Take home messages

• Soil amelioration can produce large increases in yield, but responses are highly variable
• Crop response to amelioration depends on complex interactions between soil water availability, the nature of soil constraints and where they occur in the profile
• A decision support tree (DST) focusing on soil and environmental factors to predict crop response to amelioration in SE Australia has been developed
• Use of the DST needs to be followed up by detailed soil sampling within the paddock and economic analysis
• Effective paddock zoning is critical to economic success of soil amelioration.

Background

Many subsoils in the medium (MRZ) and high rainfall zones (HRZ) of south-eastern Australia contain multiple soil physicochemical constraints that restrict the growth of crops by limiting the ability of the roots to extract water and nutrients from the subsoil. These soil constraints include poor physical structure, transient salinity and a range of specific chemical toxicities including high aluminium, boron and nutrient deficiencies (Adcock et al., 2007). These different constraints occur simultaneously in many subsoils and generally increase in severity with soil depth (Nuttall et al., 2003). Together these constraints can significantly constrain crop yields and quality and ultimately grower profitability.

Considerable research has been undertaken over the last 40 years to combat these subsoil constraints using a range of strategies broadly described as ‘genetic solutions’, ‘soil amelioration’ or simply ‘live with the problem’ (Armstrong et al., 2022). Soil amelioration using organic and other amendments, can increase grain yields significantly as well as profits (Sale & Malcolm 2015, Sale et al., 2021). Despite its potential, rates of adoption of soil amelioration have remained low due to a combination of variable (unpredictable) yield responses, high upfront costs of implementation, logistical constraints such as the limited availability of suitable organic matter sources and access to appropriate machinery (Nicolson et al., 2016).

Recent economic modelling suggests that targeting soil amelioration to paddock conditions where yield improvements are likely (Hendrie et al., 2024) can significantly alter the economic returns. Using knowledge gained from a range of recent research, this paper describes how soil amelioration affects crop growth and the conditions where soil amelioration is most likely to be an effective management strategy. This knowledge is embodied in a simple decision support tree (DST) designed to assist growers and advisers to make informed choices when considering soil amelioration.

Methods

Information sources

Knowledge was gained from a range of recent GRDC and CRC for High Performance Soils projects examining crop responses to soil amelioration. These included a series of field experiments conducted from 2017–23 that examined residual responses to an initial soil amelioration treatment; an analysis of historical field trials, (commencing between 1986 and 2015); and a current project examining responses to spatial targeting of soil amelioration based on soil types within a paddock (Hendrie et al., 2024).

The new field trials were located in southern NSW (2), Victoria (3), South Australia (2) and Tasmania (1) and encompassed different ameliorant sources (organic, inorganic and combinations of both), placement (surface vs subsoil), soil types (Vertosols, Sodosols, Calcarosols and Chromosols (Isbell  1996), and environments (annual rainfall ranging from 325 to 690 mm across the MRZ and HRZ of south-eastern Australia) and comprised a total of 40 site by year combinations.

For the ‘historical trials’, a database was compiled using grain yield results from 28 field trials across south-eastern Australia (predominantly South Australia and Victoria) previously established by different groups to investigate how organic amendments ameliorate subsoil constraints and improve crop and pasture production, supported by a variety of funding initiatives in SE-Australia. These trials compared different combinations of plus/minus deep ripping, with and without the use of amendments including animal manures, plant biomass and gypsum and different placement (surface or subsoil). The duration of monitoring crop performance in these trials varied between 2- and 35-years post application of amelioration treatments with more than 50% of the trials having crop data for at least 5 years post application. In total there was 95 trial-years in the database with most (55%) sown to wheat or barley. Soil types used included Dermosols, Calcarosols, Sodosols, Vertosols and Chromosols with average annual rainfall ranging from 364 to 730 mm.

The spatial application dataset included yield responses to 6 amelioration treatments over three consecutive growing seasons (2021–23) on two distinct soil types (Sodosols or Vertosols plus a Chromosol at one site) in commercial paddocks at four sites in the MRZ and HRZ of western Victoria.

