Seeder-based approaches to reduce the impact of water repellence on crop productivity

Take home messages

• Low-cost, low risk seeder-based strategies produced valuable benefits to wheat/barley establishment and grain yield in a severely water repellent sand in two below-average rainfall seasons.
• Several products and application strategies provided consistent and large crop establishment benefits over two years at the same site, while also producing up to 0.22t/ha (Year 1, wheat) and 1.07t/ha (Year 2, barley) extra grain yield.
• Edge-row/on-row sowing achieved the greatest benefits by exploiting existing in-furrow moisture via guided sowing, while 230mm deep furrow tillage produced similar benefits from the opener lifting moist soil deeper in the profile.
• A soil wetter provided grain yield benefits with both edge-row and inter-row seeding over the respective control, while combining the soil wetter with paired-row seeding on the row maximised the grain yield response in the trial (for example; +1.82 t/ha gain over a 0.6 t/ha control).
• Challenges remain in selecting the most effective wetting agents for a particular sand environment due to performance variability.

Introduction

An estimated 12.5 million hectares of sandy soils in southern and Western Australia are deemed at moderate and high risks of water repellence (Roper et al. 2015). These ‘non-wetting’ sands have low fertility and suffer from delayed and uneven wetting, which leads to erratic crop establishment, staggered weed germination and generally poor crop productivity due to low plant densities, low nutrient access, poor weed control and crop damage in areas prone to wind erosion.

A research project supported by GRDC investment (CSP00203) and led by CSIRO is investigating techniques of amelioration and mitigation of sandy soil constraints. A range of field trials are investigating management options available at seeding time to mitigate the impacts of water repellence. During 2018 and 2019, two trials were conducted in a 270mm growing season rainfall (GSR) zone at Murlong on the Eyre Peninsula (EP), namely a soil wetter evaluation trial and a seeder strategy evaluation trial, aiming to compare a number of seeding strategies.

The soil at the site (-33.691295S, +135.944050E) was classified as severely repellent (molarity of ethanol test results were 2.8 at 0-5cm and 3.0 at 5-10cm). In Year 1, a water repellency profile was estimated at seeding using a Water Drop Penetration Test with de-ionised water (Leelamanie et al. 2008), as follows: severe to extreme water repellency (0-10cm), ‘strong’ (10-15cm), ‘slight’ (15-20cm), and non-repellent below 20cm depth.

Soil wetter evaluation trial (2018-19)

Background

Soil wetter chemistries are varied and complex, and little is known of their individual suitability to local water repellence which appears to vary in nature depending up on the soil. Modern soil wetters typically have both surfactant and humectant properties. Surfactants lower the surface tension between water and the soil particles, which allows rainfall to more readily infiltrate into the water-repellent soil. These are penetrant-type products, promoting entry and drainage through the topsoil. Humectants are designed to counter excessive leaching in a low ‘surface area’ sands and aid moisture retention. Humectant co-polymers promote a horizontal spread of water within the sandy soil and increase moisture retained within the furrow seed zone. The benefits of applying soil wetters at seeding time have been evaluated in Western Australia (WA) over the past 10 years (Davies et al. 2019), and this work recently concluded that:

• Banded wetters are most beneficial for dry sown cereals on repellent forest gravels of the south-west with less reliable benefits for break-crops.
• Benefits of banded wetters are minimal, or at best sporadic, for dry sown crops on deep sands and there is no benefit with sowing into moist soil for any crop or soil type.
• Benefits are larger in seasons with low and sporadic germinating rains in autumn.

South Australian (SA) research at Wharminda on EP conducted from 2015 to 2017 found that the two soil wetting agents evaluated could significantly improve wheat, barley and lupin establishment and also have a positive impact on grain yield, in two years out of three (Ward et al. 2019).

Building on these results, the soil wetter trial instigated at Murlong aimed to broaden the range of soil wetter types and combinations evaluated under contrasting furrow placement scenarios.

Experimental design

The impacts of 13 different wetting agents (both surfactants and humectants), in single and dual placement configurations (furrow surface and/or seed zone) were tested over two years (2018 and 2019 seasons). The treatment costs ranged between $12 and $41 per ha (Table 1).

Table 1. Soil wetter treatments evaluated at the Eyre Peninsula, Murlong site over 2018 and 2019.

