Take home messages

• Soil amelioration with nutrient rich amendments increased grain yields by 34–65% on duplex soils in the first two years after amelioration, while on heavy textured soil sections within the same paddocks, there were no yield responses.
• Tailoring the right amelioration strategy (including not ameliorating) to each different soil type within a paddock can markedly change returns from amelioration.
• Identifying soil types and the location and severity of soil constraints within a paddock is critical when selecting amelioration strategies.

Background

Physical and chemical subsoil constraints in dryland cropping systems limit crop productivity by reducing the ability of crop roots to access soil resources, particularly subsoil water. It is estimated that subsoil constraints occur on as much as 75% of the cropping soils across Australia (Dang and Moody 2016). Seasonal conditions affect the severity of subsoil constraints on crop production, and therefore they are expected to become more limiting in our drying climate.

Attempts to manage soil constraints using subsoil amelioration with various organic and inorganic amendments in the Southern region have produced variable responses in yields, ranging from none to >60% (Armstrong et al. 2023, Uddin et al. 2022). While this experimentation has focused on singular soil types at experimental sites, soil types and associated soil constraints can vary greatly within paddocks and across landforms. Given this variability and the high upfront expense of subsoil amelioration, we sought to investigate how different soil types and landforms within paddocks will respond to different amelioration strategies. We then calculated if tailoring these different amelioration strategies to different soils in a paddock, rather than blanket application of amendments, can reduce the financial risk of subsoil amelioration by reducing amelioration costs whilst maximising yield responses.

Method

Paddock zoning

As part of identifying and categorising the relative importance of subsoil constraints on a 3-D scale in the broader VGIP2a project, three paddocks in the Wimmera MRZ and one in the Southern Grampians HRZ region were mapped into high and low producing zones (Table 1) using a combination of NDVI, electromagnetic induction (EMI) and gamma radiometric surveying (GRS). At each of the Wimmera MRZ sites (Nurcoung, Nurrabiel and Wallup) the zoning identified the two main soil types in each paddock, whilst at the HRZ site (Wickliffe) the zoning was more dependent on landform position in the paddock and the soil type (yellow chromosol) was consistent across both.

Table 1: Soil zoning and characterisation at each experimental site.

1. Growing season rainfall (GSR) is April to October rainfall

Trial design and sampling

In March 2021, six subsoil amelioration treatments were applied at a maximum depth of 35–38cm in 18m long by 4m wide adjoining plots using a custom-made subsoiler with 60cm tine spacing. At each paddock, these treatments were replicated in four randomised blocks in each of the two soil zones (n=48 plots/site). The six treatments applied at the Wimmera MRZ sites were:

A. Control

B. Deep ripping alone

C. Deep gypsum (2.5t/ha)

D. Surface lucerne pellets (12.5t/ha) + gypsum (2.5t/ha)

E. Deep lucerne pellets (12.5t/ha) + gypsum (2.5t/ha)

F. Deep matched nutrients

At the HRZ Wickliffe site, poultry litter pellets (15t/ha) were applied instead of lucerne pellets. The matched nutrient treatment contained a mixture of inorganic fertilisers to apply the equivalent quantities of N, P, K, and S as contained in the respective lucerne (400:29:264:41kg/ha of N:P:K:S, respectively) or poultry litter (436:118:267:35kg/ha of N:P:K:S, respectively) pellets. This matched nutrient treatment allows us to investigate whether the crop is responding to just improved nutrient supply, or any additional improvement in soil structure from the added organic matter.

During each growing season, crop development was monitored with biomass sampling and NDVI measurements before grain yield and components were measured at harvest. Soil moisture was measured by neutron probe and EM38 mapping. Statistical analysis of soil zone and treatment effects on measured variables was performed by analysis of variance.

Economic analysis

A preliminary analysis of the profitability of investing in subsoil amelioration was undertaken for the Nurrabiel site. The economic analysis relied on crop yields in each soil zone, grain prices and upfront costs of amelioration (Table 2). Only crop yields measured in year one of the trial were used. To account for the residual effect of the different amelioration treatments, using knowledge derived from the DAV00149 project, it was assumed that amelioration costs could be amortised/split over four years. At the conclusion of this project, a time series of yield data will be collated for a fully formulated capital budget.

