Acidity – soil sampling and lime incorporation under review

Take home messages

• Highly acidic layers between 7 and 30cm soil depth may not be detected by only sampling the 0-10cm depth.
• Recommended soil layers for pH monitoring are 0-10cm, 10-20cm and 20-30cm. Testing 0-5cm is also important if sowing acid sensitive plants and you are unsure of your soil pH profile.
• Regular liming and keeping the topsoil pH above 5.5 allows lime to move to below 10cm depth but this process is slow, even in high rainfall zones
• Strategic one-off incorporation of lime is the fastest way of ameliorating highly acidic soil layers, especially at depth.

Background

Soil acidity is a common and emerging issue in most soil types. Soil acidity results from failure of growers to replace the alkalinity lost through product removal (e.g. grain, hay, mild, meat and wool), use of nitrogen (N) fertilisers and N leaching. Soil sampling protocols have largely been designed to determine nutrient supplies in the 0-10cm layer, and not necessarily to detect subsoil acidity, which can occur in deeper layers. As a result of poor detection of acidity, especially in the subsoil, most lime decisions haven’t accounted for the slow build-up of soil acidity deeper within the soil profile.

Slow movement of lime, no-till farming systems and surface applications have had little effect on acidic layers deep within the soil profile. Infrequent applications of lime have raised soil pH in the top part of the soil but have left highly acidic layers at depths — often at 5 to 15cm. Incorporation of topdressed lime was once a widely accepted practice, however the move to no-till farming systems means incorporation is now uncommon. With the recent evidence of subsoil acidification, several questions are often asked; Do we need to incorporate lime and if so, what are the best ways to do this? And, how do we incorporate while minimising any damage to soil structure and maintain trafficability across paddocks?

A new SFS project with funding from GRDC and the National Landcare Program involves partners across the high rainfall zone in Tasmania, Gippsland and in southern South Australia (SA). The project is focused on finding ways to speed up the movement of lime and help improve the precision of where lime is applied. This paper reports on the soil sampling required to detect soil acidity and whether it’s necessary to incorporate lime.

Basics of soil acidity

Soil acidity is measured by soil pH, which refers to the concentration of hydrogen ions in the soil solution. The scale is logarithmic, so small changes in pH can mean large changes in the amount of hydrogen in the soil. All soil pH results in this paper were measuring using a calcium chloride extract which is approximately 0.7 units lower than the pH water test. This is used for reporting on soil acidity due to its consistency across seasons and reliability in neutral to acidic soils.

The desirable soil pH in the 0-10cm layer is at least 5.0 or 5.5 for acid sensitive pulses, and at least 4.8 in the subsurface and subsoil. Low soil pH and excessive hydrogen ions impact root growth, uptake of water and nutrient availability (such as N, phosphorus, potassium, sulphur, calcium, magnesium and molybdenum), and decrease soil microbial activity. As a result, N fixation by rhizobia, organic matter breakdown and cycling of nutrients are all reduced in acidic soils. In addition, aluminium becomes soluble at a pH of 4.8 in many soils and becomes increasingly toxic to root growth as pH falls. Most feeder roots are in the top 5 to 20cm of soil, so having good pH in this layer is critical. The deeper roots are important for accessing soil moisture, particularly at grain fill when topsoils have dried out.

Diagnosing soil acidity

Soil acidity develops slowly over many years and crop symptoms such as slow or reduced growth go unnoticed. Also soil pH varies across the paddock and if acid sensitive plants are sown such as lucerne, faba bean or barley, poor patches often show up. Areas of soil acidity are difficult to detect by eye and soil sampling and testing is the best method for diagnosis.

The standard soil testing depth for fertiliser decisions is 0-10cm, and deeper testing is sometimes included for N - 10-20, 20-30cm or deeper. Among 200 paddocks tested by SFS in the Corangamite and Glenelg Hopkins catchments, the main acidity issues have been at 0-10cm and 10-20cm (Table 1).  Sandy soils have a low pH buffering capacity and small additions of acid cause a large drop in pH, which is why acidity shows up first in sandy patches. Where the clay content is high, the soil pH tends to be neutral to alkaline, which in SW Victoria commonly occurs at 20-30cm. Combining soil from 10-30cm, which is sometimes recommended for subsoil sampling of nutrients, can mask acidity issues at 10-20cm.

Table 1. Mean soil pH(Ca) of 100 different farming enterprise paddocks in both Corangamite and Glenelg Hopkins catchments in SW Victoria, taken in 2018 and 2019 (Miller 2019, Debney 2019).

