Strategies to close the yield gap on three water repellent sandy soils in South Australia

Take home messages

• Crop production on many deep sands is well below potential based on their rainfall and ameliorating these soils can cause substantial increases in crop performance.
• Spading compacted sands at Brimpton Lake and Karoonda led to annual yield increases of 0.4 to 1.5t/ha and these increases persisted for 4–5 years.
• Incorporating clay into a deep sand at Cadgee had the biggest benefit to grain yields, achieving annual increases of 0.5 to 1.2t/ha which persisted for at least five years.
• Incorporating lucerne hay was only beneficial in the first two years at Brimpton Lake and Karoonda, but was beneficial two, four and five years after incorporation at Cadgee.
• Crop yields without amelioration at these three sites averaged only 32% of the attainable yield target. This was increased to 47% with spading only, and further to 60% if lucerne was incorporated in the spading operation.
• Substantial yield gaps still exist on some sands following amelioration, reflecting the complex range of constraints present.

Background

Three experiments were conducted across South Australia from 2014 to 2018 to test crop responses in poorly performing sandy soils to a range of surface and subsurface ameliorations. Sites were located at Brimpton Lake on the Eyre Peninsula, Karoonda in the Murray Mallee and Cadgee in the Upper Southeast. The constraints included water repellence, acidity, compaction, low water holding capacity and poor nutrient fertility. Twelve treatments were applied at each site to address the identified constraints.

Here we report the grain yield responses to treatments and impact of those treatments on the gap between current yields and their water-limited potentials.

Method

Treatments applied at each site included an untreated control (UC), a fertiliser package injected into the subsoil (NuPak 30), clay incorporated in the surface A horizon (Clay 10), Clay 10 + NuPak 30, and a factorial combination of four spading strategies: spading only (Sp), spading with a fertiliser package (+ NuPak), spading with clay (+ Cl) and spading with lucerne hay (+ Luc), and combinations of these, resulting in a total of 12 treatments. The influence of each of these components was assessed annually to identify the primary drivers of crop yield response at each site, the magnitude of their impact and the longevity of the effect.

Potential yield and crop water-use were calculated following the method of Hunt and Kirkegaard (2012). Crop water-use was calculated as growing season rainfall + 0.25 summer fallow rainfall. Potential yield was calculated at 12% moisture, using crop factors of 22 for cereals, 15 for lupins and peas and an evaporation factor of 60.

Rainfall data for Brimpton Lake were accessed from Yeelanna (BOM station 18005), whereas weather stations at the Karoonda and Cadgee sites provided rainfall data from August 2014 to December 2018. Local BOM station data from Karoonda and Naracoorte were used for the period November 2013 to July 2014.

Results and discussion

The impact of individual treatment components on grain yield over five production years at Brimpton Lake, Karoonda and Cadgee are presented in Tables 1, 2 and 3 respectively. Values in the tables are the predicted grain yield changes from the untreated control for NuPak 30, Clay 10 and spading, or from spading only for NuPak, clay and lucerne. For the spaded treatments, individual components are additive in each year, that is, at Brimpton Lake in 2014, spading improved wheat grain yields by 0.55t/ha above the untreated control (1.43t/ha), incorporating lucerne in the spading operation added another 0.84t/ha, so the total benefit of incorporated lucerne was 0.55 + 0.84 = 1.39t/ha higher than the untreated control.

At Brimpton Lake, spading had the most consistent impact on grain yield, with increases between 0.4 to 1.5t/ha that were sustained until year five. The addition of lucerne was beneficial in 2014 and 2015 only, adding a further 0.8t/ha each year for those two seasons (Table 1). The lack of response in 2016 to all treatments is attributed to above average rainfall in that season (Table 4).

Soil bulk density prior to spading in 2014 was 1.53g/cm3 in the 0 to 10cm layer, and 1.69g/cm3 in the 10 to 30cm layer, which reduced to 1.5g/cm3 and 1.59g/cm3 respectivelyafter spading in that year, suggesting that a major constraint at the site was physical. Clay 10 had a positive impact in 2018 only, where it increased crop establishment by 78 plants/m2 in that year.

Table 1: Annual crop type, grain yield for the untreated control (UC) t/ha and predicted grain yield change (± standard error) for each treatment component at Brimpton Lake. *denotes components that were statistically significant to the untreated control or spading only in that year: *p<0.05, **p<0.01, ***p<0.001.

Trends similar to Brimpton Lake were also seen at Karoonda. Spading increased yields in three of the five years by 0.4 to 0.85t/ha and incorporated lucerne was also beneficial in 2014 and 2015, resulting in more than 0.4t/ha of extra grain each year (Table 2). NuPak 30 was the most consistently positively performing treatment at Karoonda, whereas the incorporation of clay with spading had a negative impact on grain yield in 2014, 2017 and 2018. The inclusion of clay often led to an increase in biomass (data not shown). No individual treatment component was beneficial in the favourable season of 2016, when growing season rainfall was 48mm above the long-term average (Table 4). Positive crop responses were also rare in 2018, when water supply was less than 40% of the long-term average, suggesting that crop responses to amelioration are better in moderate rainfall seasons.

Table 2: Annual crop type, grain yield for the untreated control (UC) t/ha and predicted grain yield change (± standard error) for each treatment component at Karoonda.*denotes components that were statistically significant to the untreated control or spading only in that year: *p<0.05, **p<0.01, ***p<0.001.

