What does lime do?

Lime (calcium carbonate) is the most common soil preparation used in New Zealand. Fortunately, it is found in most areas and does not usually have to be transported large distances from quarry to farms.

Lime is applied primarily to neutralise acidification in the soil. Acidification occurs in all farmed soils - caused by normal biological processes, such as nitrogen fixation, nitrate leaching and photosynthesis. The more intensively you farm, the greater the rate of acidification. The process is also accelerated by higher rainfall, due to increased leaching.

Calcium carbonate contains calcium (Ca) at up to 20% and carbonate at 80-90%. The negatively charged carbonate ions, not the Ca ions, neutralise the positively charged hydrogen ions that are an indicator of acidification. The concentration of hydrogen ions is what is measured as pH.

Soil micro-organisms do not like acid conditions. As the pH level increases up to 6.0, i.e. becomes more neutral, conditions become more favourable for micro-organisms and they mineralise more of the nitrogen in the organic matter and make it plant available. This can sometimes show up as a greening of the pasture after lime is applied. Increasing the soil pH from 5.5 to 6.0 can result in the release of 20 to 25 kg N/ha in acid soils.

Responses to liming of pastures are often due to either a decrease in aluminium (Al) availability or an increase in molybdenum (Mo) availability. Aluminium availability increases as soil pH decreases. This can affect the availability of Ca, Mg, K, P and other nutrients. High levels of Al are toxic to plant roots - this is generally a concern at low pH.

At low soil pH (below 5.5), aluminium (Al) is present in the soil at high concentrations (greater than 5 ppm with calcium chloride extraction; greater than 1 ppm with potassium chloride extraction). This prevents clover roots growing down the soil profile and extracting water. As a consequence, clover growth is more likely to be limited in dry soil conditions where pH is low.

Molybdenum is an essential trace element for N fixation in clovers. As soil pH increases, Mo becomes more available in most soils. Soils with a pH of 5.8 or lower are more likely to develop Mo deficiencies. Pastures that are Mo deficient typically have a low clover content. Nitrogen-fixing bacteria require around ten times more Mo than most plants do, so deficiencies of Mo will be seen in legumes first.

Use of lime can also improve the absorption of rainfall into soil. This can be particularly important on dry hill country because the soil will wet up faster when a dry spell breaks, with the rain absorbing into the soil rather than running off the surface. Typically, a higher rate of lime (greater than 2.5 t/ha) is required for this to occur.

Agricultural lime requires 50% of the particles to be less than 0.5 mm in diameter, and 90% to be less than 2 mm, to be effective in increasing pH within one year after application. Fine lime (ground limestone) breaks down and increases pH much faster in the short term, especially in dry conditions, but there will be little difference in the effectiveness between ag-lime and fine lime in the long run.

Any increase in pH will still depend on the amount of lime applied, with the general rule of thumb being a 0.1 unit increase in pH for every tonne of lime applied.

The large number of benefits from lime use means that it is usually economic to use. Generally, intensively farmed pastoral land should be maintained between pH 5.8 and 6.0. In hill country, soil pH should be kept at 5.4 to 5.6 if clover is a component of the pasture. In other hill country, soil pH should not be allowed to decrease below 5.0.

It is best to apply lime in still conditions, either in spring or autumn.

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