Soil Acidity – Queensland

Key Points

Soil pH is a measure of the concentration of hydrogen ions in the soil solution. The lower the pH of soil, the greater the acidity.

Plant growth, and most soil processes, are favoured by a pH range between 5.5 and 8.

Agriculture can accelerate soil acidification, but this can be controlled through appropriate fertiliser/crop management, and lime application.

Background

Queensland has more than 500 000 hectares of agricultural and pastoral land that has acidified or is at risk of acidification. Soils most at risk are lighter-textured sands and loams with low organic matter levels, and the naturally acidic red clay loam soils commonly found in areas such as the South Burnett and Atherton Tableland. Soils least at risk are the neutral to alkaline clay soils (e.g. brigalow soils and the black clay soils of the Darling Downs and Central Queensland).

Acidic soils cause significant losses in production, and where the choice of crops is restricted to acid tolerant species and varieties, profitable market opportunities may be reduced. In pastures grown on acidic soils, production will be reduced and some legume species may fail to persist.

Soil pH

Soil pH is a measure of the concentration of hydrogen ions in the soil solution. The lower the pH of soil, the greater the acidity. A soil with a pH of 4 has 10 times more acid than a soil with a pH of 5 and 100 times more acid than a soil with a pH of 6. Plant growth and most soil processes, including nutrient availability and microbial activity, are favoured by a soil pH range of 5.5 – 8.

Figure 1: Rule-of-thumb aluminium (Al) toxicity.

Effects of soil acidity

Nutrient availability

In very acidic soils, all the major plant nutrients (nitrogen, phosphorous, potassium, sulfur, calcium, and also the trace element molybdenum) may be unavailable, or only available in insufficient quantities. Plants can show deficiency symptoms despite adequate fertiliser application.

Microbial activity

Low pH in topsoils may affect microbial activity, most notably decreasing legume nodulation. Rhizobia bacteria are greatly reduced in acidic soils. Some pasture legumes may fail to persist due to the inability of reduced Rhizobia populations to successfully nodulate roots and form a functioning symbiosis.

Aluminium toxicity

When soil pH drops, aluminium (Al) becomes soluble. When in a soluble form, aluminium retards root growth (figures 1 & 2). Poor crop and pasture growth, yield reduction and smaller grain size occur as a result of inadequate water and nutrition. The effects of aluminium toxicity on crops are usually most noticeable in seasons with a dry finish as plants have restricted access to stored subsoil water for grain filling.

Figure 2: Roots of barley grown in acidic subsurface soil (right) are shortened by aluminium toxicity.

Causes of soil acidity

Acidification can occur under natural conditions over thousands of years. However, agriculture can accelerate this naturally occurring process. Practices that accelerate acidification include:

Applying nitrogen fertilisers, particularly ammonium based fertilisers, at rates in excess of plant requirements.

Continual removal of plant and animal produce from the paddock. The impact is greatest where a large quantity of material is removed, as in the production of silage, hay and sugarcane.

Management of acidic soils

Soil testing

Ideally, soil samples should be taken when soils are dry and have minimal biological activity. Soil samples should also be taken from a number of locations across the paddock, as pH may vary from place to place. Samples should be taken at the surface (0 – 10 cm) and in the subsurface (50 – 60 cm) to detect subsurface acidity, which may underlie topsoils with an optimal pH. Samples need to be properly located (e.g. GPS) to allow monitoring. Sampling should be repeated every 3 – 4 years to detect changes and allow adjustment of management practices.

Farming practices to reduce acidification

It is most important that soil acidity be treated early. If acidity spreads to the subsoil, serious yield reduction may occur. Subsoil acidity is difficult and costly to ameliorate. Farming practices recommended to minimise acidification include:

Match nitrogen fertiliser inputs to crop demand. Soil testing should be carried out to ensure that fertiliser rates match plant requirements.

Use forms of nitrogen fertiliser that cause less acidification. Table 1 summarises the acidifying effect of different nitrogen fertilisers. Nitrate-based fertilisers such as calcium nitrate and potassium nitrate are the least acidifying, but their higher cost limits their use to high value horticultural crops.

Apply nitrogen in split applications, if practicable. Application of a crop’s entire fertiliser needs at planting time may contribute to soil acidification by allowing the leaching of nitrate nitrogen before the crop roots have developed.

Sow early after fallow to ensure more rapid utilisation of available nitrogen.

Grow deep-rooting perennial species to take up nitrogen from greater depths.

Regularly apply lime to counter the acidification inherent in the agricultural system.

Grow acid tolerant crops or crop varieties more tolerant of acid soils.

Irrigate efficiently to minimise leaching.

Table 1: Acidification potential of nitrogen fertilisers assuming

that some leaching loss of applied nitrogen occurs.

Fertiliser
Acidification Potential

Calcium nitrate, potassium nitrate
Low

Nitram, urea, animal manure
Medium

Ammonium sulphate, MAP, DAP
High

Lime or dolomite application

When soils are too acidic for a particular crop, lime or dolomite can be used to increase soil pH to the desired level. The amount of lime or dolomite required to correct an acidic pH will depend on the soil and the crop.

Soils with high organic matter and clay content will be more resistant to changes in pH and will require larger application rates. To obtain an estimate of the amount of lime required to correct an existing soil acidity problem, a soil test called ‘Lime Requirement’ or ‘Buffer pH’ should be requested. The test is used to give a lime recommendation to raise the soil pH of the surface 10 cm of one hectare of soil to a target pH that will not limit crop yield. In general, a target pH of 5.5 is suggested.

Once the target soil pH is reached, additional lime or dolomite may be required depending on the crop. The acidifying effect of cropping systems is related to the amount of material removed at harvest, the amount and type of fertilisers normally used and the amount of leaching that occurs. Table 3 gives an indication of the amount of lime required to counter the inherent acidification associated with some cropping systems. There are opportunities to decrease these lime rates by adjusting nitrogen fertiliser rates or form of nitrogen fertiliser used.

Table 2: Lime required to counter the acidification inherent in some cropping systems.

Cropping System
Lime Required (t/ha/yr)

Summer crop – winter fallow
0.1

Crop – pasture rotation
0.1

Sugarcane
0.2

Grass pasture for hay production
0.3

Fact Sheets Soil Acidity - Qld