The Power of H
The concept of adjusting soil pH has a rich history. Marcus Varro, one of the most prolific writers of Roman history, wrote in a book around 116 B.C., “They manured their fields with white chalk which they dug out of the ground.” More than 2,000 years later we are still concerned with soil pH when considering plant nutrition. Soil pH directly influences the solubility of plant nutrients, activity of microorganisms and pesticide interactions. Indirectly, soil pH influences microbial and chemical reactions that influence plant growth. In addition to typical fertility influences, low soil pH has been linked to several devastating disease outbreaks.
WHAT IS pH?
Before discussing ways to alter pH, it is important to understand the meaning of soil pH. The term pH is always written with a lowercase “p” and a capital “H.” Some early references indicate that the “p” likely originated from the word “power” as shorthand for negative logarithmic power. The “H” represents the activity of hydrogen ions. The entire pH scale is from zero to 14. It is on a logarithmic scale; therefore a pH of 5 is 10 times more acidic than a pH of 6 and 100 times more acidic than a pH of 7. All pH numbers above 7 indicate alkalinity and below 7 they indicate acidity. The common range for soils is pH 4 to 10.2.
|Current pH||Pounds of lime required per 1,000 square feet|
|Adapted from Best Management Practices for Florida Golf Courses, 2
WHAT CONTROLS pH?
In natural systems, a number of factors influence soil pH. These include soil mineralogy (parent soil components), climate and weathering. Soils in some areas contain limestone or seashells resulting in elevated calcium carbonate levels. These materials can result in pH levels between 7 and 8.2. In other areas, base-forming cations such as calcium, magnesium, potassium and sodium accumulate because of lack of rainfall, resulting in elevated pH. If an area receives high yearly rainfall, the base forming cations may be leached from the soil, resulting in a low pH. How the soil is managed may also alter the natural pH. For example, the heavy use of acid-forming fertilizers such as ammonium sulfate or ammonium phosphates can drive down pH. The soil's cation exchange capacity helps buffer pH change. The higher the cation exchange capacity, the greater the buffering capacity, thus the more resistant the soil is to pH changes.
HOW IS pH MEASURED?
While measuring pH is relatively easy with a pocket pH meter, to determine the quantity of material required to raise or lower pH requires more sophisticated laboratory procedures. This is because the buffer capacity of the soil must be measured. The buffering capacity of a soil is usually measured by slowly adding an acid to alkaline soils or a base to acidic soils, while measuring the change in pH. The amount of material required to change pH from the initial value to a target value can then be applied to field conditions. Labs typically report the liming requirement as pounds of calcium carbonate (CaCO
PLANT RESPONSE TO pH
Before changing pH, carefully consider your turfgrass, environmental conditions and the soil test. There is probably as much damage to plants by lime and sulfur applied where they are not needed, as there is damage by not applying them where they are needed.
It is good to know the soil pH that specific plants grow best in even though the vast majority of plants are tolerant of a wide range of soil pHs. Grasses are generally among the most tolerant plants to a wide range of soil pH. Most of the warm-season grasses such as bahiagrass, centipedegrass and bermudagrass do better at soil pH around 5 than 7. Most of the cool-season turfgrasses — bentgrass, ryegrass and bluegrass — perform best at pHs in the 6 to 7 range. Direct effects from pH usually do not occur unless the pH is below 5 or above 8.5. With a pH below 5, turfgrass root systems may become short, thickened and brown due to damage from aluminum and/or hydrogen. Nutrient uptake is reduced and the plant becomes more susceptible to environmental stresses. Greater thatch accumulation and algae problems are often associated with low soil pH. Soil pH above 8.5 often results in deficiencies of several micronutrients. The plant becomes chlorotic with the appearance of severe nitrogen deficiency.
Woody plants also vary in their tolerance to acidic soils. For example ligustrum, hibiscus and oleander do as well at 5 as 7. Others, like azalea, holly and ixora, actually prefer more acidic conditions. Trees such as oaks, pines and palms do well over a wide range of soil pHs. This may be due to their extensive root systems occupying large volumes of soil that may contain several pH levels.
HOW IS SOIL pH MODIFIED?
Maintaining soil pH within the desired range with liming materials or acidifying agents is a continual process. Soil reactions from surface-applied products are generally slow, so it is best to incorporate the materials into the soil before establishment when possible. With existing turf cover, the process can be hastened by incorporation after core aerification.