Economic advantage in year one was evaluated using:

• Net benefit (NB, $/ha), the additional $-returns minus each $ invested in amelioration. If NB >0, then the ameliorant is profitable.
• Benefit Cost Ratio (BCR), the additional $-returns divided by each $ invested in amelioration. If BCR >1, then the amelioration strategy is profitable.

Additional $-returns were calculated from yield gains multiplied by the 10-year average grain price, which for canola in 2021 was a net $578/t. Costs comprise of the cost of both the amendments (excluding delivery) and incorporation costs, including ‘fixed’ machinery ownership costs, and ‘variable’ costs that are proportional to machinery usage (for example, labour, fuel, and lubricants).

Table 2: Amelioration strategy amendment and application costs.

1. Total up-front amelioration costs were amortised over four years using a real 5% discount rate (7% nominal, adjusted for 3% inflation).

Results and discussion

Amelioration treatment effects on grain yields

Soil amelioration and the application of nutrient rich amendments greatly increased crop productivity and yields on some soil types and not others within the four paddocks in 2021 and 2022. The greatest yield responses have occurred in the sodosols, which consist of texture contrast sand-over-sodic clay duplexes, at Nurcoung (Figure 1) and Nurrabiel (Figure 2). In these soils, the application of the deep nutrient treatment increased faba bean yield at Nurcoung by 65% over that of the untreated control in 2021 (P=0.005), and in 2022 canola yields were increased by as much as 58% over the control (P=0.003). At Nurrabiel, the deep nutrient treatment increased canola yield by 34% in 2021 (P=0.100) and wheat yield by 43% in 2022 (P=0.055) over the untreated control treatment. As well as yield effects, large increases in canola and wheat protein contents were measured at both sites. The positive first year yield responses to the deep ripping only and deep gypsum treatments at both sites did not occur at either site in the second year, suggesting that the addition of nutrient rich amendments is required to sustain amelioration responses when ameliorating these duplex sodosols.

Figure 1. Amelioration treatment effect on faba bean and canola yield at Nurcoung in 2021 and 2022.

Figure 2. Amelioration treatment effects on canola and wheat yield at Nurrabiel in 2021 and 2022.

While large treatment effects occurred on the sodosols, no treatment effects occurred on the sodic clay vertosols in soil zone 2 at either site in 2021, while only relatively small treatment effects occurred at the Nurrabiel site in 2022. There are two potential reasons that these soils zones have been unresponsive to our subsoil amelioration treatments so far. Firstly, it is possible that ripping and incorporating amendments to a depth of 35–38cm is insufficient to ameliorate subsoil constraints which only become limiting at greater depths and to promote crop productivity on these sodic vertosols. Secondly, despite dry starts in both years, the growing season rainfall and greater topsoil water holding capacity of these soils compared to the sand-over-clay sodosols may have provided sufficient moisture in the top part of the soil profile for crop production without constraints located deeper in the subsoil limiting crop production in these years.

This latter point may also hold true for the HRZ Wickliffe site, where there were neither soil zone (P=0.467) nor amelioration treatment (P=0.672) effects on wheat yield in 2021, which averaged 8.5±0.5t/ha across all treatments (data not shown). Large early season crop growth responses to the deep manure with gypsum and deep nutrient treatments in both soil zones did not translate into grain yield benefits as the crop in these treatments experienced haying off during grain filling, but grain protein in these treatments was increased over the controls in both soil zones (P<0.001). In 2022, there were again no soil zone (P=0.621) or amelioration treatment (P=0.653) effects on the faba beans grown at Wickliffe which suffered extensive waterlogging throughout the season and yielded only 2.3±0.3t/ha on average (data not shown). A regression analysis to investigate potential slope by deep ripping effects on soil drainage was unable to show any correlation (P=0.862).

