Amelioration strategies to improve crop productivity on sandy soils

Take home messages

• All sites without significant repellence or subsoil toxicities in the Southern Region have demonstrated positive first-year responses to deep ripping ranging from 0.2 t/ha to 1.2 t/ha, with an average gain of 0.6 t/ha.
• While most experiments demonstrate multiple years benefit from ripping, yield penalties have been evident following consecutive drought years (2018, 2019), with poor season penalty risks higher in deeper ripped soils (60 cm vs 30-40 cm).
• Some sands have demonstrated extreme hardsetting behaviour which means they have very high soil strength, especially upon drying, and this may limit the longevity of ripping treatments.
• Across project experiments with water repellence and where subsoil toxicities are not present, spading treatments showed a mean annual yield response of +0.77 t/ha.
• Although spading remains the more effective amelioration approach in repellent sands, inclusion-ripping has shown smaller benefits that persist over multiple years.
• Reliable and effective inclusion of topsoil is strongly influenced by operating conditions (e.g. moisture, operating depth and speed), but design modification alongside optimising operation set-up may provide opportunities to improve inclusion-ripping outcomes.

Background

Uptake of amelioration practices to improve productivity in Southern Region sandy soils has gained strong momentum in recent years. These practices include deep ripping which aims to shatter hard/compacted layers; inclusion ripping which both shatters and ‘includes’ some of the topsoil layer at depth; and deep ploughing and spading which aims to mix and dilute repellent or hostile layers, and/or incorporate topsoil into bleached layers. Additionally, inclusion-ripping, deep ploughing and spading practices offer opportunity to incorporate amendments or fertilisers into the profile to improve soil condition or nutrient supply. First year yield responses are often positive but return on investment can require benefits over multiple seasons. Multi-year benefits can be challenging in a water limited environment with high seasonal variability, or where amelioration effects are short-lived. The impact of the quality of soil/amendment mixing and/or inclusion is often not considered. Building on previous amelioration experiments (PIRSA New Horizons est. 2014, Trengove est. 2015), CSP00203 research aims to improve the diagnosis and management of primary soil constraints across deep sandy soils in the Southern low-medium rainfall environment. Including 10 research experiments (5 years) and 18 validation experiments (3 years) the research project is working to define which sandy environments and amelioration treatments are more likely to provide strong return on investment, and where environmental risks or short-lived effects are likely to limit potential benefits.

Method - CSP00203 research and validation trial overview

A range of research experiments have been established across the Southern Region low to medium rainfall environment which have been grouped according to the primary soil constraints identified (Table 1).

Table 1. Summary of sites targeting a range of different constraints including the long-term average annual and growing season rainfall (mm) grouped according to the target soil constraints and associated treatments. Soil properties including repellence as Molar Ethanol Droplet (MED), Depth to severe soil strength (> 2.5 MPa penetration resistance measured near field capacity), surface organic carbon (% OC), surface Colwell phosphorus (P mg/kg), surface pH (1:5 H₂O) and surface exchangeable cations (ECEC cmol+/kg) are shown.*Surface is 0-10cm depth #Not analysed

All sandy sites have inherently low biological and chemical fertility with topsoil (0-10 cm) organic carbon contents of between 0.3 and 0.7%, and ECEC of 2.1-5.0 cmol+/kg, while the pH (5.9-8.8) and Colwell P (11-48) both varied across sites. The starting depth of severe soil strength ranged from 10-30 cm and total depth of the profile affected by severe soil strength ranged from 10-50 cm (Table 1).

Research experiments were established between 2014 and 2020 and include a range of deep ripping and/or ploughing approaches, with/without additional amendments (fertiliser, N-rich hay, chicken manure, clay). All experiments monitored the impact of amelioration on crop growth and yield. Experiments have different levels of measurement whereby some include more intensive measurements to understand the impact of amelioration on crop water use and soil constraints over time and others just focus on crop growth responses. This paper reports responses to deep tillage practices (ripping, spading) alone, without including responses to incorporation of amendments. Findings report the range of yield responses to deep tillage for: a) sands without water repellence issues where physical constraints have been targeted through ripping-based practices; b) water-repellent sands where approaches have focused on spading and/or inclusion ripping to disrupt repellent layers and physical constraints.