INCREASING SOIL pH
Liming is the most common method to increase the pH. A liming product contains calcium (Ca) to neutralize acidity. In some cases the liming product may also contain magnesium (Mg). These two elements displace the acid, causing ions in the soil resulting in an increase in pH. There are a number of liming materials that can be used, but calcium carbonate (aglime) and dolomitic limestone (dolomite) are the most commonly used products. These limestone products are usually in a ground powder form or pelletized. The finer the limestone is ground, the faster the reaction. Ground limestone is the most inexpensive source but is dusty and not easily spread. Finely ground limestone may be pelletized for ease of application. The pelletized limestone disintegrates quickly when wet and has about the same reaction time as the pre-pelletized ingredients. The reaction rate for limestone increases when soil temperatures are warm and soil moisture is high.
|Current pH||Pounds of elemental sulfur required per 1,000 square feet|
|Adapted from Best Management Practices for Florida Golf Courses, 2
Alternative liming products may have increased local availability. The relative purity or neutralizing value should be considered before these are applied. A liming material is rated for purity based on its calcium carbonate equivalent (CCE), which is a reference of neutralizing potential. Lime recommendations made by most labs are based on the neutralizing potential of pure calcium carbonate. Pure calcium carbonate will have a CCE of 100. The application rate of a product with a CCE less than or greater than 100 should be adjusted so that an appropriate amount of liming material is applied. A good liming source should have a CCE value of 70 or higher. Products with CCE values over 120 have a high potential to injure plants; because of this they generally aren't used for routine surface application.
For moderate acidity (pH>5), total lime requirements are often between 25 and 75 pounds of calcium carbonate per 1,000 square feet. Because of this large amount, the total amount is usually best split into several applications over a season to reach the total lime requirement. Deficiencies of Mg, K, Ca, P and slower availability of N can generally be easily handled through the fertilization program.
Excessive acidity (pH<5) causes plant problems because aluminum and manganese can be toxic to roots and nutrient deficiencies can be more widespread. For established turfgrasses, several surface applications of lime at 25 to 50 pounds of calcium carbonate per 1,000 square feet with three to six months between applications may be necessary to achieve the total lime requirement. It can take years for the liming material to penetrate deeper into the profile, therefore incorporation with aerification is beneficial.
DECREASING SOIL pH
Reducing soil alkalinity is more problematic than increasing the pH of an acidic soil. The problem is often exacerbated by the use of high pH water that continually replenishes base-forming cations. Elemental sulfur is usually the most efficient and practical amendment for lowering soil pH. Soil bacteria transform elemental sulfur to sulfuric acid, reducing soil pH. However, the acid can only neutralize so much and then soil pH will return to its original value. It is best to use small amounts frequently rather than putting on high rates, because high rates can easily cause tissue burn. A general suggestion is to not apply more than 5 pounds elemental sulfur per 1,000 square feet per application on taller maintained grasses and 1 pound or less on putting greens. Total annual elemental sulfur should be 10 pounds per 1,000 square feet or less. To increase sulfur's effectiveness, it is suggested that elemental sulfur applications are made in conjunction with turf coring or aerification.
Calcareous soils have a high content of calcium carbonate. Acids can dissolve the carbonates and lower the soil pH. If the calcium carbonate percentage is more than 5 percent then it is very difficult to influence the soil pH for any period of time because the soil is so buffered from pH change. Phosphorus, iron, manganese and zinc deficiencies may be apparent. Applications of these nutrients are commonly more efficient than trying to lower pH. Sulfur-injection systems may be beneficial if irrigation water is contributing to the high soil pH. These systems should be professionally designed and installed because sulfuric acid is dangerous to handle and is corrosive to most metals and concrete.
Neither a liming agent nor elemental sulfur should be applied to a soil or turfgrass without prior soil testing. Soils with high cation exchange capacities may require up to double the amount needed for soils of low cation exchange capacities. While most lime materials are relatively inexpensive, the quantity needed can be significant when compared to fertilizer application rates. For this reason, soil samples should be taken annually to determine nutrient and pH status. It is best to get on a maintenance program if soil pH needs to be altered so that small amounts of materials can be added to allow for slow reaction times and prevent excessive layering of soil pH.
Grady Miller is an associate turfgrass professor at the University of Florida (Gainesville, Fla.).
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