*Key: SZ=Seed Zone; FS=Furrow Surface

The range of commercial soil wetters evaluated included pure surfactants, surfactant/humectant (S/H) blends, and S/H blends enriched with organics/nutrients. Six treatments consisted of split applications and included single products split-applied at 50:50 rate or combined products applied at full rate in their recommended furrow delivery locations. All suppliers were consulted to ascertain the recommended application rates and locations of each product.

Wetting agents have variable effects in different soil types depending on the site-specific nature of repellence. Treatments were initially pre-tested on the Murlong soil under laboratory conditions showing a de-ionised water control penetration time of more than 120 minutes, whereby the soil wetters at recommended rates resulted in penetration times ranging from 2-3 seconds to 82 minutes.

Plots were 25m long by six crop rows at 0.28m spacings, and were sown at 6km/h using a deep banding knife point operating at 110mm depth, followed by twin seeding discs and a furrow stabilising V press wheel, 140mm wide. A stable consolidated furrow surface is often critical to the efficacy of surface applied soil wetters, working best on a firm settled soil, rather than mixed into loose backfill. Soil wetter treatments were applied in 100L/ha volume of rainwater with foam suppressant at 0.05% v/v, using a Teejet^® TPU1501 low angle flat fan nozzle behind press-wheels to produce a 25-30mm wide band on the furrow surface (FS). In contrast, seed zone (SZ) applications were delivered with aKeeton in-furrow seed firmer to achieve accurate co-location with the seeds.

The trial had four replications organised into a randomised complete block design. In Year 1, the plots were sown with wheat into a grazed barley stubble, while in Year 2, all plots were inter-row sown with barley into the standing wheat stubble. The 2018 treatments were re-applied to the same plots in 2019.

Some aspects of seeding agronomy are summarised in Table 2. Uniform^® fungicide at 400mL/ha and Intake^® Hi-Load Gold fungicide at 250mL/ha were also applied in furrow in 80L/ha volume to address medium/high risks of rhizoctonia or yellow leaf spot and take-all, respectively. Seeding depth in both years was targeted in the range of 3-5cm as a preferred strategy for non-wetting sands.

Table 2. Soil wetter trial seeding agronomy and season overview.

(Key: N=nitrogen; P=phosphorus; S=sulphur; Zn=zinc; Cu=copper; Mn=manganese)

Crop establishment results (2018-19)

Wheat and barley crop establishment rates at five weeks after sowing are shown in Figure 1. The inter-row control established at 24% and 12% of seeds sown (48 and 27 plants/m2, respectively), indicating very unfavourable conditions for crop establishment in this severely water repellent sand.

In 2018, the soil wetter treatments increased wheat crop establishment by 25 plants/m2 on average, with a range of 0 to 58 plants/m2. In 2019, the same treatments increased barley crop establishment by 17 plants/m2 on average, with a similar range of 0-56 plants/m2. The impact of soil wetter treatments on crop establishment was similar in both years, as confirmed by a strongly positive correlation between results in each year (r = +0.849, P<0.001, Figure 2). No correlation was found between product performance and $/ha cost, indicating that cost is not a useful indicator of performance.

Figure 1. Effect of 13 soil wetter treatments on inter-row sown wheat in 2018 (left bar within treatment) and barley in 2019 (right bar within treatment) crop establishment at five weeks after sowing, relative to a no-wetter control (NB: error bar = 1 standard error of the mean)

Interestingly, all furrow surface applied wetters performed poorly at Murlong, while the two seed zone applied (humectant) products performed better. Combining a surfactant on the furrow surface (FS) with a humectant in the seed zone (SZ) provided a synergistic response in 2019 for one combination, greater than the cumulative benefits of each single product (i.e. T1+T2 < T3), but not for another (i.e. T4+T5 ≥ T6), which did not improve benefits beyond the seed zone wetter response, in both years. Overall, five out of the six seed zone+furrow surface wetter combinations provided a benefit.

Figure 2. Correlation between 2018 and 2019 soil wetter treatment effect on crop establishment benefits relative to a 100% control (The data suggest a cluster of six products or mixes which consistently performed well at the Murlong site - details in Table 3 within this paper).

Table 3. Synopsis of top six soil wetter treatment* performances: (Snapshot crop establishment ranking at five weeks and grain yield ranking at harvest).

