Causes of Low Soil pH with Turf
Low soil pH occurs naturally throughout the Southeast. Many soils that have not been cultivated have pH's in the low 4's. The exceptions are some Coastal soils high in calcium carbonate from sea shells or irrigated with high bicarbonate irrigation water. Sediments dredged from ponds, ditches, and wetlands may have near neutral pH (around 7) when removed, but low pH develops if the sediments remain aerated. Oxidation of sulfur compounds in the sediments can result in soil pH in the low 3's. Over-application of elemental sulfur will also have a similar affect on soil pH.
Although oxidation of reduced sulfur compounds can result in precipitous and rapid declines in soil pH, declining pH in most soils is typically a more gradual process. The primary cause of declining soil pH is the application of ammonium containing or forming fertilizers, such as ammonium sulfate, ammonium nitrate, ammonium phosphate, and urea. When ammonium is converted to nitrate by soil bacteria, acidity is released. Nitrogen sources differ in the amount of acidity they generate. Ammonium sulfate is two to three times more acid than the other commonly used ammonium nitrogen sources. Calcium, potassium, and sodium nitrate do not create any acidity, but actually increase soil pH.
There are several factors that are detrimental to turf growth at low soil pH. Toxic levels of aluminum, hydrogen, and manganese may occur in the soil water. Aluminum and hydrogen damage grass roots directly, whereas manganese is toxic to the leaves and stem of the grass. At low pH, phosphorus availability is reduced and calcium and magnesium levels are typically inadequate. Deficiencies of these nutrients may occur. Microbial activity is also reduced at low soil pH resulting in thatch accumulation and slowed release of nitrogen from organic sources. Dependent on the soil type and grass species some or all of these factors may decrease turf growth.
Grasses vary in their tolerance to acid soils. Bermudagrass, centipedegrass, and seashore paspalum are moderate to highly tolerant to acid soils. Tall fescue, zoysiagrass, and bentgrass are intermediate in tolerance and ryegrass, bluegrass, and St. Augustinegrass have low tolerance. Generally, soil pH should be maintained between 5.5 and 6.5 with values below 5.8 appropriate only for the more tolerant grasses.
Liming turf soils to correct low pH
Maintaining soil pH at favorable levels is a simple but continual process. Soil sample annually and apply lime based on the recommendation. Dolomitic lime is generally preferred because it will provide sufficient magnesium as well as calcium. Both are usually low when soil pH is low. Calcitic lime can be used when soil test shows magnesium to be adequate, but is not necessary because detrimental effects due to extra magnesium are unlikely. Dolomitic lime is the least expensive source of magnesium if soil pH needs to be increased.
Surface-applied lime is slow to react and the affects move downward into the soil profile slowly. To hasten the reaction and movement of the lime, incorporation into the soil by application after core aerification is recommended. When devastatingly low pH's are encountered incorporation is required in order to increase the pH of a significant volume of the root zone. The smaller the particle size of the lime the faster it will react. The faster reaction rate of a ground limestone, rather than a pelletized lime, is particularly important when very low pH's are encountered.
Adapted from the article, 'Tales from the Annals of Disease', by Bruce Martin and Jim Camberato, which first appeared in the September-October 1999 issue of Carolinas Green.