How furrows work
Furrows are the most commonly used seeding approach for water repellent and non-repellent soils. Top soil can be thrown by seeding points or pushed, by discs and press wheels, to form a furrow. No-till and zero till methods make smaller furrows and retain more soil cover than minimum till methods with narrow knife points which can have higher erosion risks on water repellent sands. There are a number of mechanisms by which furrow sowing can improve crop establishment on water repellent soils:
- Water harvesting into the furrow
- Sowing onto subsurface moisture below the repellent topsoil
- Grading repellent top soil into ridges.
Water repellent soil in the ridges is very effective at making water shed into the furrow (Figure 1). As water pools in the furrows the hydraulic pressure can help it enter the repellent soil. Repellent soil in the centre of the ridges will tend to stay dry and will only slowly wet over an extended period of time with sufficient rainfall. Nutrients thrown into the ridge will often remain inaccessible in the dry soil and weed seeds will not germinate until the soil wets up. Wider furrows will harvest more water due to the broader ridges forming a greater catchment, but the furrow does not need to be deep. Deep furrows can increase the risk of furrow infill and seed being buried too deep as well as increase the erosion risk. More water will be harvested from standing stubble than from flattened stubble.
Sowing seed onto moisture
Zones of dry sand frequently occur in water repellent top soil, while the subsurface soil is often more consistently wet. Furrowing can place seed in or close to moist subsurface sand without making the seeding depth too great. Level sowing to the same depth may make the seed too deep from the soil surface. Deeper furrows will allow sowing onto deeper moisture, but extra care is needed to avoid the greater erosion risks which can occur as more loose sand is moved into ridges.
Grading top soil into ridges
Forming a furrow with a seeding point throws water repellent top soil aside into a ridge, potentially exposing less repellent soil in the furrow which can be sown into. Some nutrients, weed seeds, herbicides and plant pathogens can all end up in the ridge, where it may be left within a dry repellent ridge centre. Weeds and pathogens may have less opportunity to affect the crop when concentrated in ridges, especially if the ridges of water repellent sand remain dry after autumn rains. Access to nutrients in the ridges may be reduced but it can also act as slow release fertiliser as the ridges gradually wet up over the growing season and the nutrients become available to the plants.
Evaporation may be reduced in furrow-sown water repellent sands because the dry centres of ridges and inter-row soils can form an intermittent dry top soil mulch that restricts the transmission of subsurface moisture to the surface where it can be evaporated. Large amounts of retained crop residues in a zero till system will also assist moisture conservation.
Improving the effectiveness of furrow sowing on water repellent soils
Many growers, agronomists and researchers have noticed that in water repellent soils the furrows were remaining dry and that crop establishment in the furrows was poor, characterised by gaps in the crop row where the crop did not emerge. It was found that this was due to loose, dry water repellent sand flowing around the narrow knife points into the furrow rather than being thrown away from the furrow into the ridge. The water repellent surface soil falling back into the slot behind the knife point covers the seed and fertiliser. When this happens the harvested rain from the furrow can infiltrate around the water repellent sand surrounding the seed, not into it (Figure 1). The movement of top soil into the seed zone may also result in herbicides, like trifluralin, being concentrated near the seed at levels damaging to the emerging crop.
The efficacy of furrow sowing on water repellent soil may be improved by:
- Seeding point designs that do a better job of keeping the repellent soil out of the furrow
- Firming the soil using presswheels which can improve water entry
- Use of banded wetting agents to help the wetting up of repellent soil that may remain in the furrow
- Increasing the area of sown soil using paired, ribbon or broadcast seeding techniques
- Seeding on or next to the previous years crop row which often wets preferentially along the residual root systems
- Delay seeding in repellent soils until the soil already has some moisture - soil water repellence expresses more strongly when crops are dry sown
- Increase the sowing rate can result in some improvement in plant numbers.
Seeding point design
Using seeding points or boots with wings can improve grading of repellent soil out of the furrow. It has been shown that this can improve seed germination in water repellent sands by helping prevent the flow of water repellent topsoil back into the furrow. 'Superseeder' winged points and knife points with wings, such as the 'stiletto' seeding boot system, can establish crops more effectively than narrow knife points in water repellent soils. Using greater sowing speeds may also improve crop establishment from knife points when dry sowing severely repellent soils due to the greater throw of soil out of the furrow, but this requires further research.
Trial results indicate that benefits from using winged points are greater with early seeding under dry or partially dry seeding conditions in moderate to strongly repellent soils. Under these conditions yield responses in wheat have ranged from 5-15% compared to narrow knife points.
Changing points to achieve more effective furrow sowing may be one of the most cost-effective strategies for managing water repellent soil.
Firming by presswheels
Firming sand at the furrow base with a light press wheel will help create and maintain the furrow while improving water retention and nutrient uptake by the crop. Press wheels have shown increases in crop emergence and grain yields. Results from Department of Agriculture and Food, Western Australia trials show an 11% additional benefit of presswheels over furrows without press wheels for lupins in a false break of season. Press wheel pressures greater than 2kg/cm width should be used with caution on the better (yellow) sands if lupins are being sown without a deep working point.
