Soil pH is a measurement of soil acidity. A pH of 7.0 is neutral, below 7.0 is acidic and above 7.0 is alkaline. The target pH for most field and forage crops is between 6.0 and 6.5. Soils in Missouri naturally become more acidic over time. Crop removal of calcium, magnesium, and potassium play a large role in the change of pH. The breakdown of organic matter and nitrogen fertilizer also increase soil acidity.
When the pH is low or very low, essential plant nutrients, like nitrogen, phosphorus and potassium, are not available for plant growth. Phosphorus becomes unavailable because it forms insoluble compounds with aluminum and iron. Some micronutrients, like aluminum and manganese, become readily available, resulting in toxicity issues. High concentrations of available aluminum may inhibit root development and limit water and nutrient uptake. Many beneficial soil microorganisms, like rhizobia bacteria needed for nitrogen fixation in legumes, need a pH near 6.0 to function efficiently.
Soil testing is the only way to know soil pH. University of Missouri Soil Testing Laboratory
recommendations for adjusting soil pH are given in the amount of Effective Neutralizing Material (ENM) needed to raise the pH to the optimum level. The MU Soil Testing Lab measures salt pH, which is generally 0.5 units lower than the water pH. It is important to read pesticide labels that may indicate a desirable water pH range for most effective weed control.
Soil pH is raised by adding a liming material to neutralize the acidity. The most common material used is calcium carbonate in the form of crushed limestone. When magnesium is deficient in the soil, dolomitic limestone should be used. Limestone breaks down slowly, taking about six months to fully take effect.
Agricultural liming materials sold in Missouri must have an ENM rating. This number is based on the purity of the calcium carbonate in the product and how finely it is ground. Optimum soil pH required is based on the crop being grown. For example, alfalfa requires a higher pH than other forage crops.
If the liming material is not tilled into the soil, caution should be used to avoid raising pH too much at the soil surface. No more than 1500 ENM of lime per acre should be applied at one time. If more is needed, a split application should be made one year apart.
Products other than crushed limestone can be used, but are often more expensive. Pelletized lime is less dusty, but it may take more time to breakdown and have the full effect on soil pH due to the binding agents. Liquid lime will react more quickly than crushed limestone, but usually has a lower ENM value and will have an effect only near the soil surface. Hydrated lime will react very quickly with the soil, but can be difficult to handle due to its caustic nature. Although gypsum contains calcium, it is not effective at neutralizing soil acidity.
Maintaining the proper pH is critical for efficient fertilizer uptake and plant growth as well as ensuring adequate availability of essential nutrients and reducing the toxicity of micronutrients. Adjusting pH to the desired range is often the most economical soil amendment.
For more information about soil sampling and testing, contact Valerie Tate, Field Specialist in Agronomy at firstname.lastname@example.org or call 660-895-5123. University of Missouri Extension programs are open to all.
Valerie Tate, field specialist in agronomy with the University Extension.