. Virginia law specifies that at least 30% must pass through 100 mesh, 50% pass through 60 mesh, and 90% pass through 20 mesh. Thus some lime particles react quickly and some will react slowly, continuing to work for a few years.

Aglime can be dolomitic or calcitic. It can be pulverized or pelletized. Dolomitic lime contains both calcium carbonate and magnesium carbonate. Calcitic lime contains mostly calcium carbonate. If the soil is low in magnesium, using dolomitic lime will add needed magnesium. If the soil has sufficient magnesium, either kind is fine. If the soil has high levels of magnesium, it's better to use calcitic lime. Magnesium tends to tighten a soil, so using dolomitic instead of calcitic lime would help give more structure to a sandy soil (CEC<5). Conversely, a clay soil (CEC >30) or even a clay loam (CEC>15) would benefit from more calcium and less magnesium to help flocculation, assuming it has sufficient magnesium. Pulverized lime contacts more soil particles. However, pelletized lime is much easier to spread. Use the carbonate forms of lime (calcium carbonate, magnesium carbonate). The oxide and hydroxide forms can burn the lawn and can be dangerous to apply.

Retailers carry various "fast-acting" calcitic lime products. These have been ground very finely and will react quickly. Only a limited amount can be used per application. It may be claimed that each pound of the product can be as effective as 4-5 pounds of aglime. However, a pound of fast-acting lime with a CCE of 95% will not neutralize any more acidity than an aglime with the same CCE. It just works faster. Whatever the lime requirement, that whole amount is needed, whether it comes from fast-acting lime or aglime or a combination.

Lime is most effective when it is mixed into the top six inches of soil before a lawn is established. For established lawns, applications are broadcast on top of the ground. For applications on the surface, don't apply more than 50 lb/k of aglime at a time or more than 100 lb/k a year. If a fast-acting lime is used, follow the instructions on the bag. If aeration is planned, take advantage of aeration's open holes to make a lime application then.

Lime can be applied anytime the ground is not frozen. Fall is a good time so the lime can work into the soil during freeze/thaw cycles over the winter. If a large amount of lime is needed, some can be applied now, some later. Two or three applications a year can be made until the recommendation is met. It is best to apply lime well in advance of seeding. Avoid applying lime when the grass is stressed (drought, heat, disease). It's better to separate lime and fertilizer applications by two weeks or more.

Figure out your square footage so you can apply the correct amount. For example, if using 50 lb/k rate and your yard is 5,000 square feet (5k), 50 lb/k x 5 k = 250 lb.Wear shoes, long sleeves and pants, and gloves as lime can irritate the skin. If using pulverized lime, add a dust mask and googles and choose a calm day. Plan to apply the lime in at least two passes, going back and forth in one pass, then up and down in the next, so that coverage will be even. If the bag has spreader suggestions for spreading in one pass, choose the setting that is half of the suggested one. Make one pass and see how much you've applied and how much is left to apply. You can make adjustments to the setting after each pass. If the bag has no suggestions for spreader settings, open the spreader about ⅔ open for a broadcast spreader, ⅓ open for a drop spreader, to apply at a 50 lb/k rate. If applying at a lower rate, adjust the opening accordingly. Make adjustments after the first pass. The aim is to apply the lime evenly over the area. Water the lime in to get it off grass blades and to start activating it. Pets and children should stay off the lawn while it's being applied and for a day or two afterwards because lime can irritate the skin. However, it is not toxic.

Aglime will work slowly over the next three years. Low CEC soils (<5) will tend to have more rapid rises and falls in pH, so it's good to test them annually. Other soils are more buffered and can be tested about every three years. Do it in the same season of year and be consistent in the depth of soil tested.

If the soil has a high pH, it is usually not practical to try to lower pH, especially for an established lawn. Sulfur can lower pH but only 5 lb/k can be safely used on an established lawn at a time (no more than two applications a year) and it could take 2-70 lb/k to adjust to 6.5 pH, depending on the type of soil and initial pH, assuming there is no calcareous soil. If the soil is calcareous, each 1% of calcium carbonate in the soil requires an additional 130 lb/k. Ammonium sulfate fertilizer will lower pH slightly but it can burn grass if not applied properly. Adding organic matter is helpful for many reasons and one is that its decomposition will lower pH a little bit. However, if the soil is composed of limestone, it will neutralize any acid added. Instead of trying to lower pH, you can work around problems caused by high pH. Since phosphorus tends to quickly bind up with calcium in alkaline soil, an organic fertilizer with phosphorus can be used so phosphorus is released slowly as microorganisms die. Or a synthetic phosphorus fertilizer can be used, dividing up the soil test requirement for phosphorus into several applications. Iron is often deficient, but foliar sprays of iron or chelated forms will supply it (Milorganite has chelated iron). Other micronutrients can also be less available. Add some if needed.

Land grant colleges offer inexpensive soil testing and private labs also provide the service. A pH meter may not be accurate, and even if it is, it won't tell you how much lime is needed. Adding lime without a soil test can lead to overliming and problems associated with alkaline soil. The only way to know if you need lime and how much is to get a soil test.
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