Starting soil bulk density was 1.52g/cm3 in the 0 to 10cm layer and 1.63g/cm3 in the 10 to 30cm layer, reducing to 1.43 and 1.54g/cm3 respectively following spading in 2014. A major soil constraint at Karoonda is therefore physical. Given the NuPak 30 treatment was applied with a deep tine prior to sowing in 2014, we suspect it acted as a ripping operation in addition to supplying deep nutrition. Topsoil was also observed in narrow channels at 30cm in those plots, which were often colonised by roots.

Table 3: Annual crop type, grain yield for the untreated control (UC) t/ha and predicted grain yield change (± standard error) for each treatment component at Cadgee.*denotes components that were statistically significant to the untreated control or spading only in that year: *p<0.05, **p<0.01, ***p<0.001.

In contrast, Cadgee did not have high bulk density in the profile, likely reflecting its history of grazing, rather than cropping. It did, however, have the most severe water repellence (MED >3), was severely acidic, chronically deficient in phosphorus, potassium and sulphur, and had the lowest water retention (field capacity) of the three sites (data not shown).

Spading alone did not increase crop yields at Cadgee beyond the first year, as it did little to address chemical constraints. The application of clay was therefore important at this site to treat water repellence, improve nutrient fertility and water retention. Shallow incorporated clay (Cl 10) had a positive impact on yield in later years, whereas spaded clay was beneficial from 2015, adding 0.5 to 1.2t/ha of grain annually above the untreated control yield (Table 3). Lucerne also increased yields in 2015, 2017 and 2018, adding 0.3 to 0.7t/ha annually.

Given these results, the following treatments were selected for yield gap comparisons; untreated control (UC), spading (Sp) and spaded lucerne (Sp Luc) at Brimpton Lake; the UC, NuPak 30, Sp and Sp Luc at Karoonda; and the UC, Sp, spaded clay (Sp Cl) and spaded clay + lucerne (Sp Cl + Luc) at Cadgee.

Estimates of annual crop water-use, which accounts for stored fallow soil moisture as well as growing season rainfall, are shown in Table 4, along with the water-limited yield potential for each site, and an 80% yield target, which is considered attainable in most years (in the absence of any constraints or adverse events). Table 5 shows the actual yields (adjusted to 12% moisture), the yield gap (target yield – actual yield) and the per cent of attainable yield achieved (actual yield/attainable yield*100).

Table 4: Estimated annual water-use and water-limited yield potential for the three sites over five years (2014-18).

Crops on untreated soils only achieved between 15 and 60% of the attainable yield potential, but amelioration strategies increased these levels by 12 to 42% in responsive years (Table 5).

Table 5: Actual yields, the yield gap and per cent of the attainable yield achieved for the best performing treatments at each site over five years (2014-18).

Wheat grown at Brimpton Lake in 2014, 2015 and 2018 was the most responsive to amelioration, particularly with spaded lucerne, achieving 50, 75 and 74% of the attainable yield target in those years, respectively.

Similarly, at Karoonda, spaded lucerne closed the yield gap to 65 and 70% of attainable yield in the responsive years of 2014 and 2015, whereas spading only closed the yield gap to 54% of attainable yield in 2017.

Crop performance was generally poor at Cadgee, reflecting the complex range of constraints at the site. This demonstrates why broadacre dryland cropping is not common on deep sands in this environment, despite the high water-limited yield potential. A substantial yield gap of 2.7t/ha still existed with spading in 2014, achieving only 28% of the attainable yield target. Results were better in later yields. The barley grown in 2015 was the most responsive crop to amelioration, with spaded clay + lucerne achieving 72% of the attainable yield target.

Post-trial soil characterisation at Cadgee showed small improvements in soil chemical fertility with the best performing treatments, particularly where clay was included. Nonetheless, pH, P and K fertility were still below critical thresholds and would have been yield limiting. The constraints at this site were only partly treated, not fully ameliorated, except for water repellence which was eliminated when clay was included (data not shown).

Conclusion

Overcoming soil physical constraints by spading resulted in a three-year cumulative wheat grain yield increase of 3.55t/ha at Brimpton Lake and 1.87t/ha at Karoonda above the untreated control. This closed the annual yield gap on average by 21% and 24%, respectively.

There was a positive response to spading at Cadgee in the first year (0.46t/ha), which only persisted beyond this point when clay ± lucerne were included, reflecting the chemical and nutritional constraints present there. Spaded clay + lucerne was the best performing treatment in 2015, 2017 and 2018, adding a cumulative 4.35t/ha of grain above the untreated control, closing the yield gap on average by 31%.

Results confirm that the yield gap can be partially closed when major constraints are overcome in sandy soils, leading to annual yield increases between 0.6 and 1.5t/ha that can be maintained for up to five years post-amelioration. The magnitude of the crop response is impacted by the range of constraints present and is best optimised when all constraints are addressed. Substantial yield gaps were still present in some years due to residual constraints, but also climatic factors (e.g., frost, waterlogging, drought), highlighting the need for realistic yield expectations when assessing payback periods following amelioration.

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. We thank the site hosts and technical teams who delivered the experimentation for these trials. GRDC project 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, AIREP, and MacKillop Farm Management Group.

References

Hunt J, Kirkegaard J (2012) A guide to consistent and meaningful benchmarking of yield and reporting of water-use efficiency. CSIRO.

Contact details

Melissa Fraser
Melissa.fraser@sa.gov.au