Sub Surface Acidity – A widespread challenge

by

Todd Buck

Soil acidity affects approximately 50 million hectares, or about 50 percent of Australia’s agricultural land. Through the extensive mapping of pH variability (0-10cm surface soils), we know that pH varies significantly across paddocks (average of 1.5 pH units). Alongside the variation across the paddock, we also know that pH varies with depth through the soil profile. Subsurface acidity and thin acid layers are increasingly being recognised by the industry as a widespread and significant constraint to crop production.

Through research work with SAGIT, MLA, Farmlink, SFS as well as commercial sampling, we have explored sub-surface acidity across multiple regions and farming systems. From the Mid North High Rainfall Zone in SA, to SW Victoria and the Riverina NSW.

To identify acid throttles and subsurface acidity soils are sampled at a depth of 0-20cm, segmented into 4 equal parts 0-5, 5-10, 10-15, and 15-20cm. The location of samples varied across projects but considered known soil pH (0-10cm), cation exchange capacity (CEC) as well as the results of EM38 surveys.

Image above: Example of Soil pH in Mid North High Rainfall Zone in South Australia

This cropping paddock highlights the variation we frequently see in pH down through the soil profile.

In this set of results, the samples are grouped based on the CEC (0-10cm) which is often a good surrogate for pH buffering capacity. Where the CEC was less than 10 cmol(+)/kg the pH decreased from 5.7 in the surface 5cm to below 4.5 in the 5-15cm layers. In contrast where the CEC was greater than 30cmol (+)/kg there was no acidification in the sub-surface layers with pH greater than 6.5.

The above image highlights that similar results can be observed in pasture systems from SW Victoria.

Conclusion

The segmented sampling at 5cm intervals highlighted the pH stratification in the surface layers with standard 0-10cm sampling unable to detect potential acid throttles in the 5-15cm depth, or sub-surface acidity down to 20cm. Understanding sub-surface acidity is essential to ensuring that sufficient lime is applied to ameliorate the problem. Research shows that surface pH needs to be maintained above pH=5.5 for downward movement of the neutralising effect of lime.  Even when these higher pH levels are maintained, amelioration past the surface soils will take time. 

0-10cm grid mapping is a proven method for mapping the variation in pH across the paddock.

Once you have mapped this variation targeted sampling to investigate potential acid throttles and sub-surface acidity is essential and can be used to further refine your VR liming strategy.

Precision Agriculture provides surface and subsurface nutrient mapping in Eastern Australia to assist growers and agronomists test their soils to develop programs to address acidity issues

Further reading: Case Studies

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