Take home messages

• Subsurface and subsoils are likely to be acidifying, so monitor soil pH profiles to assess where acidity issues exist.
• Proactive, preventative management of topsoil pH with regular lime addition to keep pH above 5.5 remains the most cost-effective solution for addressing subsoil acidity.
• Prevention of acidity doesn’t necessarily mean high rates of lime are needed. Be guided by starting pH when making lime rate decisions and manage for possible micronutrient deficiencies when lime is used on light textured soils.

Background

Southern Farming Systems (SFS) has been involved in testing methods of ameliorating acidity and measuring the constraints of low pH to yield production since 2014. This has enabled us to create production response curves to soil pH. Through monitoring soil acidity, we realised that subsurface acidification (10-20cm) was just as prevalent as topsoil acidity (0-10cm) and occasionally, where topsoils (A horizons) were deep, soil acidity extended to 30cm but rarely beyond this. This recognition of subsurface acidity triggered further investigation to understand the reasons why, and what we needed to do to both address it and prevent it. The results of our trial work underpin the calculations of our new LimeAssist calculator. It is a lime decision tool designed to help advisors and growers calculate the economic returns from lime applications. It is available at URL https://limeassist.sfs.org.au/ This paper discusses current research findings and our local soil acidity trial near Avoca and their implications for management.

Soil pH and its effect on yield

Soil acidity is caused by excessive hydrogen ions (H+) in the soil. Soil pH results in this paper have used the calcium chloride test which is approximately 0.7 units lower than the pH water test and is used for reporting on soil acidity due to its consistency across seasons.

Low soil pH also impacts on nutrient availability by making some desirable nutrients chemically less available to plants (nitrogen, phosphorus, potassium, sulphur, calcium, magnesium and molybdenum) and by decreasing soil microbial activity. Hence, nitrogen fixation by rhizobia, organic matter breakdown and cycling of nutrients are all reduced in acidic soils. Low soil pH can also make some nutrients more available causing toxicities (aluminium and manganese).

Target pH is ideally between 5.5 and 6.0. There is no real benefit of trying to raise pH higher, soil pH above 6.4 is alkaline. Most plants are not constrained by acidity at pH 5 or 5.5 for pulses where rhizobia is sensitive to acidity (faba bean, lucerne but lupins are an exception). The biggest impact on yield is from aluminium toxicity which starts to occur when pH falls to 4.8 and aluminium levels start to rise sharply as pH falls. This effects the root’s ability to grow and take up nutrients and water. Plant species vary in their aluminium tolerance.

Condon et al. (2021) also reports that soil acidity stress increases susceptibility to biological and environmental stresses like waterlogging, herbicide injury and disease.

Know your soil pH profile

Acidification occurs mainly where roots take up nutrients, where nitrate is leached from or ammonium fertiliser is placed. The return of plant material to the surface is like liming, in that it removes the H ions from the soil. This mismatch of where H ions are released into soil and where lime or slightly alkaline plant material is returned to soil surface means the soil pH profile varies with depth and is said to be stratified.

This means you need to monitor your soil profile to identify where the acidity is occurring so you can develop a suitable liming program. A useful way to monitor soil acidity is by testing soil in 5cm increments, (0-5, 5 10cm, 10-15cm, etc). This is because you can get acidic bands at 5 to 15cm, which can be masked by 0-10 or 10-20cm testing.

In Victoria and NSW, the main soil acidity issues are confined to the top 20cm of soil or the topsoil A horizon. We refer to acidity at 10-20cm as subsurface acidity, as subsoil is generally defined as the B horizon, where clay increases. In SW Victorian soils, the subsoil starts at about 30 to 35cm where clay levels increase.

When we have deeper acidity, it is where the topsoil is deeper. The results from the SFS trial near Avoca, Tasmania indicate light textured soil down to 30cm and acidity issues to depth. Ideally, further testing would be conducted to identify the extent of the acidity.

Inadequate lime rates or infrequent liming contributes to the development of subsurface and subsoil acidity developing. Lime simply runs out, ameliorating acidity in the topsoil and none is left over to address deeper acidity. Some soil types can have naturally occurring acidic clays at depth. Aluminium content is closed related to clay content and type, so sands rarely contain much aluminium compared to loams or clays.