Results & Discussion

Ripping deep sands with physical constraints - shattering to maximise root exploration

Yield responses to ripping across experiments with physical constraints are summarised in Figure 1. Except for one non-responsive site, all sites demonstrated a positive response to ripping in the first year (Figure 1b). Yield gains ranged from 0.2 t/ha to 1.2 t/ha, with an average gain of 0.6 t/ha. These responses are similar to those reported by Dzoma et al. (2020) across 5 sandy soil experiments (Loxton, Peebinga, Buckleboo). The non-responsive example is the only project site with severe subsoil acidity (Yenda, NSW) which has shown larger responses to nutrition compared to physical interventions (ripping 30 cm, deep sweep tine) over 4 years of monitoring. Subsequent year responses to ripping across the remaining experimental sites demonstrate an average yield gain of 0.3 t/ha but also include a higher incidence of yield penalties of up to -0.6 t/ha. All observed yield penalties relate to the 2019 season and represent a consecutive year of dry seasonal conditions, likely due to extreme water deficit after crop establishment. Ripping responses in a more favourable 2020 season show benefits ranging between 0.3 t/ha and 0.9 t/ha at responsive sites, including those that suffered penalties in 2019.

Figure 1. Annual crop yield (t/ha) responses to deep ripping in sands where physical issues are considered dominant including (a) biplot demonstrating unmodified control yields against deep ripped yields; and (b) frequency distributions of yield gains (ripped yield – control yield) in the year of ripping and (c) subsequent years following ripping across CSP00203 trial sites.  Data represent treatment averages from seven research experiments (multiple years, n=4) and two validation experiments (single year, n=3) with a total of 40 response years. The linear regression has a fit with R² of 0.81 at P<0.001.

We are examining the soil processes that limit the longevity ripping effects, causing sands to return to their physical constraint over a short timeframe. The four sites that we were able to access in 2020 did not classify as having a cementing layer, but they did have a hardsetting layer that is prone to becoming extremely hard (restricting all root penetration) over very small reductions in soil water content (just 4% w w^-1) (da Silva et al. 2021). This is likely to be a critical issue in low rainfall environments. We will continue to explore this hardsetting response across a broader range of sites (when accessible) and contrast the process under different amelioration strategies.

Analysis of the role of physical disturbance for closing the yield gap (where ripping comparisons were available) reveals that at half of the sites examined the yield gap was closed (Figure 2). At 5 sites, the yield potential (denoted by stars within the stacked column at Bute_B, Kimba, Tempy, Kybunga and Warnertown) determined according to Sadras and Angus (2006) was exceeded. Many of the examples, where a substantive yield gap remained occurred in the very dry season of 2019 (Figure 2). Further analysis of experimental data has revealed that crop rooting depth and water extraction has played a key role in the yield benefits gained by amelioration treatments (data not shown).

Figure 2. Demonstration of the role of ripping (using the best performing treatment including ripping with inclusion) for closing the yield gap at sites where high soil strength is the primary constraint. The cumulative stack shows the control yield (white), the yield benefit due to ripping (dark grey) and the remaining yield gap (light grey). The yield potential calculated according to Sadras and Angus (2006) is represented as a star.

Water repellent sands – mixing to maximise water capture and root exploration

Early research experiments led by PIRSA (New Horizons 2014-2018) demonstrated that spading can have long-term yield impact on water repellent sands with physical constraints, providing subsoil chemical toxicities are not present. Five years of monitoring two research sites (Karoonda, Brimpton Lake) have shown ongoing establishment, biomass, and/or yield gains. A research site at Murlong and seven validation experiments continue to improve our understanding of amelioration responses in repellent sands, including comparing spading and alternative deep tillage practices (Figure 3a). Where subsoil toxicities are not present, there was an average annual yield response of +0.8 t/ha, including examples of substantial gains (+2.1 t/ha) as well as no response in some seasons (Figure 3b).

