Take home message

• Soil testing in 5 cm intervals to a depth of 20 cm allows informed decisions to be made regarding acid soil management
• If testing identifies the presence of acidic subsurface layers, incorporating adequate lime to the depth of acidity can provide a rapid fix
• If deep incorporation is not an option, liming soil above the acid layers to target pH_Ca 5.8 is needed to help move the lime effect to the depth of acidity
• The economic response to lime depends on the extent of soil acidity, where it is in the soil profile, the effectiveness of the lime application, seasonal conditions and what crops are then grown.

Introduction

Soil acidity is common in south-eastern Australia. Whilst some soils may be naturally acidic, agricultural production will act to decrease soil pH. The soil becomes more acidic due to acidifying processes of the nitrogen cycle and removal of agricultural products like grain, hay, animal meat and fibre.

To address the acidification caused by farming, lime (calcium carbonate) is added to the soil. Since the 1990s in south-eastern Australia, it was recommended to apply 2.5 t lime per hectare every 8–10 years and that the lime be incorporated into the soil to maintain soil pH_Ca just above a value of 5 (approx. pH_Ca 5.2) in the 0–10 cm surface layer. This advice was based on estimates of acidification rates of the local farming systems of the 1980s and the knowledge that below pH_Ca 4.8, the concentration of exchangeable aluminium (Al³⁺) increases and begins to cause plant performance problems.

With the adoption of no-till farming, less incorporation of lime now occurs. This traditional management has resulted in temporal amelioration of pH in the surface 0–7 cm but has not prevented the ongoing acidification in soil layers below. Acidification of these subsurface layers has been linked to poor plant production, particularly in pulse crops (Burns et al., 2018).

Standard soil sampling practices that take soil cores from the 0–10 cm and sometimes the 10–20 cm layer do not identify the true presence of acidic layers in the soil. The gradual acidification of subsurface layers in the region and ineffectiveness of traditional liming practices has gone undetected.

How do I know if there is a problem?

Because the processes that change pH occur in different layers and at different rates within the surface soil, we now know that sampling in 5 cm intervals to a depth of 20 cm allows the extent and location of acidic subsurface layers to be identified. Figure 1 demonstrates the detail of pH stratification that can be identified using 5 cm sampling depth intervals. Whilst 0–10 cm sampling would have returned a result of pH_Ca 5.2 for both soils, there are acid layers below a depth of 5 cm with pH values of pH_Ca 4.3 that may impact plants and soil biology.

Though sampling in 5 cm intervals results in 4 samples per paddock/zone, this does not have to excessively increase the cost of soil analysis. It is possible to analyse these samples for pH_Ca, exchangeable cations including aluminium and Colwell P for less than $200 per site (including GST, excluding sampling cost). This compares favourably to standard comprehensive soil tests costing around $180–200 for two 10 cm intervals (0–10, 10–20 cm), which may provide inaccurate information on the acidity status of the soil. It is worth noting that the cost of failing to correctly identify acidity can be far higher than the cost of soil analysis if an acid susceptible crop was grown, resulting in yield loss.

Figure 1. Soil pH profiles from sampling in 5 cm intervals identifying the extent and location of subsurface acidity in two acid soils (Soil A =▼ acid surface layer, Soil B = ● acidity to depth). Both soils have a 0–10 cm pH_Ca of 5.2

How do I use the information from soil tests?

The detail shown in the 5 cm interval sampling in Figure 1 allows evidence-based acid soil management decisions to be made. From research conducted in NSW over recent decades we know that the effect of lime will not move below the depth of placement if the pH is below pH_Ca 5.5 (Condon et al., 2021). In contrast, when lime is applied to achieve a pH between pH_Ca 5.5 and 6.5, components of the lime can remain in a soluble form (bicarbonate) allowing movement to neutralise acidity further down the profile.