If you detect low pH (<4.8) at 20-30cm, then test 30-40cm and so on, to build a picture of pH throughout the soil profile. This allows you to target areas to monitor.

Where there is limited history of lime applications, the soil pH profile is often relatively uniform. For example, Table 2 shows the soil pH profile at the Drysdale site, with no lime history, which has a consistent soil pH down to 20cm. The detection of soil acidity is more complex when lime has been applied, because this creates stratification and a favourable pH layer often confined to the top 5cm of soil. Note the difference of 1.2pH units between the top and bottom 5cm increments at Drysdale.

Table 2. Soil pH(Ca) at Hamilton and Drysdale sites, at different soil depths with and without surface lime.  Two soil sampling strategies (in 10 or 5cm increments) were compared.

Soil testing in 5cm increments is somewhat laborious and expensive but it provides the greatest detail of pH stratification and knowledge of acid barriers. This is recommended when you are planning to sow acid sensitive legumes. At Hamilton for example, lucerne was established in a paddock that had been recently limed, but large patches died out after three months. Further testing showed a soil pH of 4.9 at 0-10cm (Table 2), however, 5cm increment testing showed a pH of 3.9 at 5- 10cm and 4.1 at 10-15cm (McCaskill et al. 2009).

Increment testing at 5cm depths is difficult as you need to extract intact cores and split them into increments. It’s ideally done when there is soil moisture so samples hold together, but not so sticky that they can’t be pushed out of the coring device. Lubricants can be used which don’t interfere with soil pH. For this reason, many agronomists like to extract soil cores using a dig stick, or open facing coring device, and apply pH field test indicator along it to give a quick pH guide of the soil profile.

Where to sample in the paddock?

Precision or grid sampling is a tool that can be used to help identify variability through pH mapping.  Analysis of 350 paddocks and found the average range in pH values with paddocks was 0.95 units, ranging from 0.1 to 3.2pH units (Barlow et al. 2019). This means that with an average pH in the paddock of 5.0, pH might vary from 4.1 to 5.9. When sampling paddocks, different land management units or soil types need to be sampled separately. The Fertcare^® soil sampling guide is an excellent reference and provides information on separating out zones of different soil types and unrepresentative areas.

For general monitoring, a zigzag pattern across the paddock gives a good representative sample, lowers risk of bias, and is easy enough to do (Table 3).The straight-line transect is not quite as reliable, as you could potentially follow a pateern (e.g. a feeding line of sheep). Use GPS reference points to identify transects for future monitoring, and for monitoring trends.

Table 3. Fertcare^®’s soil sampling paddock/block sampling patterns and attributes.

Note: As the sampling characteristics improve the number of asterisks assigned increases (Source: Gourley and Weaver, 2019).

For decisions about the use of variable-rate lime, precision sampling divides broadacre paddocks into one to two hectare grids and takes eight samples within each grid along a 120 to 160m transect. Whilst statistical models may suggest more intensive sampling may be needed to inform site specific management, the costs can become prohibitive and may not be practical.

Do we sample in the row, or outside it, or both?

American research was used to inform sampling patterns where fertiliser banding occurs (Kitchen et al. 1990). Recommendations are based on row spacing. For example, for 30cm row spacings, one core from the sowing row should be combined with eight from the inter row soil to get a true average of the soil. in the absence of research on what is applicable in relation to soil acidity sampling in modern farming systems, which potentially use deep banding of fertiliser, the above recommendation remains the best advice.

Deep banding of N can cause acidification at the placement site. Unlike ammonium-based fertilisers, deep banding of urea itself does not cause net acidification unless N is converted to nitrate and leached. Deep banding is not yet a widespread practice but monitoring pH at the depth and location of where the N is being placed is probably worthwhile — especially if is beyond 10cm.

pH mapping and variable rate – is it worth it?

Soil pH mapping involves more intense sampling and identification of zones of difference to tailor lime rates to reach a target pH. VR lime application may appeal to growers if it offers cost savings by reducing the amount of lime needed compared to a blanket rate application. There is also potential for reallocation of lime, so the poor areas get more, and/or a more even pH distribution is created. pH mapping assists growers and agronomists to identify problem areas and provides greater information to make more informed decisions about the rates of lime needed across the paddock.

The question of pH variability and economic benefits of high cost intensive point sampling was investigated by Agriculture Victoria (GRDC project DAV00152), who investigated pH and lime responses in 10 paddocks in the high rainfall cropping zone. A discounted cash flow model was used to compare a high cost ‘precision strategy’ involving grid soil pH mapping and variable rate lime applications to low-cost traditional sampling, testing and uniform applications across the paddock. Cost assumptions included commercial production of a grid map at $16/ha (including laboratory analysis of two top-soil samples per ha) and VR lime application costing approximately $4/ha more than normal spreading.

