Acid soils developing ‘below plough depth’

Soil scientist Mark Conyers.

Soil scientist Mark Conyers.

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 of Australian land.

NSW Department of Primary Industries soil scientist, Mark Conyers, says the answer may lay in “biological amelioration”, using nitrate fertilisers.

“The form of nitrogen supplied to plants plays a key role in acid or alkali production.”

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

Dr Conyers says some of Australia’s best farming soils are turning acidic at a depth of more than 30 centimetres. In many cases, acidification rates are greater in the subsoil than in the topsoil.

“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.”

Dr Conyers is part of a team from NSW DPI and La Trobe University which is investigating techniques to ameliorate subsoil acidity. They include Dr De Li Liu (DPI), Associate Professor Tang (La Trobe) and Research Fellow Zaman Nuruzzaman (La Trobe, based at DPI Wagga).

Dr Conyers said subsoil acidity is partly caused by plant roots absorbing more positively than negatively charged atoms.

“The excess uptake of cations over anions is the major cause of subsoil acidity, especially when legumes are a component of farming systems.”

“Our strategy is to manage the balance in the uptake by plant roots of cations and anions, thereby stimulating root growth in acid subsoil.”

Dr Conyers said the most immediate impact of subsoil acidity is 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 that 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.”

Dr Conyers said the form of nitrogen supplied to plants plays a key role in the cation/anion relationship in plants, and hence in acid or alkali production.

“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 will greatly influence 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 ameliorating subsoil acidity and assess its impact under different soil types and climatic conditions.

Further information

Collaborating agencies

Latrobe University

Funded by

  • NSW DPI
  • Australian Research Council

Scientist

Mark Conyers
Wagga Wagga Agricultural Institute