Delivering solutions for water repellent soils
Paul Blackwell and Stephen Davies
WA Department of Agriculture and Food, Geraldton
- Some knife point seeding systems allow water repellent sandy soil to flow into the open furrow with the seed and fertiliser reducing establishment. Modifying the seeding system and use of banded in-furrow wetting agents may provide opportunities to improve the success of furrow sowing on water repellent soils.
- Recent findings show that in mouldboard ploughed soils the buried organic matter can hold more soil water at 5-20 cm where it is available to the plant and less subject to evaporation compared to undisturbed soils which also exhibit more ‘dry patch’.
- It is critical to only use mouldboard ploughs or rotary spaders on soils that are wet and to seed them immediately with a cover crop to reduce the risk of wind erosion. Incorporating a green manure crop later in the season when the soil is wet and wind erosion risks are lower and sowing a cereal cover crop or summer crop may be an alternative strategy.
Furrow sowing and in-furrow banded wetting agents
Historically furrow sowing was introduced to water repellent sandplain soils to harvest water to where the seed was which improved crop establishment. Recent evidence, however, indicates that furrow sowing using knife points is often unsuccessful (e.g. Fig. 2; Images 1 and 2) and water repellent soil is often concentrated with the seed and fertiliser in the base of the furrow. It is suggested that this may be a result of the flow of dry water repellent soil around the knife point and into the slot opened by the knife point ahead of the closer plate and press wheel. There are several approaches that could be used to improve the efficacy of furrow sowing:
- Modifying aspects of the design and configuration of the knife point seeding system to minimise the flow of water repellent soil behind the knife point. The modifications that could be made to the seeding system could include:
- Addition of small wings to the knife point or to the seeding boot which help lift the soil and make it flow away from the back of the knife point (Images 1, 2 and 3);
- Reducing the distance between the knife point and the closer plate/seeding boot to minimise the amount of soil that may be able to flow behind the point (Image 3), although this may have negative effects on stubble and vine handling;
- Increasing speed of seeding to increase soil throw away from the furrow with care to avoid soil being thrown into neighbouring furrows. This may also benefit from increased row spacing ( even up to 15") to allow better cover of treflan type herbicides and especially to allow faster sowing rate (about 10 kph).
- Changing the rake angle of the knife point so it is less vertical may result in more soil lift ahead of the point encouraging sideways flow of soil away from the knife point. It appears that knife points with a very vertical rake angle tend to ‘burst’ the soil ahead of the tyne creating a lot of fine, loose soil material which may be more prone to falling behind the knife point.
Image 1. Side-by-side comparison of seeding lupins with knife
points versus winged boots on a deep yellow sand at Balla, 2011
Using a combination of these modifications may give the best result but all of these ideas are just concepts at this stage and need further research and testing.
Image 2: Side-by-side comparison of seeding wheat with knife points vs.winged knife point/boot with paired seeding rows on a non-wetting gravel at Badgingarra, 2011
- Using an in-furrow banded wetting agent to help the base of the furrow wet up essentially avoiding very 'gappy' establishment because the surfactant wets the higher zones of the furrow base better than natural rain alone and water harvesting. Figure 1 shows how dry sown lupins using knife points on a non-wetting paddock at Nookanderri have been more successful with the use of a banded wetting agent, and more especially from sowing on the old cereal row where there is more moisture.
Image 3. Knife point to which a small wing has been added.
Note: distance between knife point and closer plate and setup for banding in-furrow surfactant behind press wheel. Note that the press wheel in the foreground is V shaped in cross section and rips out the edges of the furrow less, allowing a more reliable placement of the banded surfactant band on the furrow base.
Impact of in-furrow banded wetting-agent on lupin establishment between or on the previous season's wheat rows at Balla, 2011.
- Seeding after there has been sufficient heavy/intense rain
wet the soil surface layer may not prevent soil flow behind the knife point but it may mean that the wet surface soil is being incorporated with the seed and fertiliser (Image 4). Note that this concept needs further research and testing to be confirmed.
Image 4: Comparison of canola establishment either seeded dry before 16mm rain overnight or seeded the following day after overnight rain on pale deep sand at Badgingarra, 2011.
