Garden Soils May Need Lime

Soil testing, which includes a measure of the soil acidity, is the only sure way to determine if lime is needed in the soil. The type of crop to be grown may also influence the recommendation for the amount of lime to apply. Contrary to popular belief, lime is not a fertilizer. Lime is calcium carbonate, and while plants need calcium to grow properly, most soils contain enough calcium for good plant growth. The main function of lime is to change the soil pH. For those unfamiliar with the pH expression, a pH of 7 is considered neutral. Those soils with pH over 7 are considered alkaline, or "sweet soils." Those with less than 7 are considered acid or "sour" soils.

A pH of 5 is 10 times more acid than a Ph of 6, and a pH of 4 is 100 times more acid than a pH of 6. Most vegetable plants grow best in a soil that is in an acid range from about 5.5 to 6.8. Some plants, especially ornamentals such as azaleas and rhododendrons, need very acid soils.

Lime not only changes the acidity, but as soil becomes less acid, phosphorous becomes more available, so plant growth may be improved. Liming can also increase available nitrogen by promoting more rapid breakdown of organic matter and an increase in nitrogen fixation. Toxic elements in the soil such as aluminum and manganese are less available to plants when soils are not very acid.

Most gardeners think about adding lime in the spring, but actually a tall application is better since it normally takes five to six months for ground limestone to produce the desire pH change. Spring application, however, will still be beneficial as a gradual change occurs throughout the summer. The effect of lime does not move upward through the soil, so lime must be mixed very thoroughly with the soil, or applied to the soil surface after the soil has been prepared. Do not mix lime with manure or commercial fertilizer to apply it.

Rely on a soil test to avoid excess application. Too much may tie up phosphorous or magnesium in the soil and stunt plant growth. Soil tests may easily be obtained at your Local University Extension center, but before this service was offered, lime was still applied by home gardeners. Generally, they applied about five pounds of ground limestone per 100 square feet of garde space every four to five years.

There are several types of lime available. Ground limestone is best since it is easiest and safest to use. Ground dolomitic limestone is even better when available, since it supplies plants with magnesium as well as calcium. Other forms of lime sometimes used are hydrated (slaked) lime and quick lime (oxide of lime). These are more difficult to handle, and the chance of burning plants is greatly increased. However since the reaction is quicker, they are sometimes used at lower rates for special situations.

http://www.dpi.nsw.gov.au/agriculture/horticulture/vegetables/soil/soilpak/common-problems/Does-my-soil-need-lime.pdf