Feeding the future – sustainable agriculture
Box 1 | Acid eats into farm incomes
Soil acidity is a problem that can creep up on farmers. It develops slowly and may not become obvious for years after the soil pH falls to levels that start to affect crop or pasture growth. The main adverse effect on plant production comes when the pH falls to levels that result in aluminium or manganese in the soil dissolving and causing root damage. (Some plant species are more tolerant than others; sometimes farmers get their first hint of an acid soil problem when they see less tolerant plants such as lucerne and canola performing less well than, for example, lupins or wheat.)
Acid soils are now recognised around the world as one of the key threats to sustainable agriculture. In Australia costs are estimated at more than one billion dollars a year. 33 million hectares of farming land are highly acidic (pH less than 4.8) and 58 million hectares are moderately to slightly acidic (pH 4.9-6).
Farming inevitably tends to make soil more acidic. On undisturbed land, nutrient take-up by plants increases soil acidity, but when plants die the extracted alkalinity returns to the soil. Farming disrupts this cycle because the plant matter is removed, either as crops or eaten by livestock. In the nitrogen cycle, the conversion of organic nitrogen from plant residues (or dung and urine) to nitrate in the soil is an acidifying step, but the acid is neutralised when nitrate is taken up by the plant. In agriculture, sometimes these processes are out of phase (eg, nitrate accumulates when there are no plants to take it up). Nitrate is soluble and easily leached. If leaching occurs, the nitrogen cycle is disrupted and valuable nitrate is lost to the production system, leaving behind the acidity generated when the nitrate was formed.
Fertilising crops or pastures hastens the acid build-up. Even fertilisers such as superphosphate that do not directly affect acid levels may indirectly increase the rate of acid addition to the soil if they increase plant growth and hence agricultural production. Growing legumes to boost soil nitrogen increases production but also increases the risk of soil acidification if the supply of soil nitrate exceeds the capacity of the crop or pasture to absorb nitrate. Leaching of nitrates from the soil is one reason why acidity tends to build up faster in high-rainfall farmland than in drier regions.
Topdressing with lime remains the most effective remedy for soil acidity, but researchers calculate that more than 2 million tonnes of lime would have to be applied just to treat the worst 1.5 million hectares. This is many times the amount used each year in agriculture in Australia.
Researchers can help address the problem by producing more acid tolerant crop and pasture plants. Australian scientists have found that the roots of tolerant wheats excrete a compound that makes soil aluminium harmless, and that a single gene is largely responsible for this process. This opens up the prospect of increasing the tolerance of other crop and pasture plants via genetic engineering.
In another development, scientists have found that tree planting can slow, or even reverse, the acid build-up in some farm soils. Trees help in a number of ways. For example, they extract nutrients from deep layers in the soil – away from the roots of crop and pasture plants – and return them to the surface in fallen leaves and twigs. And they take up nitrates that would otherwise be leached from the soil. Changes to farming practices – such as reducing hay cutting and retaining crop stubble – can also assist.
Related sites
Acid soils – a ticking time bomb? (CSIRO, Australia)
Management of soil acidity in agricultural land (Department of Agriculture and Food, Western Australia) Soil acidification – the cause (by W.M. Porter, The Riverina Outlook Conference 1981, Australia)
Soil quality indicators: pH (United States Department of Agriculture)
Page updated September 2009.