Subsoil acidity is the worst form of acidity, as it can cut off roots supply to stored deep water, particularly at grain fill when soils are drying out.  Either reduced root growth is observed or J shaped roots are created, where roots have actively tried to avoid acidic areas.  The soil depth makes it harder and more costly to treat.

Table 1: Soil results of the SFS trial site Avoca, Tasmania.

*Relatively harmless levels, ideal is <5%

**Very high level. Highly tolerant plants 10% reduction in yield when Ext. Al 8 to 13.5mg/kg

***High level. Tolerant plants 10% reduction in yield when Ext. Al 4 to 8mg/kg

Preventing and treating subsoil acidification

Surface liming is currently the most common strategy to combat soil acidity and may ameliorate subsoil acidity in the long-term but is a slow process.

A lime trial run by NSW DPI from 1992 to 2010 (18 years), known as the MASTER trial, is one of several trials that found subsoil acidity amelioration only occurred when soil pH in the topsoil was kept above 5.5 (Li et al. 2019). Micro-fine lime was incorporated into the top 10cm and soil pH maintained above 5.5 for the trial duration to counteract acidification and lime movement. After four years, lime had moved to 15cm but advanced no further for another four years (2004), until in 2010, lime was detected at 25 and 30cm. Movement of lime was about 1cm per year. We have commonly recorded lime movement rates at 2cm per year.

There is evidence that direct liming into acidic subsoil produces a quicker and greater yield response. Lime incorporation into the subsoil occurs mainly via deep ripping and mixing. Deep ripping only ameliorates acidity immediately around the tynes to provide good pH areas for root growth, leaving acid subsoils between the rows.

SFS trials indicated that when topsoil (0-10cm) acidity is ameliorated, subsoil acidity at 10-20cm reduced yields by 3 to 4% in tolerant crops (wheat, oats, lupins) (unpublished trials) at median exchangeable aluminium levels of about 20% (pH 4.1). These yield reductions are similar to those recorded at other regional locations as part of the subsoil acidity project (Li 2021). If less tolerant crops had of instead been grown (barley, canola), then these crops would most likely have failed to thrive and had significant yield declines.

In recent liming trials (DAN00206), Li concluded proactive, preventative management of topsoil pH with regular lime addition remains the most cost effective solution for addressing subsoil acidity. Prevention is achieved by maintaining soil pH at about 5.8 and re-liming when pH falls to 5.5 at 0-10cm.

There can be yield advantages with growing aluminium tolerant cultivars or species (e.g. wheat and lupins) in the short term while waiting for lime to ameliorate subsoil acidity. However, failing to ameliorate acidity delays the inevitable yield decline and eventually acidity will fall to a level where it is not easily reversed (soil structure declines at pH <4.0 via clay dissolution). In addition, soil pH affects many plant and soil functions, not just aluminium toxicity.

Local liming and lime trial results

A trial site near Avoca, Tasmania was established in July 2019. The randomised and replicated trial included seven treatments: Untreated control; Aglime rates, 3, 6 and 9t/ha; and Ozcal prilled lime at 300, 600 and 200kg/ha. The 200kg/ha rate was applied 2019 and 2021.

Rates of prilled lime were recommended at 10-20% of Ag lime rates. The grower rate on the surrounding paddock was 6t/ha and so, higher and lower rates were chosen. Site was disced, lime treatments applied, then power harrowed to create a fine seed bed in 2019. Both Ag lime and prilled lime products were applied by hand to plots 4m by 10m.

The agricultural lime used was a hard calcite rock lime from Mole Creek (high in calcium, low in magnesium) and tested by Agrifood technology with a neutralising value (NV) of 92.8%. Particle size distribution in Table 2 showed most of the lime (42%) was greater than 1mm. Hard rock limes ideally need to be very fine to have fast reaction in the soil. Soft rock limes (made of shells/fossils that are geologically younger in age) react about 15% faster than hard limes. Ideally you would want 50% of the lime finer than 0.25mm and very little above 1mm. This lime may cause clumping when wet as Victorian Limestone Producers Association site reports this lime to have 21% greater than 1.0mm. This lime, because of its coarseness and minerology would, be slow to dissolve.

