A practical approach to sub-surface acidity in the mid-north

The primary objective of this project was to increase the capacity of producers and advisors in the Mid-North region of South Australia to cost-effectively measure and manage sub-surface acidity and acid throttles.

Producer: Ben Plueckhahn
Location: Manoora, SA
Annual Average Rainfall: 480 mm
Soil Type: Red Sodosol/Black Vertosol
Enterprise: Dryland Cropping – Beans – Durum wheat – Wheat

Subsurface acidity is increasingly recognised as a widespread and significant constraint to crop production. Soil acidity affects approximately 50 million hectares, or about 50 per cent of Australia’s agricultural land (NLWRA 2001). In South Australia it’s estimated that more than two million hectares of agricultural land is susceptible to soil acidification, and under current farming systems, the area prone to acidification is expected to double over the next 40 years. Soil acidity issues have historically occurred in the higher rainfall areas including the South East, Adelaide Hills, and Kangaroo Island, but are becoming more widespread in the medium and high rainfall cropping areas such as the Mid North and on the Eyre Peninsula (GRDC Acid Soils SA).

While there is increasing awareness of the potential implications of sub-surface acidity and thin acidic layers (often known as acid throttles) to crop production, there has been minimal research on how to measure or spatially define these constraints.

The primary objective of this project was to increase the capacity of producers and advisors in the Mid North region of South Australia to cost-effectively measure and manage sub-surface acidity and acid throttles. Specifically, the aim was to investigate the variability in sub-surface soil pH and its correlation with topsoil pH maps, whether alternative soil data layers such as electrical conductivity, elevation, and radiometrics can be used to more accurately target strategic sub-surface sampling, and what has been the effectiveness of historic lime applications in ameliorating soil acidity throughout the profile?

Soils were sampled at nine locations in one of Ben’s paddocks to a depth of 0-20 cm, segmented into four equal parts, i.e. 0-5, 5-10, 10-15, and 15-20 cm depths. Locations were selected according to data from an EM38 survey and an existing surface pH map, largely avoiding areas where the surface pH was not acidic, as it was considered unlikely that subsurface acidity would be present where surface pH was at or above neutral.

The separate soil segments were analysed for pH (CaCl2), exchangeable cations, and CEC.

Figure 2. Showing EM38 ECa and sample location points.
Figure 3. Showing 2020 surface soil pH and soil sample location points.

From the pre-existing 2020 pH mapping, about 37 ha of the paddock (ca. 50%) had a surface pH of 5.5 or less, and another 11 ha between 5.5-6. In response to the pH mapping, variable rate lime was applied in 2020 at 2T/ha on the areas with pH 5-6, and 4T/ha on the areas with pH < 5.0, and a further 1.5T/ha was applied on the latter in 2021.

Soil pH in the 0-5 cm layer of the nine locations sampled ranged from 5.3 to 6.8, i.e. at or somewhat above the agronomic optimum. However, subsurface acidity was present in eight out of the nine locations, with pH decreasing to below 5 at five locations and below 4.5 at two locations. The change in pH between the surface 0-5cm and the 5-10cm layers was up to 1.5 units in multiple locations. While there was a slight increase in soil pH between the 5-10 and 10-15cm layers, pH was still below 5 at several points in the paddock. Given the lime applied over the previous 2 years, this highlights the slow rate at which lime works its way down the soil profile, and the impact of hidden acidity below the usual sample depth which means the calculated lime requirement is an underestimate of the true amount needed.

The one location where the subsurface pH increased rather than decreased was where the surface pH was highest amongst those sampled (point C). This location falls within a part of the paddock with black vertosol soil, which is characterised by a gradual increase in soil pH with depth. Point C also had the largest CEC out of all the sampling locations, and there was a reasonably good relationship between CEC and soil pH across the paddock (r2 = 0.79).

Figure 4. Showing stratified soil pH in 5 cm increments to 20 cm depth, at nine locations in the paddock.
Figure 5. Showing the relationship between soil pH (calcium chloride) and soil cation exchange capacity, across all sample depths and locations.

The segmented sampling at 5 cm intervals highlighted the pH stratification in the surface layers, with standard 0-10 cm sampling unable to detect potential acid throttles in the 5-15 cm depth. With a typical sowing depth of 2.5-5 cm, seeds may inadvertently be placed directly above an undetected hostile layer, which is likely to impact on root development and hence grain yields, especially if more acid-sensitive crops such as pulses or canola are part of the rotation.

Ben commented that he was interested to find that subsoil acidity was so high. His father had been relatively proactive with lime over the last 10-20 years, with some of the paddock having lime applied in 1997, 2000 and 2010, and so he thought this would have moved through the profile and maintained the pH at a slightly higher level.

Research has shown 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. More rapid amelioration of pH to depth of the acidic sub-surface soils requires incorporation to at least 10-15cm depth. This would need to be conducted with care however, as the subsoil for the Red sodosol is typically quite hostile, with potential issues including high boron levels, high sodium levels, and salinity. Lime rates also need to be increased where sub-surface acidity is identified due to the greater volume of soil requiring lime.

This project was supported by the Mid North High Rainfall Zone grower group, through funding from the South Australian Grain Industry Trust. We would like to thank all of the landholders involved in the project for their cooperation and support.

Click here to download a printable copy of this trial.

Previous ArticleNext Article