Soil Acidity
Lime application rates and incorporation techniques may require reviewing in light of recent survey work by NSW DPI. Many of the points below are not new, but it is important to revisit such topics, as they can be forgotten. Nearly all modern farming practices have an acidifying effect on the soil. The leaching of Ammonium Nitrogen below the rootzone, plus the removal of plant material are major contributors. The more produce that is removed, the greater the rate of acidification.
Lime has long been known to increase soil pH, reducing the negative impacts of Aluminium ions on sensitive plants. Lime applications have mostly targeted pH 5.2 (CaCl) based on soil tests taken from 0-10cm depth. Two important messages from the original research appear to have been forgotten:
There is no movement of alkalinity down the soil profile, unless the topsoil pH is greater than pH 5.5. Liming to pH 5.2 will ameliorate soil acidity in the layer of incorporation but will not improve the soil below. This potentially restricts root growth, especially in sensitive crops, depending on soil type.
Liming without incorporation will do very little to ameliorate soil acidity other than at the soil surface.
Sub-surface acidity at 10-15cm was found in many soils surveyed by NSW DPI, including many with recent applications of lime. Testing 0-10cm in most cases would not have identified this issue. It is likely that this acidity is restricting production of canola, barley and wheat, even though symptoms may not be obvious. This “acid throttle” will definitely impede the production of acid sensitive species such as faba beans, lentils and chickpeas, as shown below.
Most legume rhizobia prefer a pH of 5.0 to 6.0, so nodulation will be reduced in many soils, even on plants that can grow in acidic soils. Acid tolerant rhizobia strains are under development. However these are not a silver bullet, plus they are still a number of years away and do not replace the need to ameliorate soil acidity.
To identify an acid throttle, soil testing for pH should be carried out at 0-5, 5-10 and 10-15cm intervals down the profile. Where pH is low and Aluminium % is high at depth, lime applications should aim to reach pH 5.5, which is likely to require an increase of 0.5 to 0.8 t/ha above applications targeting pH 5.2.
Any applications of lime must be incorporated, ideally the year prior to growing canola, barley or sensitive pulse crops. Incorporation should result in thorough mixing within the top 10-20 cm. Incorporation with narrow points or a speed tiller may be less effective than more aggressive forms of cultivation.