Table 2: Particle size distribution of agricultural lime December 2019.

In comparison, the prilled lime is superfine and is recommended by suppliers to be applied at one third of the Aglime rate due to its fast reaction with soil. However, this means only one third of the neutralisation of H ions is possible. Also, about 100% soil coverage is achieved with 2.5t/ha of Aglime, so not all soil will have contact with prilled lime.

Figure 2. Avoca 2019 results of poppy alkaloid concentration (%) and straw (t/ha).

Trial results indicate limited response to lime in 2019, despite poppies being regarded as acid sensitive. This is not unusual because lime is slow acting. However, the highest rate of lime 9t/ha appears to have suppressed poppy alkaloid content (P=0.064) at 90% significance level and the wheat yield (non significant) (see Figures 2 and 3). Due to the low cation exchanged capacity of the topsoil, it is suspected that applying high rates of lime has induced micronutrient deficiencies.

Where soil acidity issues exist, there can be a temptation to want to quickly address it and a belief that this is only achieved by using high rates of lime. However be careful as application of high rates of lime has been reported to cause micronutrient deficiencies (for example, copper, zinc, boron and manganese) especially if these elements are already marginal. Incorporation of lime can reduce this risk compared with top dressing lime. A soil test indicated that, of these micronutrients, copper was low but boron content was not tested.

High rates of pure lime (NV 100%) are thought to be in excess of 2t/ha for a sand, 3t/ha for a sandy loam or 4t/ha for a loam or clay loam soil. While these deficiencies can be overcome by the application of appropriate granular fertiliser or foliar sprays, if high rates of lime are required, it is best to split applications over a period of three or four years or incorporate the lime.

Figure 3. Avoca 2020 results of wheat yield (t/ha) and grain protein content (%).

Another notable result is that the prilled lime yields were similar to the control yield. This result is similar to results of other prilled limes (for example, Calciprill) at Bairnsdale, Kybolite and Sherwood (unpublished). Final soil pH tests from samples taken in June 2022 will be used to estimate success of treatments at changing soil pH and acidification rates. However, it is expected that the prilled limes, despite being potentially fast working, aren’t provided in sufficient quantities to raise pH to remove yield constraints. Therefore, they may not necessarily be suited to poor pH recovery but could possibly play a role in maintenance liming. A suggested use of prilled lime has been to place it down the sowing shoot to ameliorate acidic subsurface layers formed in the sowing row (Condon et al. 2021). This follows its successful use in direct placement into the acid subsurface providing improved yields compared to incorporation of Ag lime in one season.

Conclusions

Soil pH profiles are naturally stratified and therefore monitoring of the soil pH down the profile needs to occur to identify areas likely to cause yield constraints. The top 10cm of soil, where most of the roots occur, will have the highest rate of acidification, but is also likely to be the area ameliorated by liming. Infrequent lime use, surface application or inadequate rates of lime to match acidification rates will exacerbate the occurrence of subsurface and subsoil acidity. Prevention of acid layers developing at depth occurs by maintaining soil pH above 5.5. However, if high rates of lime are needed in light textured soils for pH recovery, consider split applications and monitoring of micronutrients to avoid inducing deficiencies. Prilled lime appears ineffective at recovery of broadacre low pH but may be effective in treatment of acidity occurring in the drill row. Incorporation of lime speeds up recovery times.

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. The author also acknowledges the support of the Australian government through the National Landcare program and team at SFS.

References

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), 1-11.

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

Li G (2021) Final technical report: Innovative approaches to managing subsoil acidity in the southern grain region. NSW Department of Primary Industries.

Tang C (2002) Causes and management of subsoil acidity. In: SuperSoil 2004: 3rd Australian New Zealand Soils Conference, 5 – 9 December 2004, University of Sydney, Australia.

Contact details

Lisa Miller
Southern Farming Systems
23 High Street, Inverleigh VIC 3321
0488 600 226
lmiller@sfs.org.au