How do I manage acidic soils?

Prevention and amelioration

Management needs to aim at maintaining a profitable farming system and improving soil condition to minimise the degradation caused by soil acidification.

Managing acid soils is most important for the long term sustainability of farming systems and the soil resource. 
Effective management of acid soils will involve a combination of these three management approaches:

1. Minimise acidification
2. Apply a liming product (amelioration)
3. Use acid tolerant crop and pasture varieties

It is understood that agricultural production is an acidifying process. For your chosen production system, management options available to minimise soil acidification include the types, rate, placement and timing of fertiliser application(s).

The application of limestone or other alkali material that neutralises soil acidity is the only practical amelioration technique. Its effectiveness is determined by product quality, rate and application method.

Acid tolerant plant species can be used to maintain some production whilst implementing amelioration.  It is important to remember that acid-tolerant plants can also be responsive to increases in soil pH.  In the case of legumes it is important to consider not only plant impacts but also the possible effects of soil pH on nitrogen fixation.

There can be considerable hidden costs in choosing a tolerance rather than amelioration pathway to manage soil acidity.

Minimise acidification

Where soils are at risk of becoming acidic the future impact of soil acidity can be reduced, but not eliminated, by slowing the rate of acidification. This can be achieved by choosing fertilizers that are less acidifying and managing the movement of nitrate nitrogen in the soil.

1. Use less acidifying fertilisers

2. Reduce the movement of nitrate nitrogen

Apply Lime

To get the best effect, high quality lime should be finely crushed, evenly spread and incorporated into the part of the soil where acidity is a problem (or as deep as practicable).
Lime moves very slowly through the soil.  Incorporation is recommended where possible and where erosion risk is low. Where soil acidity occurs at depth incorporation will hasten the liming effect. 
Any liming needs to be done at least a season prior to sowing a sensitive crop to ensure lime has had time to react. Lime needs to be applied to the entire layer where the pH is suboptimal to ensure amelioration.  
The amount of lime required should be determined using pH in CaCl₂ soil test results from an accredited lab and the table below. Soil samples should be collected using a valid sampling strategy.

Historically lime was applied to achieve a soil pH of 5.2 (0-10cm) to remove the effect of toxic aluminium on plant production. However this will not stop acidification further down the soil profile, nor is it optimal for most commercial rhizobia strains. There are significant benefits when pH is maintained above 5.5. 
Where acidity occurs below the depth of lime incorporation the neutralising effect will only progress deeper into the soil profile if the surface soil is maintained above pH(CaCl₂) of 5.5

To ensure that a response to liming is likely, particularly when putting in a lime trial or test strip, make sure:

1. Areas and depths of acid soil (<5.5) using test results (eg at 0-5, 5-10, 10-15, 15-20 cm etc depth intervals) are identified.
2. Soil pH is the most limiting manageable soil factor to plant growth.
3. The liming product to be used is sufficiently fine and with a high neutralising value, or is the most cost effective product available.
4. The lime rate is sufficient to achieve a soil pH(CaCl₂) of 5.5
5. Take-all  has been controlled when planting wheat or triticale.
6. The timeframe is long enough for applied lime to raise the pH to the desired level, especially where growing plants sensitive to acidic soils.

Grow acid tolerant crops and pastures

If a paddock is already acidic, particularly where both the surface and subsurface soils are acidic, the economic and offsite impacts can be reduced by growing acid tolerant crops and pastures. 
However acidification will continue despite the use of acid tolerant plants unless other management options are used to ameliorate soil acidity and or prevent further acidification.