In 2019, the additional on-row sowing control resulted in crop establishment well above the best soil wetter treatment (+85 plants/m2), which indicates that access to soil moisture under the stubble row is critical in achieving uniform and fast germination in this non-wetting sand. This trial did not combine on-row sowing + soil wetter, but this was done in a second trial at the same site (see the seeder strategy trial).

Table 3 ranks the top six soil wetter treatments used at Murlong, which were consistent across both years. This indicates these products may prove reliable over many seasons on this particular soil type. Anecdotal evidence suggests that some of the other wetting agents not in the top six at this site have performed well in other areas of the state, so a broad evaluation across other types of water repellent sands is advisable.

Grain yield results (2018-19)

Figure 3 shows the grain yield results for both years. In 2018 (decile 2 GSR), the untreated control had an average wheat grain yield of 1.02t/ha. In the first year, grain yield responses to soil wetter treatments ranged from 0 to 21%, with a maximum response of 0.22t/ha. There was a significant positive correlation (r = +0.76, P<0.01) between grain yield and plant density at 38 days after sowing (DAS).

Figure 3. Effect of thirteen soil wetter treatments on grain yield (kg/ha), relative to a no-wetter control(NB: error bar = 1 standard error of the mean)

The earlier break of the season and slightly drier season in 2019 saw larger barley crop responses to soil wetters, with the grain yield of the inter-row sown control averaging 1.10t/ha. Yield responses to the wetter treatments ranged from +23 to +97%, with a maximum increase of 1.07t/ha. In comparison, the on-row control introduced in 2019 yielded the highest (2.15 times more than the inter-row control), providing a 1.26t/ha grain yield benefit. A strong positive correlation (r = +0.883, P<0.01) was obtained between grain yield and plant density at 36DAS. The greater yield responses to soil wetters in 2019 may have been influenced by the stability of the water harvesting furrows produced by the seeding system (Figure 4), compared to 2018 when the challenging post-seeding period resulted in early backfilling of the furrows.

Figure 4. Left: Precision tine-disc seeding system used in the soil wetter evaluation trial; Right: Stable water-harvesting furrows still apparent at 54 days after sowing during 2019.

Overall, the grain yield responses across all treatments were similar for both years, with a strong positive correlation (r = +0.815, P<0.01) between the two data sets (Figure 5). This is encouraging and suggests the better treatments may be recommended to growers in this environment.

Figure 5. Soil wetter treatment grain yield correlation, relative to control during 2018 and 2019 (NB: arrows indicate ±1 std error of the mean control for both years).

Table 3 provides a synopsis identifying the top six performers overall for both crop establishment and grain yield for this site. This evaluation was conducted using a precise split seeding system (knife point + independent dual seeding discs) where co-location of the wetter and seed was assured, and a stable wide furrow was created for the surface wetters applied in a 30mm wide band (Figure 4). The lower performance of a less accurate seeding system used in Trial 2 (see seeder strategy trial) suggests seeding system accuracy had a likely impact on securing these results.

Seeder strategy evaluation trial (2019)

Background

In 2019 a dry 11-12cm thick repellent top layer was present in the inter-row zone at seeding, but with consistent moisture below 16-17cm, which was separated by a patchy transition zone. This situation was similar to conditions seen at sowing in 2018. However, there was good moisture 4-5cm below the existing stubble rows in 2019. Measurements quantified 9mm more water stored in the 0-40cm layer in the stubble row zone compared to the inter-row zone, with the majority in the top 25cm layer. This additional soil moisture under the stubble row at sowing was consistent with observations made in a water repellent sand at Lameroo, where 7-9mm of extra water was measured in the 0-40cm layer under the row in 2018 and 2019.

Experimental design

This trial was sown to barley in 2019 into wheat stubble plots established in 2018. Real-Time Kinetic (RTK) AB-line technology ensured high accuracy when sowing row-guided treatments (Table 2). Plot dimensions, sowing and wetter application techniques were the same as the soil wetter evaluation trial, but this trial was sown five days later (20 to 22 May 2019). Eleven experimental treatments with four replications were organised in a randomised complete block design, and consisted of:

• Six treatments assessing the impact of a selected seed-zone soil wetter (SACOA SE14^® at 3L/ha) under inter-row, edge-row and on-row sowing configurations, at a common 110mm depth of furrow. Different seeding systems were used to achieve edge-row, inter-row and on-row sowing, as shown in Figure 6.
• Two soil wetter treatments assessing the additional impact of a 230mm deep furrow till under inter-row and edge-row sowing.
• Two soil wetter treatments contrasting the impact of an inverted T opener (95mm wide) and of paired-row sowing (75mm spread) at the common 110mm depth of furrow and under on-row sowing configuration.
• One additional contrast to the no-wetter control under inter-row sowing, assessing the impact of a proportion of in-furrow fertiliser; nitrogen (N) and phosphorus (P) (6N+12P) applied with the seeds.

