Subsurface acidity – how far has the research advanced?
Take home messages
- Subsurface acidity and stratification are emerging as serious constraints to crop production across NSW, Victoria, SA and WA. Identification and treatment is being explored through multiple GRDC and State and Federal government investments.
- Traditional soil sampling strategies can lead to misdiagnosis of subsurface issues; strategic sampling at specific depth intervals is required.
- Lime rates need to be adjusted to account for subsurface pH, changes in soil texture and organic carbon content down the profile.
- Strategic incorporation/tillage can aid the efficacy of lime application for treating subsurface issues; other subsoil constraints (e.g. compaction) should be taken into consideration to maximise treatment impact, along with the risks associated with soil disturbance.
- Options for treating subsurface and stratification issues are being examined in a new GRDC project in SA.
Soil pH is largely a function of soil type, rainfall and farming system, and can be inherently variable both horizontally and vertically in the profile. A soil pHCa between 5.2 and 7.5 provides optimum conditions for most agricultural crops, though plant species differ in their tolerance to acidity and alkalinity.
Whereas soil acidification is a natural process, primarily driven by the leaching of nutrients (especially nitrates) from topsoil, it is accelerated under productive farming practices. Where no lime is applied, the topsoil becomes acidified and the acidic layer spreads down the soil profile, retarding penetration of roots of acid-sensitive species, and ultimately reducing crop yield.
Subsurface acidity is the acidification of the soil below the top 10cm. The development of acidic subsurface layers can induce nutrient deficiencies and/or toxicities, limit crop responses to fertiliser application, and adversely affect root growth, water uptake, nodulation, plant vigour and the N fixation potential of acid-sensitive pulses (Burns et al. 2017b). For acid-sensitive crops like pulse legumes, rhizobia survival and nodulation are compromised at pHCa below 5.0. Acidic conditions also contribute to the suppression of organic matter breakdown and cycling of organic N within the subsurface layer (Paul et al. 2003).
Much of SA’s 4.4 million hectares of productive farmland has a topsoil pHCa below 5.5 or has the potential to develop acidity (Figure 1; see colour copy of this paper on the GRDC website). The potential for acidic layers at 5 to 15cm or deeper across these areas is high particularly where the A horizon is thicker than 20cm. Remedial action is required to curb its development. When it comes to subsurface acidity, prevention is better than cure.
Figure 1. Map of South Australia showing areas currently affected by surface acidity (blue; see colour copy of this paper on the GRDC website) and areas at risk of developing surface acidity in the future. Note, all of these soils are also at risk of developing subsurface acidity.
The delineation between surface and subsurface acidity is important as monitoring and treatment options will vary, becoming increasingly complex at depth. Subsurface acidity cannot be detected with conventional topsoil sampling methods (0-10cm), and targeted sampling to depths at suitable increments is required.
There have been multiple GRDC and State and Federal government investments in recent years across NSW, Vic, SA and WA aimed at exploring subsurface acidity and its treatment. This paper serves to present a summary of that work and its relevance in the South Australian context, including recommendations for sampling and treatment. Note, this paper is an extract of a literature review being prepared for the GRDC project ‘New knowledge and practices to address topsoil and subsurface acidity under minimum tillage cropping systems of South Australia’ (DAS1905-011RTX). Contact Brian Hughes (Brian.Hughes@sa.gov.au) for the complete version.
Causes of subsurface acidity
The causes of soil acidification, either in the surface or subsurface layers are similar, however there are some differences.
The key environmental factors that can affect the difference in pH between surface and subsurface layers are soil fertility, initial soil pH profile before clearing, rainfall and fluctuations in soil moisture content. In duplex soils, the changing soil clay content which drives pH buffering capacity can have an impact on the speed of development of acidic subsoil layers (Paul et al. 2003). The higher soil organic matter content in surface layers may also buffer against pH changes, maintaining a higher pH than the underlying soil. Conversely, the lack of organic matter in light textured sandy subsoils can mean that severe acidity can develop quickly.
Topsoil acidification of cropping soils is largely driven by nitrification from either ammonium-based fertilisers or organic forms of N from plant residues and the subsequent leaching of nitrates. The removal of alkaline farm products is the other major contributor. Stratified acidic layers at 5 to 15cm are becoming increasingly common under no-till systems in the high and medium rainfall regions of southern Australia at the depth where N fertiliser is applied, even where topsoils have been limed (Angus et al. 2019, Burns et al. 2017a, Paul et al. 2003, Scott et al. 2017).
Subsurface acidity can occur when surface acidity goes untreated, gradually extending down the profile, and can be exacerbated by the production of acids, especially from leguminous plant roots. Plants maintain their electrostatic charge by excreting acid (H+) where the charge of the cations taken up exceeds the charge of the anions. Non-legumes take up significant quantities of nitrate (NO3-) so the excess of cations over anions and acid production is usually low. By contrast, legumes generally fix most of their N internally and have a greater uptake of cations over anions. Thus, legumes produce more acidity in the deeper soil profile than non-legumes (Tang 2004).
