Tech Notes Issue 3

At pH values below 5.4 soil aluminum toxicity potential increases  

When soils are tooa cidic, aluminum that is locked up in clay minerals dissolves into the soil as toxic, electrically charged particles called ions, making it hard for most plants to grow. In fact, aluminum toxicity in acidic soils limitscrop/turf production in as much as half the world's arable land in Africa, Asia and South America.

Excess soluble/available aluminum (Al+3) is toxic to plants. Very little Al+3 in soil solution are required to cause turf damage.Turfgrasses cannot tolerate > 1.0 ppm of Al+3 in solution and often have serious problems at 0.50 ppm. The most common turfgrass problem is stunted root growth. At 0.5 ppm the root system can be stunted at more than 50% compared to no soluble Al present. Additional Al +3 toxicity symptoms include:

• Reduces the availability of phosphorus (P), through the formation of Al-P compounds

• Reduces the availability of sulfur (S), through the formation of Al-S compounds

• Reduces the availability of other nutrient cations through competitive interaction

Physiologically, it is believed that Al +3  impacts phosphate /nucleotide metabolism, cell wall structure and function, and membrane transporters to name a few.

Soluble soil Al is a serious problem at soil pH's below 5.0. Between 5.0 and 5.5 soluble Al is likely to be a problem but not as severe. In general Al +3 is most severe at pH below 5.5 on soils with low base saturation and low or deficient in calcium and magnesium. At pH's greater than 5.5 soluble Al is not a concern for turfgrass growth under most, if not all conditions.

The availability of Al+3 is not completely understood. However, in addition to soil pH, the total amount of Al present in a soil type, the type and amount of clay in the soil and soil organic matter greatly affect the availability.

Liming is the most commonly recommended solution soluble aluminum in the topsoil. Through liming you are trying to raise the pH so that soluble Al is not present. In established turfgrass systems liming is usually surface applied and only affects the top few inches. If soluble Al needs to be addressed deeper in the soil profile, one option is the application of gypsum (CaSO4). Gypsumis not a liming agent, and cannot neutralize acid. However, calcium applied as gypsum is a competitive cation for Al +3, which can allow for leaching of Al+3 deeper into the soil profile as long as enough water passes through the rootzone.