Results and discussion

Crop response to soil amelioration

Soil amelioration produced relative grain yield responses ranging from 0 (or even negative) to greater than 200%. There was a wide range in ‘control’ grain yield, from 0.9 t/ha to 9.1 t/ha (wheat) throughout the trial period with yield responses as large as 1.9 t/ha. These large increases however were the exception and smaller increases (10–20%) lasting multiple years at ‘responsive sites’ were the norm. Largest yield responses were mainly in the HRZ (Figure 1A). Some MRZ trials produced no or negative responses to treatments involving deep soil disturbance, reflecting poor establishment in the period after amelioration with rainfall insufficient to consolidate the seedbed following disturbance (Figure 1B). These low or negative yield responses in the new field trials generally coincided with very dry seasonal conditions (Deciles 1-3). Deep ripping without amelioration did not increase grain yields. As reported in previous GRDC Updates (Uddin et al 2022), the two field trials in southern NSW (Rand and Grogan) consistently produced some of the largest yield responses, although the magnitude of responses varied with season/crop type.  Large relative responses did not always produce large production increases expressed on an absolute basis (i.e. t/ha). As a consequence, the relative potential benefits of soil amelioration need to be larger in environments with lower yield potential e.g. MRZ than in those with high yield potential to achieve a net economic benefit from amelioration.

Figure 1. Relative grain yield response (%) of crops (control = 100%) to different soil amelioration strategies with amendments applied either to the soil surface or subsoil (deep) at four sites in: (A) the HRZ (2019) and (B) MRZ (2021) in SE Australia. Amelioration treatments were imposed in 2018 except for Rand and Condowie, which were imposed in 2017.  GSR = growing season rainfall (mm). ‘NEOM’ = animal manure pellets; ‘green chop’ = lucerne or field pea hay pellets. Value above Control represents absolute grain yield (t/ha).

Mechanisms underlying crop response to soil amelioration

A range of mechanisms have been proposed to explain crop responses to soil amelioration using organic matter. These include improved nutrition, especially nitrogen (Celestini et al., 2019), and increased root growth in the vicinity of where the ameliorant is placed (Wang et al., 2020). Controlled environment studies (Uddin et al., 2022) have demonstrated that soil amelioration with organic matter can improve soil structure in the immediate zone surrounding the ameliorant; however, there is minimal evidence from fields trials that the application of organic matter (in the absence of a calcium source such as gypsum) will directly mitigate physicochemical constraints such as dispersion in the lower sections of the subsoil.

The primary mechanism whereby soil amelioration increases grain yield in dryland cropping systems appears to be via increasing the size of the soil water bucket. This increase occurs by increased ability of roots to extract soil water, most likely from enhanced root growth in the layer where the ameliorant is placed (typically 20 to 50 cm depth) rather than deep in the profile. (Wang et al., 2020; Uddin et al., 2022). In addition, sub soil amelioration with organic matter increases the size of the soil water bucket by improving infiltration of rainfall (see Figure 2). This finding may be of particular benefit on the many soils in the northern grain region with a tendency to ‘seal’ following rainfall or due to slaking.

Figure 2. Effect of subsoil amelioration using gypsum (5 t/ha), NEOM (chicken manure pellets applied at 15 t/ha), or wheat straw (applied at 15 t/ha) plus N and P fertiliser, compared to control (no amelioration) or deep ripping, on infiltration rate at 25 cm depth at Horsham five years after initial application. Vertical bars indicate l.s.d. (P = 0.05). Source: J Jian, Pers. Comm.

Predicting where soil amelioration is likely to increase yields

The historical trials indicated that soil type strongly influenced response to amelioration, with the largest increases in yields occurring on Sodosols and Calcarosols, and the lowest increases recorded on Vertosols and Chromosols. There was one inconsistency between the effect of soil type on response to amelioration. Whereas the historical trials suggested that subsoil amelioration was effective on Calcarosols, more recent experimentation (N Wilhelm, pers. comm.) suggests that generally crops do not respond to amelioration on this soil type.

