Background
Numerous options exist for managing soil water repellence on deep sands including improved furrow sowing methods, strategic deep tillage and clay spreading or delving. For the water repellent gravel soils it is less clear which options are consistently effective and some of the existing options may not be appropriate.
Aim
To determine which management options can successfully overcome soil water repellence and improve crop yields on water repellent forest gravel soils.
Trial details
| Property | Upwood, north Kojonup |
|---|---|
| Soil type | Loamy forest gravel |
| Crop/variety | Hindmarsh barley |
| Paddock rotation | 2014 - canola, 2013 - barley |
| Treatments | 1) Control – untreated (off-row sown from 2016) 2) On-row seeding (from 2016) 3) Wetter banded on furrow at 2L/ha (on-row sown from 2016) 4) Wetter banded on furrow at 2L/ha (off-row sown from 2016) 5) Wetter banded with seed at 2L/ha 6) Blanket wetter (applied every second year) 7) One-way disc ploughing – with pre-emergent herbicide 8) One-way disc ploughing – no pre-emergent herbicide 9) Mouldboard ploughing – with pre-emergent herbicide 10) Mouldboard ploughing – no pre-emergent herbicide |
| Replicates | Three |
| Sowing date | >4 May 2015 |
| Seeding rate | 100kg/ha |
| Fertiliser (kg/ha) | Pre-emergent: 100kg/ha Urea; 120kg/ha Gusto Gold Post-emergent: 50L/ha UAN |
| Growing season rainfall | April – October: 251mm |
| Herbicide | Pre-emergent: 1.8L/ha Glyphosate; 2L/ha Trifluralin (excluding selected plough treatments) Post-emergent: 670mL/ha Velocity; 300mL/ha Axial |
| Insecticide | Pre-emergent: 500mL/ha Chlorpyrifos; 200mL/ha Bifenthrin Post-emergent: 100mL/ha Transform |
Results
The soil wetter treatments did not significantly reduce the topsoil water (0-5cm) repellence but one-way disc ploughing reduced repellence by 43% while mouldboard ploughing completely removed it (Figure 1).
Establishment in the control plots was very patchy; average plant density was 68 plants/m2 (Figure 2).
Plants continued to emerge over several months following the initial germination.
Where banded and blanket soil wetters were applied, plant establishment increased plant numbers to 75-77 plants/m2, an increase of 10% compared to the control.
The strategic tillage options had the best early establishment with mouldboard ploughing increasing plant numbers by 24% (84 plants/m2) and one-way disc ploughing by 36% to 93 plants/m2 relative to the control (Figure 2).
Both soil wetters and strategic deep tillage increased grain yield (Figure 3).
Furrow banded wetter increased grain yield by 0.72t/ha (26%) over the untreated control while seed banded wetter and blanket wetter increased yields by just over 1.0t/ha (38%; Figure 3).
Responses to one-off deep tillage were even greater. One-way ploughing increased the barley yield by 1.36t/ha (49%) and mouldboard ploughing by 1.71t/ha (62%; Figure 3) compared to the control.
In this experiment the ploughed treatments were tested either with or without pre-emergent herbicides because, in other experiments, pre-emergent herbicides had caused significant crop damage reducing yields. In this experiment, in the 2015 season, there was no significant impact of pre-emergent herbicides.
The applicability of these research trial results to growers is further verified by a large scale on-farm demonstration site on repellent loamy gravel to the south of Kojonup (refer to Soil wetting agents for water repellent forest gravel soils 2015 trial report). In this trial the entire site was seeded using banded wetter but one-off one-way ploughing increased barley grain yields by 0.83t/ha, from 4.33t/ha in the no-till control to 5.16t/ha in the ploughed. The ploughing reduced the topsoil water repellence at the site from very severe (MED = 3.2) to low (MED = 1.0).
Comments
In these trials one-off strategic deep tillage on repellent gravels was highly effective at improving establishment and grain yield of cereal crops.
In other experiments on these soil types, soil inversion with mouldboard ploughs have given mixed results, possibly due to crop damage from pre-emergent herbicides, but this did not impact yields in this trial in 2015.
Soil wetting agents were also very effective in improving establishment and increasing grain yields. This result is supported by other research and grower experience (refer to New opportunities for soil wetting agents on repellent soils.).
The relative cost of the banded soil wetting agent mitigation treatments used in the trial is relatively low, costing about $16/ha, but they need to be repeated annually, however, the returns measured make this highly profitable.
In contrast the strategic tillage amelioration options are slow to implement and have a relatively high cost, approximately $60-120/ha, however, it is a one-off cost that should provide multiple year benefits.
In this experiment the yield gains from strategic tillage more than paid for the cost of the treatment.
Amelioration of water repellent gravels with deep tillage can be limited by rocks and cemented gravel boulders and can leave the paddocks rough.
One-way disc ploughs appear to be an effective tool for ploughing gravel soils and may be better suited to these soil types than other ploughs.
Based on this data the best-strategy would be to use banded soil wetters as a low-cost, high return mitigation approach over all the affected soils on a farm (Smart no-till furrow sowing to optimise whole-farm profit on non-wetting soil) while assessing the benefits of strategic tillage using test strips, and progressively ameliorating suitable and responsive areas over time.
Acknowledgements
The 'Delivering enhanced agronomic strategies for improved crop performance on water repellent soils in WA' project, is funded by DAFWA and Grains Research and Development Corporation (GRDC) and is part of GRDC's Soil Constraints West Initiative.
Thanks to Justin and Deb Elliott for hosting the trial, technical support from DAFWA and Vincent Gallucio (Living Farm) and support from the Souther DIRT grower group.