The problem with aluminium
A key impediment to plant growth in acidic subsoils is the potential for aluminium (Al) toxicity. Aluminium is a component of many soil constituents including clays and oxides, and is also present on the surface of soil organic matter. As soils acidify, Al becomes available from the soil constituents, increasing the concentration of Al ions in the soil solution, typically once pHCa falls below 4.8. However, some soils can have low pH without Al toxicity.
High Al severely damages plant root hairs and impairs the uptake of water and nutrients. This may produce symptoms of drought and nutrient deficiency which can be difficult to relate to soil acidity and Al toxicity in the absence of soil testing data (Yang et al. 2013). Crops such as canola, barley, annual medics, lentils, faba beans and lucerne are very sensitive to Al toxicity. Often when plant roots encounter a toxic Al layer in the subsurface, the damaged roots will respond by growing sideways.
Is there a subsurface problem?
In 2019 there were various reports of patchy legume crops (faba beans, lentils and chickpeas) across SA soils that were widely considered to be alkaline. Soil testing in the patches revealed that they were no longer alkaline and a stratified acid band mostly at 5-15cm was the culprit behind the poor legume growth. Often subsurface acidity isn’t uniform across whole paddocks, but rather appears in certain soil types or positions in the landscape.
Its presence is often masked by traditional 0 to 10cm soil sampling, with the alkaline 0 to 5cm layer diluting an acidic 5 to 10cm layer, resulting in an overall pH result that doesn’t cause alarm. Where pH stratification and/or subsurface acidity is present, traditional soil fertility sampling may not accurately reflect pH variability and its extremes in the profile.
Crop grain yield maps and/or mid-season normalise difference vegetation index (NDVI) images, particularly in the legume phase of a rotation, can help identify ‘productivity zones’, or areas of good and poor plant growth that can be used to target soil sampling.
A soil pH indicator kit, purchased from your local hardware shop or plant nursery, can be used to quickly and cheaply determine whether acidity is contributing to poor plant growth, following this method:
- Use a yield or NDVI map to locate zones of ‘good’ and ‘poor’ production in a paddock.
- In each zone, dig a few holes to 40cm using a shovel or front-end loader, creating a flat vertical soil profile face.
- Apply the pH indicator liquid on the profile down to 30cm and then apply the powder and let the colour develop. Alternatively, you can use a Dig Stick soil probe (Spurr probe) to remove an intact soil core and apply the same procedure to determine the change in pH down the profile.
- Once the colour reaction is complete, use the pH colour indicator card to determine the pH down the profile and a tape measure to identify the positions of any pH changes. Any acid layers will be visible as bright green or yellow colours.
- Take a photo, including the tape measure for reference.
If acid areas have been identified using the pH indicator kit, careful soil sampling and more accurate laboratory pH and other analyses are recommended.
- Depending upon the position of the acid layer, soil depths might include: 0-5, 5-10, 10-20 and possibly 20-30cm. If the layer is more common in the 5-15cm layer the following depths may be more appropriate: 0-5, 5-15 and 15-25cm.
- Within each productivity zone, collect multiple samples from 10 to 15 cores and combine samples from each depth using a clearly labelled bucket. The number of zones (usually 2-6) that should be sampled will depend upon the variation within the paddock and its size.
- Thoroughly mix the samples for each layer depth for each zone and bag a sub-sample; send to the lab for pHCa, organic carbon % and a soil texture assessment. Aluminium (measured in CaCl2) is also warranted.
Alternatively, precision soil sampling approaches, such as grid-based or on-the-go Veris® pH mapping can provide more detailed data on the variability in surface pH and possible stratification, which can identify areas of potential subsurface acidity for further sampling.
How much lime will I need?
Subsoil pH can be increased slowly over time by liming sufficiently to maintain pHCa at 5.5 or more in the top 10cm (Burns et al. 2017b, Conyers and Scott 1989, Scott and Conyers 1995). Lime rates required to achieve a target pH are influenced by the buffering capacity of the soil which is determined by the soil texture and organic matter content. The rough rules of thumb to change the pH by one unit for each 10cm depth of soil are: 2t/ha of lime for a sandy soil; 3t/ha for a sandy loam; and 4t/ha for a loam/clay loam. Where organic matter is low (common in subsurface layers and/or lower rainfall areas), rates can be substantially reduced and will have the same effect.
Lime quality is important when it comes to determining rates, with particle size (fineness) and purity (neutralising value) driving its effectiveness to counteract acidity. Recent work in SA compared different sources of lime, broadcast at 3.0 or 6.0t/ha without soil incorporation. Fine lime was found to move slightly further down the soil profile over 4 years (7 to 10cm) than coarser lime, which only moved to 5cm (Hughes and Harding 2019).