Findings showed that nine out of 10 sites had a greater Net Present Value (NPV) when the precision strategy was employed compared to applying the traditional strategy, but only when an acid sensitive pulse was included in the rotation, such as faba bean (Stott et al. 2019). The size of the additional net returns ranged from 2% to 14% for the case study paddocks examined. NPV was low when the precision strategy was applied at the Seaspray site, which had a mean pH (Ca) of 4.2 and a co-efficient of variation (CV) of 4.8% — exhibiting less variation than most sites. Eight of the 10 sites had considerable variation with a CV ranging from 6.2 to 11.9%. However, the traditional uniform strategy had a higher NPV for all 10 sites when a more acid tolerant rotation was used, such as barley-wheat-canola.

This analysis indicated that if the paddock has a pH >4.5 and a CV >4%, and high-value acid-sensitive crops are planned in rotation, then the net returns from precision pH mapping and variable rate liming would be superior to traditional methods (Stott, pers. comm.).

Do we need to incorporate lime?

The decision to incorporate lime or leave it on the soil surface is dependent on where the acidity issue is and how fast you need to ameliorate soil acidity, given that lime moves approximately one to 2cm per year at best. In the examples given below it can take up to 20 years to address acidity down to 30cm. If soil pH is poor at 5cm and beyond, and you are using acid sensitive or high value crops, then incorporating lime helps ensure establishment and additional yield. Currently, deep-banded acidity is not easy to implement and considered a poor investment (Conyers et al. 2019).

There are two long-term lime trials that provide evidence of how higher rates and frequent applications of surface applied lime have addressed subsurface acidity without incorporation. Firstly, a trial near Wagga Wagga ran for 23 years, with a starting pH of 4.1 (0-10cm) and 4.2 (10-30cm). This trial involved applying superfine lime every six years, which reached and maintained soil pH above 5.5 in the surface 0-10cm after 11 years (Li et al. 2019). Only after this occurred did alkalinity leach into the layers below. Another finding of this trial indicated the acidification rate at 10-20cm was very low (0.005 pH units per year) suggesting not a lot of lime is needed at depth to match and exceed the acidification rate. Soil pH under the lime treatments increased at 0.04 pH units per year as the alkali moved vertically over time. This was a sandy soil, and high clay content soils will have higher pH buffering capacity, slower soil pH amelioration rate and will require higher rates of lime to change pH.

In the sandy soils of WA, several acidity trials have been monitored over the long-term. A trial at Wongan Hills was established in 1994 and tracked pH change after recurring applications of surface applied lime (Azam et al. 2019). The sandy soil was limed three times over the 23-year period and had higher soil pH throughout the top 30cm compared to the untreated control. This was achieved by maintaining soil pH above 5.5 in the top 10cm, allowing movement of alkalinity to the subsurface soil. However, a large proportion of the applied lime was undissolved and still sitting in the top 4cm, mainly from the most recent applications of lime. In this case the undissolved lime was related to its coarseness. When the acid soil particles react with the edge of the lime partciles, the surrounding pH increases until it reaches pH (Ca) 5.4 when dissolution slows and stops as the soil becomes alkaline. The pH adjacent the soil needs to further acidify before the lime starts dissolving again. Through incorporation, this lime was further distributed through the profile to correct acidity and achieve greater grain yields.

Although topdressing lime on the soil surface can address deeper acidity, it is far better to avoid subsoil acidity in the first place by maintaining topsoil pH (Ca) at 5.5 or above.

The pros and cons of incorporation

Some downsides of incorporation include additional costs, reduced trafficability, and potential negative effects on soil structure. Mark Conyers, former NSW Agricultural research scientist discussed the results of a strategic tillage project at the 2017 Bendigo GRDC Update. Conyers acknowledged that repeated tillage is not ideal and while tillage should be minimised, a strategic ‘one-off tillage’ or lime incorporation can address agronomic or soils issues and will not undo 20 to 30 years of no-till. In the worst cases on red and grey clays of southern NSW, he found that strategic tillage set soil structure back by two to four years, and that adding fresh residues, green manures or a pasture phase hastened the recovery. Additionally, strategic tillage either did not affect yields, or improved them (Conyers et al. 2017).