Mouldboard ploughing and rotary spading
- Establishment and soil moisture benefits
Mouldboard ploughing and rotary spading are one-off soil amelioration options for water repellent soil that are often successful but are also expensive and irreversible. For this reason it is important that the benefits persist with few if any negative consequences. A number of the farmer demonstration trials established in 2010 were sown to lupins in 2011.
Figure 2. Impact of mouldboard ploughing or rotary spading on lupin establishment in the second season after treatments applied compared with unsuccessful furrow sowing.
Figure 3. Impact of mouldboard ploughing on soil moisture content compared with dry patch or wet soil in a highly repellent untreated pale deep sand at Badgingarra. Organic matter buried by the mouldboard plough occurred from 5-20cm in the soil profile.
In general significant improvements in lupin establishment were measured (Fig. 2). These sites will continue to be monitored and yield differences between treatments determined.
At one of the sites, mouldboard ploughing of a highly repellent pale deep sand resulted in the moisture content of the subsoil at 10-20 cm being significantly wetter than the untreated control (Fig. 3). Furthermore dry patch affected approximately 10-15% of the soil volume in the untreated control soil and this dryness extended from the water repellent surface soil into the subsoil (Fig. 3). Similar findings have been observed at another mouldboard plough trial site at Mingenew.
- Minimising the wind erosion risk
Strong pre-frontal winds coupled with exposed soils following mouldboard ploughing or spading resulted in more wind erosion problems in 2011 (eg. Table 1).
||Average wind speed (km/h)
||Maximum wind speed (km/h)
||Hours wind speed > 29 km/h
||Daily rainfall (mm)
While it is impossible to completely avoid the wind erosion risk after mouldboard ploughing and spading, particularly with such strong and persistent high wind events (Table 1), the following steps can be employed to minimise the risk and damage:
- Only mouldboard plough or spade the soil when it is wet and seed as soon as possible – this may mean leaving spading or ploughing until after the rest of the seeding program has been completed and seeding these areas late or even doing it in spring and seeding a cover crop or maybe a summer crop.
- Only sow cereals in the year the soil has been mouldboard ploughed or spaded due to their greater tolerance and capacity to recover from sandblasting, do not sow canola or lupins. It may even be advisable to grow 2 years of cereals after ploughing or spading to build stubble levels and soil cover more quickly.
Image 5: Soil infill of vehicle wheel marks in trial plots by severe wind erosion prior to rain at the end of May 2011, Nookanderri farm.
The middle section was shallow spaded without the smoothing roller to help incorporate clay, the wings had very little clay (<30 t/ha) and the pitted rough surface left by the spader significantly reduced soil levelling and erosion, compared to the deep spading with smoothing roller or the inversion (mouldboard) ploughing.
- When rolling or firming the soil after ploughing be cautious about creating a very smooth soil surface which will increase the erosion risk (Image 5). Using a ‘sheeps foot’ roller or similar which leaves lots of depressions can reduce wind erosion as can not using or reducing the impact of the smoothing roller on the back of some of the spaders (Image 5).
- Avoid grazing stubbles in the first summer after mouldboard ploughing or spading as it is safer to get several years’ worth of stubble cover before subjecting the site to grazing.
- Pale fine sands with few coarse sand particles and vey little clay content, such as those found on the south-coast, may be particularly vulnerable to wind erosion and extra care should be taken. Loamier sands with higher clay contents and sands with a broader range of particle sizes including coarse grains may resist wind erosion better.
- Mouldboard ploughing or spading in a green manure crop or pasture in spring when the soil is wet and winds are less then seeding with a cereal cover crop or summer crop may be an alternative option.
It should be remembered that water repellent soils are often subject to wind erosion over summer due to poor establishment and improving establishment and crop/stubble biomass by spading or mouldboard ploughing may in the long run reduce erosion risk over these soils but the extreme risk in the first year or two needs to be managed carefully.
This water repellence research is funded by GRDC through the ‘Delivering agronomic strategies for water repellent soils in WA’ (DAW00204) and DAFWA. Thanks to all the growers who have allowed us to be involved in there trials and farms – it is greatly appreciated. Special thanks to the Ford family at Nookanderri farm, Andrew Matthews and CRT Great Northern Rural Services. Thanks to the Northern Agri, West Midlands and Mingenew-Irwin grower groups.
Appendix. Northern Agri Group -Water Repellance Survey Results Summary
KeePad Session Autumn Field Day, 6 April 2011.