Soil acidification

Soil acidification is a process where the soil pH decreases over time. This process is accelerated by agricultural production and can affect both the surface soil and subsoil.

Queensland has more than 500,000 hectares of agricultural and pastoral land that is acidified or is at risk of acidification. The higher-rainfall coastal areas used for intensive agriculture are most at risk.

Contributing factors

Some contributing factors to soil acidification include:

• the application of high levels of ammonium-based nitrogen fertilisers to naturally acidic soils
• leaching of nitrate nitrogen, originally applied as ammonium-based fertilisers
• harvesting plant materials (plant material is alkaline so when it is removed the soil is more acidic than if the plant material had been returned to the soil).

Effects

Excessively acidic soils may lead to a dramatic decline in crop and pasture production because the pH of the soil changes the availability of soil nutrients.

Acidic soils may have some or all of the following problems:

• helpful soil micro-organisms may be prevented from recycling nutrients (e.g. nitrogen supply may be reduced)
• phosphorus in the soil may become less available to plants
• deficiencies of calcium, magnesium and molybdenum may occur
• the ability of plants to use subsoil moisture may be limited
• aluminium, which is toxic to plants and micro-organisms, may be released from the soil
• levels of manganese may reach toxic levels  
• uptake by crops and pastures of the heavy metal contaminant, cadmium, may increase.

Control measures

It is most important that soil acidity be treated at an early stage. If acidity spreads into the subsoil, serious yield reduction may occur. Subsoil acidity is difficult and costly to control.

There are a number of ways to minimise the soil acidification process, including:

• the use of less acidifying farming practices—considered when soils show signs of acidification
• applications of agricultural lime—applied to counter the acidification caused by cropping systems

More information

Read the soil acidification fact sheet.