Soil acidity and liming : basic information for farmers and gardeners

SoilFacts Soil Acidity and Liming : Basic Information for Farmers and Gardeners Situation in North Carolina Nearly all North Carolina soils are naturally acidic and need lime, which neutralizes the acidity, for optimum growth of crops, forages, turf, trees, and many ornamentals. Even though most of these soils have been limed in the past, periodic additions of lime based on soil tests are still needed. Soil-test summaries and field records compiled by the North Carolina Department of Agriculture and Consumer Services (NCDA&CS) emphasize that poor management of soil pH accounts for a high percentage of the “crop problems” in North Carolina. Nature and Cause of Soil Acidity residues are added to the soil. Lime will neutralize this acidity by dissolving, where¬ upon it releases a base into the soil solution that reacts with the acidic components, hydrogen and aluminum. Soil pH is an indicator of “soil acidity" (Figure 1). A pH of 7.0 is defined as neutral. Values below 7.0 are acidic, and values above 7.0 are basic or alkaline. Small changes in numbers indicate large changes in soil acidity. A soil with a pH of 5 is 10 times more acidic than a soil with a pH of 6 and 100 times more acidic than a soil with a pH of 7. Most plants can grow in slightly acidic soils, so the goal of liming is not to raise the pH to neutral (7.0), but to avoid crop problems related to exces¬ sive acidity. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. North Carolina State University and North CarolinaA&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North CarolinaA&T State University, U.S. Department of Agriculture, and local governments cooperating. “Soil acidity” is the term used to express the quantity of hydrogen (H) and aluminum (Al) cations (positively charged ions) in soils. When levels of hydrogen or aluminum become too high — and the soil becomes too acid — the soil's negatively charged cation exchange capacity (CEC) becomes “clogged” with the positively charged hydrogen and aluminum, and the nutrients needed for plant growth are pushed out. This is why root growth and plant development suffer when soils become too acid. Over time, soils also become acidic because calcium and magnesium leach out, because hydrogen is added to soils by decomposition of plant residues and organic matter, or because nitrification of ammonium occurs when fertilizer (UAN solutions, urea, ammonium nitrate, ammonium sulfate, anhydrous ammonia), manure, or plant Soil pH 4 5 6 7 8 I _ I _ I _ I _ I _ I _ I _ I _ L ACIDIC NEUTRAL BASIC l I I I I I I I r Figure 1. General relationship between soil pH and acidity. Benefits of Proper Lime Use Proper liming provides a number of benefits: ■ Plants develop healthier roots because they are exposed to less potentially toxic aluminum. Better root growth may enhance drought tolerance. STATE UNIVERSITY Л&Т STATE UNIVERSITY COOPERATIVE EXTENSION Helping People Put Knowledge to Work