Liming: a best practice to neutralise soil acidity

By PRADEEP KUMAR, graduate agronomist, AgriWest Rural Cooma

MOST of the land in the Monaro region is dominated by acidic soil.

According to a report, the estimated cost of soil acidity in agricultural production is $1,585 million per year, which is noticeable in the high and moderate rainfall regions of NSW, Victoria, and WA.

Technically, any soil with a pH below seven can be considered acidic soil, although many plant species that are tolerant to acidity can perform very well in these conditions.

Soil acidification is a result of hydrogen ion accumulation in the soil through the build-up of parent rock material or organic matter.

Another common cause of lower soil pH is when the paddock has excess nitrogen in the soil.

For example, if a crop/ pasture cannot utilise all the nitrogen applied through nitrogen or ammonia fertilisers, or the soil has abundant organic matter available; it produces a lot of its own nitrogen so that nitrogen is also in excess. This can happen even in a year when we don’t grow any crops. Now the question is, if there is zero crop/pasture in the paddock, and the soil is producing a lot of nitrogen, where does that nitrogen go?

When we have rainfall and mainly excess rainfall, a lot of that nitrogen converts over to nitrate. Then most of that nitrate converts to nitric acid and it strips out calcium out of the soil.

As a result, soil pH starts getting down gradually and when it goes below 5.0 (CaCl2), the productivity of even tolerant crops and pastures starts to drop. That can also lead to aluminium toxicity and a significant decline in soil microbial activity.

At the same time, most essential soil cations such as phosphorus, calcium, magnesium, and molybdenum become less available to the plants.

To get better crop and pasture production effective management of acidic soils is very important that can be achieved by adopting the combination of three management practices mentioned below.

1. Minimise acidification – by applying less acidifying fertiliser and managing nitrate movement in the soil

2. Application of agricultural lime to increase soil pH

3. Grow acid-tolerant crops and pasture varieties

Initially, to identify this issue we need a soil test in which we look at two important aspects: first soil pH and second, exchangeable cation percentage, specifically hydrogen.

The only reason we look at this is because hydrogen and calcium correlate together. Consequently, the higher the hydrogen number, the lower the pH would be.

The solution to that is to add more calcium (CaCo3) to the soil through a lime application.

Other sources like dolomite and alkali material will also help neutralise soil acidity. So, the calcium carbonate combines with the excess hydrogen in the soil to produce water, carbon dioxide and free calcium.

Lastly, using acidtolerant species can be another solution to overcome this problem. Species such as subterranean and arrowleaf clover, ryegrass, tall fescue, serradella, chicory etc. are tolerant to aluminium toxicity.

Although growing tolerant species can be a useful stopgap, it does not rectify an acid soil problem, which may get worse over time.

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