Solutions to Soil Problems: Soil Acidity

January 20, 2015

Soil pH

	Rhododendrons are very tolerant of acid soils and grow best in soil with a pH between 4.5 and 5.5. Photo credit: Uwe Fischer Flickr CC BY-NC-SA 2.0
	Influence of soil pH on the relative availability of plant nutrients in a typical mineral soil. Image source: Chart of the Effects of Soil pH on Nutrient Availability
	Surface application of lime to a field. Plants should be irrigated after lime application to ensure the foliage is free of the amendment. Photo credit: Dwight Sipler Flickr CC BY 2.0

Soil pH is a measure of the acidity or alkalinity (basicity) of a soil, and is reported as a value between 0 and 14. A soil test for pH measures the concentration of hydrogen ions in the soil solution.
A pH of 7.0 is considered neutral. A pH value below 7.0 indicates that the soil is acidic, with lower values representing increasing acidity. A pH value above 7.0 indicates that the soil is alkaline (basic), with higher values representing increasing alkalinity.
The pH scale is logarithmic, so a change in 1 pH unit reflects a 10 fold change in acidity or alkalinity. On average, garden and landscape plants grow best in soils with pH values between 6.0 and 7.2.

Causes of Soil Acidity

Soil acidity can be caused by a number of factors:

• Soils in areas with large amounts of rainfall tend to be acidic because the water leaches basic cations (calcium, magnesium, sodium, and potassium) out of the soil profile, and these cations are then replaced by acidic cations (hydrogen and aluminum).
• Carbonic acid formed from carbon dioxide and water acidifies soils in high-precipitation areas.
• Acidic soils tend to be high in iron and aluminum oxides, as they are the slowest minerals to weather in soil. Aluminum in these increasingly acidic soils is solubilized and will combine with water to release additional hydrogen ions (acidity).
• The soil parent material (or mineral types from which the soil developed) can be a source of acidity in soils.
• Nitrification of ammonium fertilizers yields hydrogen ions.
• Acid rain contains nitric and sulfuric acid.
• Added elemental sulfur oxidizes to form sulfuric acid.
• Plants take up, and thus remove, basic cations from the soil.
• Plant roots excrete hydrogen ions in exchange for nutrients in the soil.

Detrimental Effects of Soil Acidity

• Soil acidity can lead to elemental toxicities for plants by aluminum, iron, manganese, and zinc due to the increased solubility of these elements at low pH values.
• Soil acidity can cause limited availability of some macronutrients and micronutrients such as phosphorus which binds to iron and aluminum oxides in acidic soils.
• Other elements in their plant-available forms, such as molybdate, exibit decreased solubilities at low pH values.
• Microbial activity drops off in acidic conditions which can lower nitrogen (the key plant nutrient) concentrations, reducing nitrogen fixation and nitrogen mineralization, two processes vital to creating plant-available forms of nitrogen.
• Organic matter decomposition by soil organisms slows.
• Calcium, magnesium, and potassium deficiencies develop.

Treatments for Soil Acidity

Soil acidity can be ameliorated and the pH of the soil increased by the addition of lime/limestone (calcium carbonate) and similar compounds that have been ground fine for use. Types of lime-like amendments include:

• Dolomitic limestone
• Quicklime
• Hydrated lime
• Marl
• Chalk
• Oystershell
• Wood ashes
• Fluid lime

Each lime-like amendment has its benefits and drawbacks, such as effectiveness, price, and purity. Lime is most effective at neutralizing acidity when it is incorporated/tilled into the soil to the full depth of the plow layer or root zone.

Benefits of Liming:

Lime treats acidity by combining with carbon dioxide gas, water, and hydrogen ions to form free calcium ions and carbonic acid (weak acid). The carbonic acid then dissociates to form carbon dioxide gas and water, ridding the soil of hydrogen ions. Liming is also effective at accomplishing the following:

• The calcium addition by the lime displaces aluminum and hydrogen off the soil particle surfaces and replaces calcium in the soil (dolomitic lime furnishes magnesium as well).
• As the pH of the soil increases, excess metals, such as aluminum, iron, manganese, and zinc, precipitate out of the soil solution and are no longer plant-available.  
• Phosphorus solubilizes and become plant-available.
• Molybdenum solubility increases.
• Microbial activity resumes.

Other lime-like amendments neutralize acidity, but may follow different reaction paths. For instance, calcium oxide combines with water and hydrogen ions to immediately form free calcium ions and water.

Applying Lime to Acidic Soil

The amount of lime needed to treat the soil acidity depends on the following:

• Crop tolerance to acidity/alkalinity and the lime requirement of the selected plants
• pH of the untreated soil and the desired pH of the treated soil
• The amount of soluble and exchangeable acidity
• Cost of the lime amendment will affect choice of product
• Less product is needed when applying a lime-like amendment that has a large calcium carbonate equivalent
• Fineness of grind of the lime amendment
• Amount of organic matter in the soil
• Type of clay present in the soil

The SMP, Adams-Evans, and Mehlich buffer methods are used to determine the lime requirement of soils. These different methods were developed for distinctly different soil types. Other methods, such as titratable acidity and reactive aluminum, may also be used to determine soil lime requirements. A soil sample can be submitted to an analytical laboratory to determine the lime requirement, which will be given in the results report.

Time of Application

Lime should be applied a few months before planting, approximately one time per year. Lime applied to turf should be irrigated after application to wash any lime off the leaves. Lime should not be applied if a soil test indicates that liming is not necessary; similarly, caution must be taken to avoid over-liming. A soil with too high of a pH poses a whole new set of problems.