Barley crop establishment

On-row sowing alone increased barley plant density by 39 plants/m2 over edge-row sowing and by 95 plants/m2 over inter-row sowing (Figure 7). Edge-row sowing was much more variable than on-row, indicating the sensitivity of this strategy to optimum position which may be a barrier to adoption. Crop establishment with inter-row sowing was 21 plants/m2 less than the inter-row control in the 2019 soil wetter evaluation trial, which had used a more accurate seeding system (Figure 5 left). The placement of 6N+12P fertiliser with the seed created a small additional loss to an already poor crop establishment in the control (NB: 0.28m row spacing, approximately 10% seedbed utilisation).

Figure 6. Seeder strategies evaluation trial: Left: Baseline double shoot seeding system used for inter-row and on-row sowing; Right: Side banding double shoot seeding system used for edge-row sowing.

The addition of soil wetter increased plant density by 22 and 29 plants/m2 in the inter-row and edge-row sowing treatments, respectively. In contrast, soil wetters provided no benefits with on-row sowing, where the stubble row soil was already sufficiently moist to achieve good germination. This stands in contrast with a single plot unreplicated test conducted in the soil wetter evaluation trial combining treatment (T10) with on-row sowing, which produced a total 119 plants/m2 more than the control, also resulting in the most vigorous and uniform crop growth during the season.

In this case the benefit of the soil wetter (SACOA SE14^®) with inter-row sowing was slightly less than that measured in the soil wetter evaluation trial (22 plants/m2 compared with 36 plants/m2), which may be due to better seed placement and water harvesting by the better furrows obtained in the soil wetter evaluation trial. This perhaps emphasises the importance of considering a range of furrow management issues when looking at the suitability of soil wetters as a mitigation approach.

Deep furrow till to 230mm had a major positive impact (extra +74 plants/m2) under inter-row sowing with a soil wetter, whereby the associated deep moisture delving strongly benefited an otherwise dry seed zone. No corresponding benefit occurred with edge-row sowing, where a 26 plants/m2 decrease was recorded. This may be due to the differences with the side-banding seeding system using a long steep knife point to reach 230mm depth which was probably less effective at lifting moisture up and the extra disturbance may have also reduced the uniformity of seed placement.

Figure 7. Impacts of various inter-row, edge-row and on-row sowing strategies on crop establishment at five weeks after sowing in barley at Murlong in 2019.

Deep furrow till was not evaluated with on-row sowing. However, a positive response to the inverted T opener (+20 plants/m2) was measured, indicating that the extra quantity of moist furrow from soil lifting and mixing benefited seed germination. Under on-row sowing with the soil wetter, the paired row system (T25) did not improve crop establishment over the single row equivalent (T27), both using a knife point opener.

Barley grain yield (2019)

Barley grain yields ranged from 0.5t/ha to 2.42t/ha, with inter-row, edge-row and on-row sowing controls yielded 0.59, 1.45 and 2.0t/ha, respectively (Figure 8). All on-row treatments yielded 2t/ha or more, with paired row sowing (T25) yielding 2.42t/ha. The edge-row sowing treatment benefited from the soil wetter (+0.22t/ha) and the 230mm deep-furrow till (+0.24t/ha). Inter-row sowing also benefited from the soil wetter (+0.37t/ha), and considerably more from the 230mm deep furrow till (+1.16 t/ha). The soil wetter had no effect on grain yield when applied on-row where furrow moisture was sufficient to achieve good germination, while a minor grain yield benefit from the inverted T opener was measured (+0.1t/ha).

Figure 8. Impacts of various inter-row, edge-row and on-row sowing strategies on barley grain yield at Murlong in 2019.

Overall, grain yield responses to treatments were very highly correlated (r = +0.950; P<0.01) with plant densities established early in the season, indicating higher plant populations was a key factor driving barley grain yield under the trial conditions. The inter-row control in the soil wetter evaluation trial yielded significantly more (+0.5t/ha) than in this trial, which may be explained by the combined benefits of five days earlier sowing, greater water harvesting and stable furrows, more precise seed placement and soil wetter co-location achieved by the tine-disc seeding system.