Using this knowledge and the data from 5 cm sampling, we can see that the examples in Figure 1 would require different liming interventions. Both soils are typical of paddocks with long-term, traditional liming programs, with elevated pH greater than pH_Ca 5.5 in the surface 0–5 cm and an acidic subsurface layer (pH_Ca 4.3) in the 5–15 cm layers. However, Soil A has a relatively shallow layer of acidity (▼ Figure 1) which then trends towards pH_Ca 5 in the 15–20 cm layer, and Soil B is severely acid to depth.

Whilst it is possible that with time, some alkali present in the surface 0–5 cm could move down to the 5–10 cm layer (as the pH >5.5 in the 0–5 cm layer), most or all will be consumed raising the pH of the 5–10 cm layer. Therefore, lime addition is required to provide enough alkali to increase pH in the 10–15 cm layer.

Subsurface acidity in the 5–15 cm of Soil A could be ameliorated relatively quickly if required rates of lime were applied and incorporated to a depth of 15 cm. If incorporation was not an option, application of lime to hit a target of pH_Ca 5.8 in the 0–10 cm layer would enable movement of the lime effect to the 10–15 cm with time. The problem will be overcome with either option with the difference being the time in which amelioration occurs.

Soil B (● Figure 1) will require follow-up lime applications to maintain soil pH at a target of pH_Ca 5.8 in the layers above the deeper acidic layers that are being targeted. This will require a program of monitoring the surface 0–5 and 5–10 cm layers and adding lime whenever the pH drops below pH_Ca 5.5, with the understanding that acidity will remain at depth until such time that enough alkali is added to address that acidity. On a severely acidic site at Book Book, NSW this amelioration took 18 years (Li et al., 2019), though more recent research with FarmLink and Holbrook Landcare Network have reported more rapid changes to depth on less acidic soils albeit over the last few wet years (2021-2022). Regardless, liming to a target of soil pH_Ca 5.8 in the soil above acid layers may be a suitable and effective management intervention for the long term.

The knowledge of what acidity is present and where it is in the profile is necessary to develop an appropriate liming strategy and better match soil conditions to plant requirements and the growers’ cropping programs. However, ongoing monitoring is also necessary to ensure that any lime application has achieved the desired outcome, and to guide re-liming intervals, ongoing lime requirements and selection of suitable crops.

Crop response and return on investment

It is important to remember that crop types and cultivars vary in their sensitivity to soil acidity and that the nature of soil acidity differs in terms of severity and where in the profile it occurs. The GRDC investment in acid soil research in southern NSW spans sites at Morven and Methul, with severe acidity that would normally be limed in traditional practice, and at Canowindra and Burrumbuttock on soils that have not yet reached pH values to limit production (Table 1). The latter have been included to study the benefit of proactive management of soil acidity, that is, to fix it before it becomes a yield limiting problem. Hence, the more acidic sites required greater lime rates to achieve targeted pH outcomes. Canowindra had relatively more clay than other sites and whilst acidity was not severe, it had high pH buffering capacity and the calculated lime rate required was relatively high for the modest pH changes required.

The proactive approach adopted at Canowindra and Burrumbuttock is particularly relevant if incorporation is not an option. If liming were delayed until the subsurface pH drops to critical values in layers below 10 cm (i.e. pH_Ca <4.8), crop performance may be compromised, and amelioration efforts will become increasingly difficult, more expensive and less efficient.

Table 1. Soil pH_Ca measured in 5 cm intervals to a depth of 20 cm in unlimed plots (control) at four NSW experimental sites, and the rates of lime applied to raise pH_Ca to 5.2 (0–10 cm) or 5.8 (0–10 cm) at those sites*.

As expected, the crops grown on severely acid soils (Morven and Methul) responded to lime applications (Table 2). Whilst lime should be considered a capital investment, the benefits of which span multiple years, the yield response at Morven paid for the lime investment in one year.

