Most soils in the eastern part of the United States are too acid for the best growth of vegetables and many other kinds of ornamental plants. Most vegetable crops do best at soil pH levels of 5.5 to 6.5. A soil pH of 5.5 is too acid for many kinds of vegetables, and above pH 6.5 is not acid enough.
The pH scale is the method is used to express the acid or alkaline level of the soil; a value of 7.0 is neutral, lower values are avid, and higher values are alkaline. The increase at the lower levels of the pH range is quite rapid because the numerical values are based on a logarithmic scale. For example, pH range is quite rapid because the numerical values are based on a logarithm scale. For example, pH 6.0 is rated as being slightly acid but is 10 times more acid than pH 7.0, and a pH of 5.0 is 10 times more acid than 6.0 and 100 times more acid than 7.0. Most garden plants grow best in slightly acid soil with a pH of 6.0 to 6.5 because more nutrient elements are available for their use at this value. A higher or lower levels, certain elements become chemically bound in the soil complex and are unavailable for plants.
Some plants such as azaleas, rhododendrons and blueberries grow best in a strongly acid soil at about pH 5.0. Under less acid conditions the leaves become yellow, with green veins. This is because an inadequate supply of iron is available in soils at the higher pH levels.
In soil too acid, for example, string beans bloom but pods are not set on the plants. In less severe cases, bean pods are pale yellowish-green and the leaves are covered with brownish spots. Other crops are also affected but symptoms may not be as prominent or noticeable.
Soil samples sent to the University of Maryland Soil Testing Laboratory have shown that some soils are too acid even for azaleas. Many lawns are not as good as they should be because the soils are too acid for good grass growth.
Certain processes are continually active which contribute to soil acidity. These include leaching from normal rainfall and irrigation, crop removal, soil erosion, use of large applications of acid-forming fertilizers, and, to some extent, the breakdown of organic residues.
The way to find out about your own soil is to have it tested. Residents of Maryland can have it done free of charge at the University of Maryland, residents of Virginia at Virginia Tech, and residents of the District at the University of the District of Columbia.
Contact the Cooperative Extension Service of the university for information on how to take the soil samples and where to send them.
Lime is applied to neutralize soil acidity and it supplies calcium and magnesium. The amount of lime to apply depends on the acidity of the soil and the kinds of plants to be grown. Too much lime can be as harmful as too little.
Unlike fertilizer, lime is a slow-acting product that takes several months to neutralize acid soils. If left on the soil surface it is not going to react very rapidly. It should be dug in and mixed thoroughly with the top six to eight inches of soil.
Research has shown that limestone pulverized to particles of 100 mesh or finer will react almost completely within a couple of months after application if dug in, while those of 60 mesh will react the first year or so.
If you are starting a garden in an area that has been so used, or if you have not limed your garden or lawn for four or five years, lime is probably needed. The lime needs to be dug in. If you do not get the results of your soil test soon enough to get started, apply about 10 pounds of ground limestone per 100 square feet and mix it thoroughly with the top six inches of soil.
Water moving through the soil will leach out the basic elements and cause the acidity to increase gradually. Therefore, you may need to apply lime every three or four years to maintain the best level for your particular garden or lawn.