At Wallup, the deep ripping only and the deep gypsum treatment had no effect on wheat yield in 2021 (Figure 3), whereas the application of the nutrient rich surface and deep applied lucerne with gypsum, and deep nutrient treatments, increased wheat yields by 34–42% in the calcarosol in soil zone 1 and by 10–33% in the grey vertosol in soil zone 2 (P<0.001). Wheat protein was also increased by an average of 30% and 23% in soil zones 1 and 2, respectively by these treatments (P<0.001, data not shown). In 2022, these positive treatment effects persisted for faba bean yields in the calcarosol (P=0.023), with the deep ripping only and the deep gypsum treatments also producing greater yields than the control. However, the effects of the amelioration treatment occurred in the grey vertosol in soil zone 2 in 2021, did not occur again in 2022.

Figure 3. Amelioration treatment effects on wheat and faba bean yield at Wallup in 2021 and 2022.

While positive responses to each of the deep ripping treatments have occurred in the calcarosol at Wallup, the most productive treatment overall has been the surface applied lucerne with gypsum. This likely indicates that the greatest factor driving crop yield in this soil zone is nutrition and yield benefits can be achieved without the need for expensive deep incorporation. This result highlights the importance of understanding what constraints, whether physical, chemical, or nutritional, are the most limiting to crop production before undertaking subsoil amelioration.

Economic analysis

The NB and BCR of applying all combinations of the six different amelioration strategies to the two different soil types at Nurrabiel in 2021 were calculated and ranked on NB (Table 3).

Without any amelioration, the gross margin of the canola at Nurrabiel in 2021 across the entire paddock was $1,835/ha. If the deep lucerne with gypsum treatment, the most expensive amelioration strategy, was applied to the entire paddock, the grower would have been $205/ha worse off. If, however, the sodosol soil in zone 1 is ameliorated with deep nutrients and the sodic vertosol soil in zone 2 (where there was no yield response) is not ameliorated, then a net additional benefit of $461/ha on top of the initial $1,835/ha gross margin would be achieved, producing a benefit cost ratio of 3.3.

Table 3: Benefit Cost Ratio (BCR) and net benefits (NB, $/ha) in 2021 for blanket applied and soil zone tailored subsoil amelioration strategies (top 10 only) ranked on net benefits at Nurrabiel.

Conclusion

Preliminary results from the first two years of this project suggest that yield responses to soil amelioration differ between soil types within paddocks. Therefore, tailoring different amelioration strategies to these soil types will produce substantial savings in upfront amelioration costs and increase net returns on investment, as shown for the Nurrabiel site. Continued monitoring is required to measure residual effects of the amelioration and to collect durable data for a more robust economic analysis of the returns from tailoring subsoil amelioration treatments to different soil types within paddocks.

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 thanks the McRae, Mewett, Vallance and Wilson families, and Agriculture Victoria.

References

Armstrong R, Dunsford K, O’Leary G, Beverly C, Pritchard F, Stott K, Hendrie D, Willhelm N, Hughes B, Tavakkoli E, Das B, Madovan M (2023) Soil amelioration in medium and high rainfall regions: where will it pay dividends? GRDC Grains Research Update, Bendigo: 21-22 February 2023.

Dang YP, Moody PW (2016) Quantifying the costs of soil constraints to Australian agriculture: a case study of wheat in north-eastern Australia. Soil Research 54, 700-707.

Uddin S, Pitt W, Armstrong D, Hildebrand S, Aslam N, Poile G, Oates A, Fang Y, Armstrong R, Newton D, Jia Y, Sandral G, Lowrie A, Lowrie R, Tavakkoli E (2022) Amelioration of hostile subsoils via incorporation of organic and inorganic amendments and subsequent changes in soil properties, crop water use and improved yield, in a medium rainfall zone of south-eastern Australia. GRDC Grains Research Updates, NSW: 15-17 February 2022.

Useful Resources

3D mapping profiles soil-based constraints 

Contact details

Daniel Hendrie
Agriculture Victoria Research
Grains Innovation Park, Horsham VIC 3400
0470 621 486
daniel.hendrie@agriculture.vic.gov.au