Two important factors appeared to influence the relationship between soil type and response to amelioration. Firstly, in the Vertosols, which were confined to MRZ environments (<425 mm annual rainfall), the severity of soil constraints e.g. sodicity and boron toxicity, typically tend to occur deeper in the profile, making it logistically more difficult to physically place the ameliorant near the constraint. APSIM (V. 7.10) simulation modelling indicated on these fine textured soils (i.e. with large plant available water range), there was often insufficient rainfall in continuous cropping systems to wet the subsoil where the ameliorant was placed, effectively ‘stranding’ the ameliorant (Dunsford et al., 2024). This lack of subsoil water meant that soil amelioration could not provide any yield advantage via increasing the rooting depth as subsoil constraints only impact on yields if there is subsoil water (Nuttall and Armstrong 2010). Secondly, on dense clay soils it is logistically challenging to physically place amendments at depths greater than 40 cm, especially if the soil is dry. In contrast, where topsoils are sandy, it is much easier to physically place ameliorants into the subsoil, and yield responses to amelioration are much more frequent (Hendrie et al., 2024). Incidentally, deep ripping without amelioration on clay soils did not increase yields. In contrast recent research in other GRDC projects suggests that in the absence of chemical toxicities or nutrient deficiencies, deep ripping can increase crop productivity on coarse texture soils (UOQ2308-006RTX; Unkovich et al., 2023). This relationship between soil type and response to soil amelioration provides an opportunity to spatially target soil amelioration to soil conditions within a paddock using a variable rate strategy (Hendrie et al., 2024) that can significantly increase the economic benefit of soil amelioration (see Figure 3).

Figure 3. Effect of soil amelioration on grain yield of faba beans (2021), canola (2022) and wheat (2023)
on different soil types within a paddock at Nurcoung, Victoria.

Crop type appeared to have little effect on responses to amelioration, although data sets were dominated by cereal crops (wheat and barley).

Decision support tree

Using the knowledge gained from research encompassing a wide range of experiments, soil and crop types and seasonal conditions using both organic and inorganic amendments we formulated a simple decision support tree (DST) to assist growers and advisers determine where soil amelioration is most likely to produce the best outcomes (Figure 4). A basic starting point of the DST was to determine the need for soil amelioration based on the ability of crops to achieve high water use efficiencies (Sadras and McDonald 2012). We note that there have been some exceptions where the DST would indicate a strong likelihood of response to amelioration but generally no responses were recorded over multiple years of experimentation (e.g. a highly dispersive, hard setting Sodosol in a HRZ environment).

The experimentation underpinning this DST was confined to MRZ and HRZ environments with dense clay soils in south-eastern Australia. As such, the DST should be used with caution before adaptation to other farming systems, including extrapolation to northern region conditions. The DST focused on key biophysical factors (soil type and environment) influencing yield responses of grain crops. It is intended to be used as a precursor before more detailed soil mapping is undertaken, economic factors e.g. cost of ameliorant and rate of application, and logistical factors (e.g. availability of suitable machinery), are considered.

Figure 4. Decision support tree for identifying where soil amelioration is likely to produce yield benefits in SE Australia. For the water use efficiency criteria, use a critical value of 90% in MRZ and 80% in HRZ environments.

Conclusions

Soil amelioration can produce large yield responses and increase profitability but only when it is targeted at appropriate environments. Key factors to consider include soil type, long-term soil water availability, the nature and extent of soil physicochemical constraints and in particularly where they occur within the profile. Before commencing soil amelioration, it is important to undertake a soil survey of the paddock to target only those parts of the paddock where responses are likely, as well as an economic analysis.

Acknowledgements

The authors wish to acknowledge the generous support of the many growers on whose land trials were conducted and the financial support provided by the GRDC and our respective host organisations through projects DAV000149/ DAV1606-001RMX; DJP2209-002RTX, NLP4-GKXBV5N/DJP2204-011SAX, UOQ2308-006RTX. We are particularly grateful to the many technical staff who managed these trials and associated data collection.

References

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Contact details

Prof. Roger Armstrong
Agriculture Victoria Research
Grains Innovation Park, Horsham VIC 3400
Ph: 0417 500 449
Email: roger.armstrong@agriculture.vic.gov.au

Date published
February 2025