The neutralising value (NV) is the carbonate component of lime that neutralises acid in the soil, and therefore, the proportion of carbonate in the liming material is important as it impacts the effectiveness of the product. The higher the NV, the greater the material’s capacity to neutralise acidity. Pure calcium carbonate has a NV of 100%; good quality liming materials should have a NV greater than 80% (Harding and Hughes 2018). Lime rates need to be adjusted to reflect NV.
Registered agricultural lime suppliers are required to provide purchasers with a laboratory analysis of the neutralising value, particle size and calcium and magnesium content of their liming products.
Calculators are available to assist with lime rate decisions and assessment of lime quality from different sources (contact Brian.Hughes@sa.gov.au for a copy), though these decision support packages were developed to target surface acidity only (0 to 10cm). These calculators will be reviewed as part of the new project to calculate lime rates that account for subsurface acidity.
How can I increase lime movement in the soil?
The current industry practice of spreading lime without incorporation under no-till or zero till management is relatively ineffective at treating subsurface acidity because of the slow movement of lime down soil profiles (Burns et al. 2017). Surface applied lime is often concentrated in shallow surface layers (0 to 2.5cm) with little further downward movement in the short to medium term (Burns et al. 2017d).
Lime particles need to react with the soil and the by-products leaching into the soil. The speed at which this occurs is related to rainfall, the soils texture and buffering capacity, and the fineness of the lime. Depending on these factors, it can take anywhere from 4 to 15 years before lime applied on the surface moves beyond 10cm, but incorporation has been shown to increase liming efficacy (Conyers et al. 2003). It appears more aggressive application or incorporation methods may be needed to achieve rapid changes to pH at depth (Li and Hayes, 2017). The more vigorous the soil disturbance with lime applications, the faster subsurface acidity will be neutralised (Angus et al. 2019). Deep lime placement has been tried in several experiments with mixed results.
Strategic cultivation with a tyned or disc implements every 4 years or more interspersed with no-till can be beneficial on a range of soil types, overcoming a number of production constraints, not just acidity. The timing of the cultivation is critical to minimise impact on soil structure and to reduce the risk of erosion. The benefits of this strategic cultivation need to be weighed against the potential cost and risks (Conyers et al. 2019). While occasional strategic tillage conflicts with the philosophical ideal of zero disturbance of soil, it may provide a tool for flexible management of weeds and pests within a conservation agriculture approach (Conyers et al. 2019).
Deep tillage and soil mixing – sandy soils
Many soils of southern Australia contain a range of physicochemical constraints, often occurring concurrently in the top and/or subsoils (Davies et al. 2019). Strategic deep tillage and/or soil mixing that extends beyond the top 10cm can be used to alleviate multiple soil constraints (for example acidity, water repellence and compaction), effectively spreading the cost and risk of incorporating lime across several soil constraint benefits and maximising the potential gains in production. Azam and Gazey (2019) demonstrated benefits to root growth and water use efficiency, doubling grain yield by incorporating lime to >30cm with a rotary spader, overcoming both acidity and compaction.
Types of deep tillage include: deep ripping (with and without inclusion plates); delving; soil mixing (spading, large offset discs); and, soil inversion (mouldboard plough, modified one-way disc plough). A summary of each approach, working depth, constraints addressed, and approximate cost can be found in Davies et al. 2019. These approaches are best suited to sandy soils and some still require validation in SA. Cultivation and deep tillage assessments will be made in this project across a range of soil types and cropping systems in SA.
Organic amendments generate alkalinity as they decompose, and mixtures of lime and organic material can improve the response to lime by creating favourable conditions for the movement of lime through the soil (Butterly et al. 2018a). Organic wastes such as compost, animal manures, lime-treated sewage sludge and plant residues have been trialled and found to give some effect in reducing acidity when the wastes themselves have some alkaline content (Butterly et al. 2018b; Condon et al. 2018; Nguyen et al. 2018). Organic matter can also reduce Al toxicity, even when no pH change is detected (Antonangelo et al. 2017; Li 2018). Ongoing research in the southern region is examining the benefits of organic amendments for soil acidity and other constraints.
Subsurface acidity is becoming increasingly prevalent across SA’s cropping land, leading to patchy plant growth and reduced grain yields, especially in pulses. Its presence often goes unnoticed until it is well developed, due to limited or inaccurate subsurface soil sampling and assessment. A strategic soil sampling approach is proposed to adequately identify stratified and subsurface bands of acidity, particularly in no-till systems. Lime application rates need to be developed that take into consideration the degree and depth of acidity, soil type and organic matter content and lime quality. Growers should consider methods to incorporate applied lime to increase its efficacy in treating subsurface issues. PIRSA is working on developing new calculators to assist lime rate decisions to treat subsurface acidity and will assess incorporation methods suited to South Australian soils.
The research reported here is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC. We’d like to thank them for their continued support.
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GRDC Project code: DAS1905-011RTX, UOA1905-015RTX
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