Soil acidification is a natural process accelerated by agriculture. Soil acidifies because the concentration of hydrogen ions in the soil increases, the main cause of soil acidification is inefficient use of ammonium nitrogen, followed by the export of alkalinity in produce (meat/grain)

Poor pasture production is a common and direct result of soil acidity. Major issues include: Loss of productive species, reduced soil microbial activity, reduced rhizobia (Nitrogen fixing bacteria) activity and nutrient deficiencies and Toxicities

Aluminium toxicity – Soils with a pH of below 4.8 are consistently showing toxic levels of Aluminium (Al). Al inhibits plant growth by; damaging plant roots and reducing the plants capability to explore for water and nutrient. Al also restrict the uptake and mobilisation of important nutrients such as phosphorus and at very high levels can also restrict the uptake of Calcium and Magnesium.

Molybdenum (Mo) is a very important trace element in our pasture systems. Mo is required in the legume nitrogen fixation process, but is also required by grasses for the production of proteins. Mo becomes very insoluble and thus not available for plant use in acid soils.

What to look for? Stunted plants, often purple in nature; clovers with pale, small leaves; sensitive plants such as Lucerne will be spindly in nature; and premature pasture death.

Management to overcome soil acidity – Sowing acid tolerant species is an effective way of maintaining production on acidic soils. Some soil types, the depth of acidity, topography and budget may limit the ability to correct the low soil pH. Note that growing tolerant species will enable a certain level of production but will not correct the issue.

Acid tolerance of commonly sown pastures around Wellington: Highly sensitive – lucerne, Balansa Clover, Persian Clover; sensitive – Phalaris, red clover, snail medic; tolerant – Tall Fescue, rye grasses, Haifa White Clover, Subterranean Clovers; and highly tolerant – Serradella, Cocksfoot, Premier Digit.

Soil testing and lime application – Regular soil sampling to improve fertilise efficiencies and monitor acidification is an important factor to guard against long term degradation. Lime is an effective tool which can be used to correct acidification. The amount of lime required will depend on the soil pH profile, lime quality, soil type and farming system.