It is worth noting that, in another project trial conducted in a non-wetting deep sand at Lameroo during 2017-2019, significant benefits of edge-row and on-row sowing on wheat and barley crop establishment and grain yield were also obtained, and significant biomass and grain yield responses to 230mm deep furrow till were also measured (Desbiolles et al., 2019). These reinforce the findings of the trials at Murlong.

Conclusions

Two trials conducted over 2018 and 2019 in a highly water repellent sand and under well below-average rainfall conditions at Murlong SA demonstrated:

• Seeder-based strategies for reducing the impact of water repellence can deliver large benefits on crop establishment and grain yield. The strategies evaluated focussed on accessing stored moisture under existing stubble rows, the deeper moisture found below a dry non-wetting topsoil and maximising in-season rainfall infiltration and use.
• Specific technologies were required to implement these strategies, such as: precision guidance (on-row, edge-row sowing), liquid dispensing (soil wetters), seeding system attributes (adjustable depth of furrow till, stable water-harvesting furrows, precision placement of seed and liquids (in-furrow, paired-row seeding, seed-fertiliser separation).
• Combining technologies can deliver additive benefits to crop establishment and grain yield, thus have the potential to form the basis of best practice. However, adoption of some strategies is likely to be limited if major investments are required by the grain grower. Other complications include the fact that water repellent sands usually occupy a part of large paddocks, and variable tracking accuracy with commercial scale machinery.
• Some of the benefits summarised could be achieved with low-technology options such as upgrading seeders with capability for deep moisture delving and seeding at a small angle to existing stubble rows (without RTK guidance) to maximise the benefits of furrow moisture.
• Additional factors that may influence the cost-effectiveness of a soil wetter include optimising-its furrow location, application rate and water volume per ha. These factors may require further experimentation on a product by product basis.
• Project validation activities in 2020 will work with growers to evaluate which seeder-based strategies can be effectively implemented at farmer scale in different sand environments.

Acknowledgements:

The research undertaken as part of this project has been made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC; the authors would like to thank them for their continued support. The authors would also like to acknowledge the technical assistance from Dean Thiele (UniSA) and Ian Richter (SARDI) for trial implementation data sampling and site management, and also support from the soil wetter suppliers listed in Table 1; Syngenta Australia, Nufarm/CropCare, Incitec Pivot Fertilisers and Wengfu Fertilisers.  The broader CSP00203 project team input is also gratefully acknowledged.

References:

Davies S, Betti G, Edwards T, McDonald G, Hall D, Anderson G, Scanlan C, Reynolds C, Walker J, Poulish G, Ward P, Krishnamurthy P, Micin S, Kerr R, Roper M and Boyes T (2019). Ten years of managing water repellent soils research in Western Australia – a review of current progress and future opportunities. GRDC Grains Research Update, 25-26Feb2019, Perth WA. 
2019 Grains Research Updates - GRDC

Desbiolles J, McBeath T, Barr J, Fraser M, Macdonald L, Wilhelm N, Llewellyn R (2019)  Seeder-based approaches to mitigate the effects of sandy-soil constraints. Agronomy Conference, Wagga Wagga (25-29 August 2019) - Agronomy Australia Proceedings 2019-soil-management-and-constraints

Leelamanie D.A.L., Karube J and Yoshida A (2008). Characterizing water repellency indices: contact angle and water drop penetration time of hydrophobized sand. Soil Science and Plant Nutrition, 54: 179-187.

Roger M, Davies, S, Blackwell P and Hall D (2015). Management options for water-repellent soils in Australian dryland agriculture. Soil research 53(7): 786-806. Management options for water-repellent soils in Australian dryland agriculture - CSIRO

Ward P, Wilhelm N, Roper M, Blacker T, Kerr R, Krishnamurthy P, Richter I, Micin S (2019) Low-risk management strategies for crop production on water repellent sands. Agronomy Conference, Wagga Wagga (25-29 August 2019) -
Agronomy Australia Proceedings 2019-soil-management-and-constraints

Contact details

Dr Jack Desbiolles
Agricultural Machinery Research and Design Centre, University of South Australia,
jack.desbiolles@unisa.edu.au
0419 752 295