Lime is primarily used in New Zealand farming systems to increase soil pH. As a result of the positive change in pH, there are beneficial changes to soil chemistry and biology along with production. So, why do soils need lime and how does it work?

Soil acidification is a natural process that happens over time. However, this process is accelerated within farming systems which include legumes, the addition of some fertilisers and the leaching of nitrate nitrogen (N). Fertilisers such as urea, diammonium phosphate, sulphate of ammonia and sulphur are the most acidifying, with most others creating a relatively minor to no direct impact on the acidification process.

The availability of macro and micronutrients, as shown in Graph 1, can be dictated by pH. For example, aluminium (Al) and manganese (Mn) become more soluble and available in soil solution at a low pH and the result is Al becomes toxic to plants.

On the reverse, as pH increases, so does molybdenum (Mo) availability. As Mo is an essential element for legumes, Mo-deficient soil liming can increase clover growth and N fixation. However,
high levels of Mo increase the risk of copper (Cu) deficiency in animals. For this reason, it is important to monitor animal Cu and plant Mo levels following liming.

The pH of soil also impacts its biology. A below-optimum pH can cause lower worm populations as worms are sensitive to a low pH. So, finding the recommended pH level in turn helps increase the worm population. Other soil organisms also benefit from a lift in pH to optimum levels, which in turn, increases the rate of nutrients added to the soil.

How does lime work when applied to soil? Lime is calcium carbonate (CaCO3). When it comes into contact with carbonate and calcium, the lime gradually breaks down with soil moisture, and it is the carbonate proportion that acts to neutralise the soil's hydrogen ions and raise the pH. Lime is slowly soluble, so the full effect of the increase in pH takes time to move through the soil
profile. The main factors that impact lime response are rainfall and irrigation. The greatest pasture response to applying lime is seen in the summer or autumn.

Several visual observations may indicate lime is required, though the most accurate way to check the soil’s pH status is to complete a soil test. All basic soil tests include soil pH when testing other macronutrients. If a below-optimum pH issue is identified, a plan to correct the pH with a lime can be made.

To avoid your soil pH impacting production, contact your local PGG Wrightson Technical Field Representative to organise a soil test.

1 Cornforth, I. (1998). Practical Soil Management. Lincoln University Press.