Conyers et al. 2019 outlines the main agronomic reasons for tillage, one of them being to incorporate lime, overcome stratification and correct deeper acidity. Ignoring a deep acidity problem just to maintain soil structure may not be prudent. Soil structure is improved with organic matter breakdown, microbial and earthworm activity and fine roots — all of which will be restricted in acid soil layers.

Trafficability after incorporation is a concern in the high rainfall zone. Deep ripping earlier in the year may help decrease soil moisture and dry out rip lines to reduce trafficability issues. This can be beneficial where winter waterlogging can be an issue, but it may increase the risk of erosion. By leaving tillage later in the season, soil moisture might also affect the result, with soil smearing on clay soils rather than mixing or shattering.  There are also concerns about tillage in sodic subsoils — bringing sodic soil to the surface might cause poor structure, surface sealing and reduce plant establishment.

What incorporation methods can be used?

The incorporation method most suitable depends on where the acidity is located in the profile and what other agronomic constraints could be addressed in the one operation. Agronomic constraints might include treating compaction, re-distribution of nutrients, soil borne diseases slugs/snails, and burying weed seeds.

Not all tillage methods are effective at mixing lime and vary with their effects on soil structure and trafficability. Burns and Norton 2018 report the ineffectiveness of the Speedtiller^®, set to a depth of 8-10cm, where 4t/ha of fine grade lime applied over five years was still confined to the shallow surface soil (0-5cm), leaving a pH of 4.4 and a 4.2 from 5 to 10cm. Light harrowing has also had poor results (Scott and Coombes, 2006).

SFS have used different methods of lime incorporation, with results and issues shown in Table 4. The results from trials are indicating some reduction in establishment using tillage treatments in the first year following incorporation, compared to no-till. However, by harvest time the yields from incorporation methods tended to be either the same, or slightly higher than the no-till treatments — especially after the first year. With shallow soil acidity at 15-20cm depth, the use of offset discs or a tined implement to mix lime to 10 or 15cm is preferred, with lime rates capable of raising pH to 5.5 so lime can move further downwards if required. Tined or offset disc implements have created few issues and the paddocks have been trafficable afterwards.

Deep ripping has been used to address acidity down to 30cm depths. Ripping on 50cm spacings between tines is suitable for addressing deep acidity as it can create preferential pH pathways for roots to follow. However, this leaves areas un-limed between the rip lines which will continue to acidify. For this reason, lime still needs to be applied across the paddock and either surface applied or incorporated in a separate pass. Experimental implements with the capacity to band lime within the acid soil layers have proven difficult to use of a broad scale.

A NSW DPI purpose-built machinery was used at subsoil acidity trials in Victoria. Deep ripping with rip lines 25cm apart was used at Stawell after being validated at Ferndale, NSW in a subsoil acidity trial. Deep ripping placed lime about 10 to 12cm either side of the rip line and 25cm rip spacing allows intersection of lime. However, the combined ripping at 25cm spacing followed by offset discing to incorporate lime at 10cm overworked the light sandy soil, causing loss of structure and traction.

Table 4. Summary of Southern Farming Systems lime application method trials (Miller 2016 and Miller 2018).

Conclusion

Being able to identify areas of soil acidity in three dimensions (i.e. spatially across the paddock and down the soil profile) supports more informed decision making regarding suitable lime strategies, especially if acid sensitive legumes (e.g. faba bean or lucerne) are part the rotation.

Subsurface acidity issues are most likely to occur from 7-20cm and can be missed through standard 0-10cm sampling methods. Therefore, sampling at 0-10cm, 10-20cm and 20-30cm sampling is recommended as a first step. In some cases, further sampling in 5cm increments may be warranted for a more detailed investigation.

Incorporation of lime gives rapid amelioration of acid topsoil layers and will move into deeper layers over a period of 5-10 years provided the top-soil pH is kept above 5.5. However, the time frame to treat subsoil acidity and the loss of yield while waiting for surface applied lime to reach the subsoil, makes it an economically unattractive choice when the subsurface acidity is severe.

Incorporation of lime through some form of strategic tillage can address subsoil acidity constraints and increase yields, especially for acid sensitive or high value crops.

Acknowledgements

This research is a collaborative project between the GRDC and the Australian Government’s National Landcare Program. 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 acknowledges support of GRDC in soil acidity research since 2014 and for recent investment into SFS led project ‘Building the resilience and profitability of cropping and grazing farmers in the high rainfall zone of Southern Australia –soil acidity.’

Thanks to Nathan Robinson from Federation University, Kirsten Barlow from Precision Agriculture and Kerry Stott, Agriculture Victoria, who provided valuable information for this paper.

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Contact details

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