Nitrate-based fertilisers could hold the key to addressing a deep-seated environmental problem - acid subsoils, which are estimated to affect at least 23 million hectares across Australia.

NSW Department of Primary Industries soil scientist, Mark Conyers, says some of Australia’s best farming soils are turning acid at a depth of more than 30 centimetres. In many cases, acidification rates are greater in the subsoil than in the topsoil.

He says the answer may lie in biological correction using nitrate fertilisers.

"The form of nitrogen supplied to plants plays a key role in acid or alkali production," Dr Conyers (pictured) said.

The potential of nitrate-based fertilisers for overcoming subsoil acidity was being investigated in a new three-year $297 000 project funded by the Australian Research Council.

"In southern NSW, even fertile red earth soils, which are regarded as the good cropping soils, are developing an acidic layer below the normal plough depth,” he said.

Dr Conyers is part of a team from NSW DPI and La Trobe University investigating techniques to ameliorate subsoil acidity, other members including Dr De Li Liu (DPI), Associate Professor Tang (La Trobe) and Research Fellow Zaman Nuruzzaman (La Trobe, based at DPI Wagga) Dr Conyers said the most immediate impact of subsoil acidity was to decrease root length, resulting in plants developing a poor root system and absorbing less nutrients and water.

"In most parts of Australia’s grain belt, moisture in top soil is depleted late in the growing season, meaning that plants are forced to rely on water and nutrients from deeper soil layers.

"If the subsoil is acidic, plant growth is limited and so are final grain yields."

Dr Conyers said the yield loss will not necessarily be immediately apparent, but as the subsoil turns more acidic he estimates a farmer will lose 500 to 1000 kilograms yield from his crop each year.

He added that current methods to ameliorate soil acidity by applying lime materials and gypsum had limited value for subsoils and were impractical for broadacre application.

"Organic materials such as undecomposed plant materials, composts, manures, peats and coal products may be effective, but are only practical if available in large quantities.

"Chemical amelioration reduces acidity but is difficult with subsoil acidity, particularly where rainfall is low and profit margins small.”

"Soil alkalisation is associated with nitrate-based (NO3) nutrition, and acidification is associated with ammonia-based (NH4) nutrition."

Dr Conyers said the balance in these two types of nitrogen in the soil greatly influenced alkalinity in a plant.

"Our research indicates that nitrate is able to slow and possibly reverse the acidification process, especially subsoil acidification in low to medium rainfall areas.

"Better management of other types of nitrogen fertilisers may also minimise subsoil acidification."

The research project will seek to quantify the effects of this new method of reducing subsoil acidity and assess its impact under different soil types and climatic conditions.

Contact Mark Conyers, Wagga Wagga, 02 6938 1830, mark.conyers@dpi.nsw.gov.au.