Banded wetting agents
Wetting up of the furrow can be improved by spraying a narrow band of soil wetting agent into the base of the furrow behind the press-wheels. This can improve the infiltration of water harvested into the base of the furrow, especially in relatively dry and more repellent conditions and reduce the ‘gappiness’ of crop establishment in furrows.
Paired-row, ribbon and broadcast seeding
Seeding methods that place seed throughout a greater volume of soil rather than in tight, narrow individual rows may be of benefit in water repellent soils. The theory behind this is that because the soil does not wet evenly, spreading the seed through more of the soil may increase the likelihood that some of the seed ends up in preferential flow areas that are wet or wet up more easily. Paired-row and ribbon seeding points and boots also incorporate wings which improve grading of the repellent soil out of the furrow. Some seeding boot systems have keel-shaped extensions below the seed outlet to firm the soil upon which the seed is placed. Some growers have tried broadcast spreading a proportion of the seed (up to 20%) at the front of the seeding bar ahead of the seeding tynes. These approaches reduce the risk of large bare gaps that often occur when single narrow rows pass through dry patches of repellent soil and can also improve crop competition with weeds. Under responsive conditions, dry or partially dry seeding with moderate to strongly repellent soil, wheat yield benefits have ranged from 10-20% or more compared with narrow knife points.
Seeding on the previous years crop row
The residual root systems of previous crops can act as preferred pathways for water entry into repellent soil (Figure 2). Seeding on or right alongside the previous years crop row can improve crop establishment because these areas wet up more readily. Disc seeders or narrow knife points minimise disturbance of the residual roots and coulters may help with stubble management.
Delay sowing and increase sowing rate
Soil water repellence can be expressed more strongly when repellent soil is disturbed while it is dry. As a result delaying sowing until there has been some rain can reduce the expression and impact of soil water repellence. In one such case canola sown dry before rain overnight had very poor establishment while what was sown the following day after rain established much better (Figure 3, below). Increasing the sowing rate can also imporve crop establishment and for some cereal and lupin crops the cost of additional seed can be quite low.
The risks of furrow sowing
- Erosion risk can be increased, especially large loose ridges with sparse groundcover and when dry sowing down steep slopes. Wind shear at the ground surface and raindrop impact can erode soil from the ridges into the furrows, especially before and during the first autumn rains. The resulting furrow infill can bury seed deeper than the original seed depth. Sufficient water movement downslope along a furrow can cause rill erosion and expose or remove seed. Plant residues, stubble and root-bound clods help to control ridge erosion and splash when cover levels are above 30-50%. No-till and zero till methods of crop establishment minimise the erosion risks by maintaining more soil cover. Sowing on the contour may reduce the water erosion risk with all sowing methods.
- Herbicide damage to crops can occur when herbicides become concentrated in the furrow as a result of furrow infill. High stubble levels, narrower furrows, lower ridge height and spraying onto wet soil considerably reduce the risk. Check herbicide recommendations for this situation with your adviser.
- Weed seed and herbicide locked up in the dry repellent ridges can carryover into the following crops
- Leaching and waterlogging can be increased due to the water harvesting effect of furrows in higher rainfall environments or seasons, particularly in soils with poor capacity to retain nutrients and in wetter growing seasons. Wider furrows have a larger catchment and will more easily induce leaching in higher rainfall environments. Splitting fertiliser application and top dressing nitrogen fertiliser onto furrow sown cereals after heavy rain can reduce the effects of leaching. Top-dressing fertilisers before seeding can result in the fertiliser being thrown into ridges where it is 'locked-up' in water repellent soil and less available in dry seasons but may be available more slowly in wetter seasons as the ridges wet up. Sowing crops in the base of furrows on shallow duplex soils which are prone to waterlogging can exacerbate the waterlogging, especially on the south coast of Western Australia. In these situations seeding into the side of the ridge may reduce the risk but may also reduce the establishment benefit in water repellent soils.
- Incomplete germination with lower rainfall occurs when the seed receives enough moisture to imbibe (swell) and start to germinate but has insufficient moisture to complete germination and emergence. In these cases the seed can end up rotting in the soil. Little can be done to control this risk. Re-seeding may be necessary once the soil has wet up. The risk of incomplete germination is increased when seeding into repellent soil with marginal soil moisture and with little forecast rainfall in the short-term. In these instances it may be better to delay seeding until the soil is drier or to wait for more rain to better wet the soil before seeding. While delaying seeding may reduce yield potential, better establishment with the later sown crop and improved weed control may negate some of the loss.
Soil water repellence research is supported by DPIRD and Grains Research and Development Corporation through DAW00244: 'Delivering enhanced agronomic strategies for improved crop performance on water repellent soils in Western Australia' and other initiatives.