In New Zealand, our pastoral soils are typically slightly acidic. Over time soils become too acidic due to a variety of factors, and the only way to counteract these natural processes of acidification is to apply lime.
The acidity of the soil can increase from plant uptake of essential nutrients, nutrient leaching, decomposition of organic matter (OM) by microbes and the application of certain fertilisers such as elemental sulphur. Once soils become too acidic, pasture and crop production can suffer.
When applying lime, the most commonly used materials are calcium carbonate (CaCO3) or magnesium carbonate (MgCO3). The carbonate portion is active in neutralising acid in the soil as opposed to calcium or magnesium. In simple terms, the carbonate added to the soil reacts with water, producing an alkali (OH-) that neutralises acid (H+). As acidity is neutralised, the pH increases and the more carbonate added, the more acid is neutralised and the greater the pH increase.
A well-known, handy rule of thumb when looking to increase pH in soil is that one tonne per ha of quality lime with 80 percent calcium or magnesium carbonate, will raise the pH of soil by 0.1 units to a depth of 7.5 cm. Using this rule of thumb, if you want to move your pH from a below-optimum 5.7 to an optimum 5.8 it will take one tonne of quality lime.
There are two facts you need to understand about this rule of thumb. Firstly, the rule relates to a depth of 7.5 cm, the pasture soil testing depth. So when looking to increase pH to a depth of 15 cm – cropping soil testing depth – this rule needs to be increased to approximately two tonne of quality lime for each 0.1-unit change. This is because there is now twice the amount of soil and, therefore, approximately twice the amount of acid to neutralise.
Secondly, this rule of thumb is based on soil having a cation exchange capacity (CEC) of 20 which is considered a medium level. The actual pH change observed in soil tests following a lime application will be dependent on soil type.
Soils with high clay content and/or high OM with a CEC greater than 20, need more lime than the rule of thumb for each 0.1-unit change. Whereas course textured or sandy soils with a CEC of less than 20, with low clay and OM levels, will need less lime to enact the same 0.1-unit change.
To maintain an optimum pH level in the soil you’ll need to apply sufficient lime. For assistance, contact your local PGG Wrightson Technical Field Representative.
