Liming: The Why’s and How To’s

What is soil pH and why should you care? Why would you want to bother liming in the fall? Soil pH is an indication of the acidity (sourness) or alkalinity (sweetness) of a soil. It is defined as the negative logarithm of the hydrogen ion concentration. The pH scale goes from 0.0 to 14.0, with 0.0 being most acid, 14.0 being most alkaline and 7.0 being neutral (neither acid nor alkaline).

 

Why is this important? Because soil pH has a strong influence on plant growth. It affects soil bacteria, nutrient leaching, nutrient availability, toxic elements and soil structure.

 

Bacterial activity that releases nitrogen from fertilizers and organic matter is particularly sensitive since bacteria operate most efficiently when the pH is between 5.0 and 7.5. When the soil is strongly acid, bacteria are hindered, resulting in an accumulation of organic matter and the tie-up of nutrients, particularly nitrogen, that are held in the organic matter.

 

Of the 17 essential plant nutrients, 14 are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. A pH range of 6 to 7 promotes the most ready availability of plant nutrients. Extremely and strongly acid soils (pH 4.0 to 5.0) can have high concentrations of soluble aluminum, iron and manganese, which can be toxic to the growth of some plants.

 

Plants such as azaleas, rhododendrons, blueberries, white potatoes and conifer trees require acid soils to grow well. Others do well only in slightly acid to moderately alkaline soils. A pH above 7.4, for example, causes a problem with the availability of iron to pin oak and a few other trees. This causes yellowing of the leaves, putting the tree under stress and possibly leading to decline and death.

 

The structure of your soil, especially if it’s clay, is affected by pH. In the optimum pH range (5.5 to 7.0) clay soils are granular and easily worked. But if the soil pH is either extremely acid or extremely alkaline, clays tend to become sticky and hard to cultivate.

 

Soils tend to become acidic as a result of rainwater leaching away basic ions (calcium, magnesium, potassium, and sodium) as a result of carbon dioxide from decomposing organic matter and root respiration dissolving in water to form a weak organic acid, and as a result of the formation of strong organic and inorganic acids (such as nitric and sulfuric acid) from decaying matter and the oxidation of various fertilizers. Strongly acid soils are usually the result of the action of these strong organic and inorganic acids over the years.

 

What’s the solution? Dolomite lime is usually added to acid soils to increase soil pH. The addition of lime not only raises pH eliminating most major problems associated with acid soils, but it also provides 2 nutrients (calcium and magnesium) to the soil. Liming materials are relatively inexpensive, comparatively mild to handle and leave no objectionable residues in the soil. They can be applied slowly (over a period of months) to raise your soil pH. If you apply them now, in the fall, they will have a chance to begin raising the pH of your soil over the winter.

 

One accurate method of determining soil pH is with a pH meter. A second method involves the use of indicators or dyes. Many dyes change color with an increase or decrease in pH making it possible to estimate soil pH. Kits containing the necessary chemicals and color charts are available at garden centers. Or a soil sample can be sent to the Analytical Services Lab ( Spaulding Life Science Center , Room G-54, 38 College Road ) at UNH in Durham (03824). Depending on the quality of your soil, pH test should be made every one to three years to determine the need for lime.

 

To obtain a soil testing form call UNH Cooperative Extension’s Family, Home & Garden Education Center ’s Info-Line toll free at 1-877-398-4769. Trained volunteers are available to answer your questions Monday through Friday from 9:00am to 2:00pm .

 

Making a limestone application is simple. Use either a drop spreader or a rotary spreader. Uniform coverage is the key as lime is very insoluble and essentially stays where it’s put. To insure even coverage, one half of the lime should be applied in one direction, and the remainder applied in a perpendicular (crisscross) direction. If you are using ground lime, it is simple to determine if coverage is uniform because of the visible white color. More care must be taken if pelletized lime is used.

 

Liming is not a “cure-all” remedy for plants that aren’t growing well. However, it is essential to help provide the best possible environment for plants to grow in.

 

Call the UNH Cooperative Extension's Family, Home & Garden Education Center 's Info-Line toll-free at 1-877-3984769 for "Practical Solutions to Everyday Questions." Trained volunteers are available to answer your questions Monday through Friday from 9 am to 2 pm .