At Methul, it is important to note the 2022 lentil crop responded to lime application even though lime was applied and incorporated with a speedtiller in May of that year, just before sowing. This did not allow enough time for the lime to increase pH in the most acidic layers. Despite this, the yield response recorded from the first three years of the trial covered the cost of the lime. This outcome was possible even with moderate yield increases (6–10%) owing to the high value pulse and the probable nitrogen benefit from that pulse to the barley yield in the following year contributing to the measured yield response.

The amelioration of soil acidity allows acid sensitive pulses to be grown, effectively nodulate and fix nitrogen, so capitalising on the investment in lime. The system benefit of improved nitrogen fixation by legumes grown on ameliorated soil is likely to be substantial and should result in economic savings from lower input cost of fertiliser nitrogen. Continuation of crop rotations limited to selection of acid tolerant species may limit the potential financial benefits from the ameliorated soil. Furthermore, the combined effect of amelioration and legacy effects of pulses produced profitable agronomic outcomes for wheat grown in 2024 (Table 2). Although the 2024 lentil crop was impacted by frost and moisture deficiency during grain fill, lime once again provided significant yield increase over the unlimed control.

The sites selected to study proactive management (Burrumbuttock and Canowindra) did not show significant yield response in the relatively short-term experiment. However, unlimed plants showed sub-clinical acid soil toxicity symptoms compared to limed plots, including reduced root development in canola at Burrumbuttock and stunted root systems in barley and reduced nodulation in chickpea at Canowindra. This warrants long term monitoring of plant response and soil fertility at these sites, particularly the comparison between the unlimed soils and those being managed to prevent formation of acidic subsurface layers.

It is also worth noting that the 2023 and 2024 seasons had below average rainfall from June to November resulting in water limited yield outcomes of experiments. The Methul site was frost affected in 2024, with the lentil plants in the limed plots showing less severe symptoms of frost damage.

Table 2. Grain yield from four NSW experimental sites with replicated treatments: unlimed control, limed to pH_Ca target of 5.2, and limed to pH_Ca target of 5.8. Statistically significant yield increases are indicated by percentage increase relative to the control (%) being listed in brackets, with the gain in revenue ($/ha) due to liming presented based on estimated commodity price.

Conclusion

Acidification is an outcome of agricultural production. It needs to be monitored and managed to prevent long term degradation of the soil resource. To effectively manage soil acidification and acidity in our cropping systems, soil sampling in 5 cm intervals to a depth of 20 cm is imperative. The data gathered by such sampling allows for tactical use of lime, and incorporation where required, to effectively manage subsurface acidity, and arms growers with confirmation of the effectiveness of their acid soil management strategies.

Significant crop yield response from high value crops sown onto severely acidic soils soon after lime application provided rapid payback on the amelioration investment, under good growing conditions. Reliability of yield from these crops is likely to improve as the condition of the subsurface layers improve. The removal of soil acidity as a constraint to production broadens the crop options available to growers allowing them to capitalise on higher value crops or crops that provide additional system benefits.

References

Burns HM and Norton MR, (2018). Legumes in acidic soils. Maximising production potential in south eastern Australia. Grains Research Development Corporation, Canberra.

Condon J, Burns H, Li G, (2021) The extent, significance and amelioration of subsurface acidity in southern New South Wales, Australia. Soil Research 59, 1-11.

Li GD, Conyers MK, Helyar KR, Lisle CJ, Poile GJ, Cullis BR, (2019) Long-term surface application of lime ameliorates subsurface soil acidity in the mixed farming zone of south-eastern Australia, Geoderma 338, 236-246.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. This research was possible in collaboration with researchers from FarmLink, Central West Farming Systems, Holbrook Landcare Network and the NSW Pulse Research team led by Mark Richards and the support and long-term commitment of collaborating growers.

Contact details

Jason Condon
Charles Sturt University
School of Agriculture, Wagga Wagga, NSW 2678
Email: jcondon@csu.edu.au

Date published
February 2025