Although proven to have long-term effect in repellent sands, spading offers practical challenges including trafficking and managing seed depth to successfully establish the following crop.  One-pass operations to simultaneously spade and seed, when conducted into a moist profile, can have advantages including minimising erosion risks, securing uniform crop establishment and increasing flexibility of when in the crop rotation spading might be implemented. While spading is the most effective approach to mix and dilute repellent layers, alternative deep tillage practices can offer some benefit by disrupting water repellent layers, or by overcoming co-occuring physical constraints to root growth. Comparison of spading to inclusion ripping at a severely repellent sand (Murlong), had intermediate benefits from inclusion ripping. A cumulative three-year benefit of 2.9 t/ha was been achieved from spading under a wheat (+1.4 t/ha), barley (+0.9 t/ha), and vetch (+0.6 t/ha) rotation while inclusion ripping showed cumulative gains of +1.4 t/ha and 2.2 t/ha for 30 cm and 40 cm depths respectively.

Figure 3. Annual crop yield (t/ha) responses to spading, ripping (<45 cm) and inclusion ripping (>45 cm) in sands where repellency and physical issues combine presented as (a) biplot of unmodified control yields against deep ripped yields (t/ha) with the dotted line representing 1:1, and (b) frequency distributions of yield gains (ameliorated yield – control yield).The linear regression has a fit with R² of 0.85 at P<0.001.

Although inclusion ripping may appear an attractive option, topsoil inclusion and crop response variability alongside elevated running costs pose challenges for reliable return on investment.  Experiments in WA and SA Mallee sands have shown higher draft requirements (+24% to +40%), reduced workrate (-24%), and extra fuel use (+3.7 L/ha) with baseline inclusion ripping compared to ripping alone (Parker at al. 2019). Simulation modelling with field validation has been used to optimise the design of inclusion plates and has shown that effective depth and included volume of soil could be increased by lengthening the inclusion plate (Ucgul et al. 2019).  A validation trial (2020) conducted on a repellent SA Mallee sand has demonstrated yield benefits of 0.8 t/ha from inclusion-ripping (modified long plates, 60 cm) over and above deep ripping alone (3.9 t/ha) where the control yield was 2.8 t/ha.  While effective inclusion of topsoil is strongly influenced by operating conditions (e.g. moisture, operating depth and speed), opportunities exist to improve this amelioration approach through design modification alongside optimising machinery set-up and operation.

Conclusion

Grouping our sites according to primary constraints and reviewing the ability of amelioration strategies to close the yield gap has revealed that physical disturbance techniques closed the yield gap at half of the sites analysed. All sites which targeted physical constraints (without significant repellence or subsoil toxicities) have demonstrated positive first-year responses to deep ripping ranging from 0.2 t/ha to 1.2 t/ha. While most experiments demonstrate multiple years benefit from ripping, yield penalties have been evident following consecutive drought years (2018, 2019), with poor season penalty risks more likely in deeper ripped soils (60 cm vs 30 cm). Experiments with spading treatments on water repellent sands showed an average annual yield response of +0.8 t/ha. Although spading remains the more effective amelioration approach in highly repellent sands, optimisation of inclusion-ripping is currently being examined considering significant responses on moderately repellent sands.

Acknowledgements

This research has been enriched by preceding research experiments, the significant contributions of growers and consultants across the Southern Region, and the support of the GRDC. CSP00203 research and validation activities are a collaboration between the CSIRO, the University of South Australia, the SA Government Department of Primary Industries and Regions, Mallee Sustainable Farming Inc., Frontier Farming Systems, Trengove Consulting, AgGrow Agronomy and Research, AIREP, and MacKillop Farm Management Group.

References

da Silva et al. (2021) Physical constraints in sandy soils: identifying and understanding reversible strengthening behaviour. The Soil Science Australia and the New Zealand Society of Soil Science Joint Conference, Cairns, June 28-July 1.

Dzoma et al. (2020) Recommendations for deep ripping sandy soils. GRDC Updates, Adelaide 2020.

Parker et al. (2019). Advice to match design of inclusion plates to soil type for optimum effect. GRDC GroundCover Supplement: Soil Constraints Part 1, Nov-Dec 2019.

Ucgul et al. (2019) Science of deep ripping. InGrain magazine, 1(5): 20-25.

Contact details

Therese McBeath
CSIRO Agriculture & Food
Locked Bag 2, Glen Osmond, SA, 5064
0422500449
therese.mcbeath@csiro.au