The soil pH is a reading taken from a scale that measures the hydrogen ion concentration of the soil medium. The scale consists of 14 divisions from 1 to 14, with the value 7 being neutral. Values below 7 are said to be acid while those above 7 are termed basic. Other terms for acid are sour or low pH's, while other terms for basic, are alkaline, sweet, or high pH's. 

The scale is logarithmic, not linear. A pH of 8.0 is 10 times more more alkaline than a pH of 7.0, while a soil with a pH of 5.0 is 100 times more acid than a soil with a pH of 7.0. While some type of plant growth occurs at pH's over a range of 3.5 -10.0, most plants grow best in a soil that is neutral to slightly acid.

Nutrient availability is the major impact that pH has upon plant growth. Moderately to very acid soils may contain toxic levels of aluminum and manganese, but inadequate nitrogen, phosphorous, potassium, sulfur, calcium, magnesium, and molybdenum. Very alkaline soils are often high in total soluble salts, while moderately alkaline soils are low in available nitrogen, phosphorous, iron, and manganese.

The pH of a soil can be adjusted with certain materials. To raise the pH of a soil or make it less acid, it is common to apply a material that contains some form of lime. To lower the soil pH, or make the soil less alkaline, a source of sulfur is added to the soil.

RAISING THE pH OF A SOIL

There are several ways in which soils become acidic.

The parent rock was itself acidic in nature.

Excessive rainfall leached the soluble calcium.

Roots themselves excrete acid-forming ions to assist nutrient uptake.

Roots secrete carbonic acid to speed up the natural breakdown of organic matter nutrient release.

The frequent addition of acid-forming nitrogenous fertilizers and sulfur-containing fungicides.

The acid nature of rainfall in the northeast United States.

Liming materials are applied to overly acid soils for a variety of reasons in addition to merely raising the pH.

As a means of supplying calcium and sometimes magnesium.

To increase the availability of molybdenum, potassium, phosphorous, and sulfur by raising the soil's pH.

To reduce the possibility of aluminum and manganese toxicity that may occur at a low soil pH.

To enhance the beneficial microbial activity in the soil especially nitrogen-fixing bacteria.

To temporarily improve the structure of clay soils through the clay micelle aggregating action of the added calcium.

The pH of acid soils may be raised by the addition of a variety of different calcium and/or magnesium carbonate, hydroxyl, or oxide sources. The most common used material is ground agricultural or dolomitic limestone.

Calcitic limestone and dolomitic limestone are naturally occurring deposits that are mined and ground or pulverized for direct use. Burnt lime is made by heating limestone to a very high temperature, while hydrated lime is made by adding water to burnt lime.

Marl, a less commonly used liming material is a naturally occurring crumbly soil composed of clay, sand, and calcium carbonate. It's purity is highly variable. Ground oyster and other shells are totally "organic" liming material.

All of these liming materials supply calcium, raise the soil pH, improve nutrient availability, and reduce aluminum, manganese, and iron toxicity. The choice of which one you might use is determined by availability, cost with regard to purity translated as oxidizing potential, and the urgency of raising the soil pH as it relates to fineness of grind.

The amount of liming material you will need is dependent upon the the existing pH of the soil and the units of change desired to reach the pH needs of the plant being grown, the soil type or texture class, and the potency of the liming material you are using. For this reason, always test the soil to determine your needs.

Potency is measured as total oxide content expressed as calcium oxide and/or magnesium oxide. Limestone contains the equivalent of 50% total oxides; hydrated lime, 70% total oxides; and burnt lime, 90% total oxides. Burnt lime is seldom used because it is caustic. Ground shell and marl are equivalent to and therefore interchangeable with calcitic limestone, but are only very slowly available.

Lime requirements vary with soil type. The higher the organic content of a soil the greater the amount of limestone it will take to effect a pH change. The more finely ground the liming material, the more rapid the pH changes but, the pH change will not be as long lasting. Spread liming materials uniformly and mix them into the top 6 - 12 inches of soil when preparing vegetable gardens, flower beds, or new lawns. Limestone can be added to the soil or applied to a lawn at any time of year. It is not wise to apply a readily soluble nitrogenous fertilizer and lime simultaneously, as this will lead to a rapid release of the nitrogen. This rapid release might lead to a burn of the roots and/or a rapid leaching of the nitrogen by precipitation or irrigation. Liming lawns in late fall enhances the movement of limestone into the soil as the freezing and thawing and wetting and drying of the soil increases the porosity of the soil. Wood ashes are sometimes used to raise the pH of a soil. It usually takes 2 times as much wood ashes to raise the soil pH a given amount as it does agricultural limestone. Spread no more than 10 lb. of wood ashes per 1000 sq. ft. annually to avoid any toxicity problems. Do not let wood ashes come into direct contact with the roots of plants, especially germinating seedlings. Never use coal ashes. A side benefit of using wood ashes is that they are a good source of potassium.

LOWERING THE pH OF A SOIL