Soil testing is a quick and accurate method to determine the relative acidity of the soil (pH) and the level of several essential nutrients (phosphorus, potassium, calcium, magnesium, sodium, sulfur, manganese, copper, and zinc) needed. The test results will aid in plant selection, soil preparation and fertilization.
They will help you avoid overfertilization, which can stimulate excessive plant growth and increase the likelihood of some diseases. It can also help reduce pollution of water supplies. Excess nutrients, applied but not used by plants, may run off into surface waters during storms or leach into groundwater. By applying the correct grade and amount of fertilizer, you will avoid unnecessary pruning of excessive new growth and have healthier, more productive plants.
Get pH tested
A soil test is the only reliable method to determine soil pH. Most soils in North Carolina are acidic, and some are as acidic as vinegar.
Soil pH is a measure of the hydrogen (acid-forming) ion activity of the soil solution. The pH scale of measuring acidity or alkalinity contains 14 divisions known as pH units. It is centered at pH 7, which is “neutral.” The lower the number, the more acidic the soil. The higher the number, the more alkaline. The pH scale is not a linear scale but a logarithmic scale. A soil with a pH of 4.0 is 10 times more acidic than soil with a pH of 5.0 and is 100 times more acidic than soil with a pH of 6.0.
Soil pH affects the availability of nutrients in the soil as well as those applied as fertilizer. Low pH can cause some elements to become chemically bound to soil particles, which makes them unavailable to plants. Microorganisms responsible for the decay of organic matter may be limited or inactive in highly acidic soil. The ability of legumes to fix nitrogen is also reduced. But when the pH rises above 6.5, trace elements such as iron, manganese, copper, and zinc become less available. The availability of most nutrients is greatest at pH 6.5.
Plants require different pH levels for optimum growth and productivity. A slightly acidic soil (pH 6.0 to 6.5) generally is considered ideal for most plants in North Carolina. However, blueberries, rhododendrons, mountain laurel, and centipedegrass grow best in soils with a pH between 5.0 and 5.5. If the soil pH is above the preferred range for a plant, growth will be slowed or the plant may develop stress problems, such as diseases, insects, nutrient deficiency symptoms and dieback.
How to test soil
Most inexpensive, commercially available soil test kits are not reliable. Even if they accurately measure pH, they do not indicate the amount of lime needed. Soil texture, organic matter content, crop to be grown, target pH, soil acidity level, cation exchange capacity (CEC), type and amount of clay, and current pH are factors to consider in determining the amount of lime needed to raise the soil pH.
Consistently reliable results can only be obtained by submitting samples to a soil-testing laboratory. The N.C. Department of Agriculture and Consumer Services Agronomic Division will analyze soil samples free of charge or for a small fee. Forms and boxes for samples are available from the Jackson County Cooperative Extension Center. See https://jackson.ces.ncsu.edu/ or call 586-4009 for more information.