Subsoil Acidity

The problem

Most Western Australian wheatbelt soils are acid down to at least 20cm in sandy loams, and can extend down to 60cm in deep sandy soils. In some cases, acids were formed by the native vegetation (e.g. wodgil soil). However, normal farming methods also cause both topsoil and subsoil to become acid.

Acid in the soil starts to dissolve aluminium as pH drops below 5.5. This dissolved aluminium is toxic and destroys the growing tips of plant roots. Affected roots can’t reach nutrients and moisture in the subsoil, resulting in poorer growth and early haying off at the end of the season through moisture stress. Some plants (e.g. canola, barley, some wheat varieties, peas, faba beans and medics) are very sensitive to dissolved aluminium and can be affected by the relatively low levels that occur when the pH is in the low 5’s to high 4’s. Dissolved aluminium also combines with phosphate making it unavailable to crops and pastures.

The research

Subsoil acidity is traditionally treated in other areas of the world by maintaining topsoil pH around 6.0 – 6.5 through regular liming. To find out if topsoil liming could also treat subsoil acidity in WA soils, Aglime of Australia surveyed 11 of their long term Aglime trials and 2 of Agriculture WA’s trials located in the central, eastern and northern wheatbelt. Sandy loam soil results Sandy loams (e.g. tammar) usually have very acid topsoils (around pH 4.5) and are extremely acid (around pH 4) 15 to 20cm deep. Below 20cm the pH increases as the soil becomes more clayey. When 2.5t/ha was first applied to a sandy loam at Goomalling, the topsoil pH increased from 4.6 to 6.1. After 7 years the topsoil pH had dropped by 1 unit but the pH increased as far as down as 30 to 35cm (Figure 1).

Deep yellow sand results

Deep yellow sands often have moderately acid topsoils (around pH 5) with extremely acid subsoils (around pH 4). At Mingenew 1.25t/ha of Aglime first increased the topsoil pH from 4.9 to 5.8. After 4 years the topsoil pH had declined slightly and the subsoil pH increased by about 0.1 units down to 40cm (Figure 2).

More recent research by the University of WA and others indicates that at topsoil pH levels below 5.3 all the applied lime is being consumed by acid. However, as the topsoil pH increases above 5.3, some Aglime is available to slowly move into the subsoil. This means that maintaining topsoil pH around 5.0 will not remedy subsoil acidification. However, maintaining soil pH between 5.5 and 6.0 does allow for excess Aglime to move into the subsoil and treat or prevent subsoil acidity.

Conclusion

The survey clearly demonstrated that Aglime applied to the topsoil could move into the subsoil and neutralise acidity. It is evident that WA farmers need to combat the continuing development of subsoil acidity by regular liming.

From research by Aglime of Australia and others, it is apparent that soils at risk of subsoil acidity should be maintained between pH 5.5 and 6.0 in the topsoil by regular liming. This would provide sufficient excess Aglime to move down into the subsoil and treat subsoil acidity.

Recommendation

Treat subsoil acidity by liming regularly to maintain topsoil pH between 5.5 and 6.0.

Action to take

• Find out if your subsoils are acid. Sample your own soils or use Precision SoilTech’s soil sampling and soil pH testing service

• Apply Aglime to the topsoil regularly