Reduced evaporation:
Autumn weather may have periods of drying conditions following autumn rains. The dry periods can be severe enough to allow evaporation to drought and kill young seedlings.
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 mulch in the topsoil.
Wider row spacings will allow dry mulch to be formed more easily. Loose dry sand spread over the base of a firmed furrow will also reduce water loss from the seed zone by evaporation. Large amounts of retained crop residues in a zero till system will also assist moisture conservation (Figure 4).
Increased overnight soil temperatures:
Low night temperatures in the soil can considerably slow autumn growth. The long wave radiation of heat to the sky at night causes a net loss of heat and lowers soil temperatures.
Furrows reduce the loss of radiation at night and improve soil temperatures. Retaining prostrate stubble also increases the protection against lower night temperatures.
The risks of furrow sowing:
Erosion:
Furrow sowing can increase the risk of erosion, especially on large, loose ridges and in more windy areas. Wind shear at the ground surface and raindrop impact can erode ridge material into furrows, especially before and during the first autumn rains.
This can bury seed deeper than the original seed depth. Pathogens and herbicides can also be splashed onto plants from bare soil. 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 to 50 per cent. Erosion risks, especially in pasture, are increased by larger interrow ridges. Deliberately sowing seed shallowly in the furrow can minimise the risk of over burial.
No-till and zero till methods of crop establishment minimise the erosion risks, but still need sufficient cover; for example, a base of no-tilled sand can still erode. Sowing on the contour will reduce the water erosion risk in all sowing methods.
Herbicide concentration:
The post emergence application of simazine and diflufencian have resulted in severe lupin damage after furrow sowing. High stubble levels, narrower furrows, lower ridge height and spraying onto wet soil considerably reduce the risk.
Much safer methods use metribuzin-diflufenican mixtures onto a wet crop with eight to ten leaves. This treatment has been very effective in northern areas of the Western Australian wheat belt for radish, double gees and cape weed control with no yield penalty.
Weed seed and herbicide lock up:
Carry-over of grass weeds into a cereal crop can be a disadvantage if rain during the lupin season fails to wet up the ridges. The location of herbicides, fertilisers or nutrients in the ridge may or may not be beneficial. Simazine or trifluralin applied just before seeding can help to improve weed control between rows of some crops.
Wider furrows grade larger ridges, which should isolate more surface material away from the seed placement zone and persist longer than smaller ridges made between narrower furrows.
Ridges pushed up by discs and presswheels will not bury weed seed or re-seeding herbicide (Figure 4). Wider rows or lupins, even up to 450 mm, and narrow points or blades will also minimise grass weed and herbicide burial.
Leaching:
Higher rainfalls may accentuate leaching of nutrients from around plants growing in a furrow, especially 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 zones.
Wider rows and narrow points or blades will minimise furrow size and reduce leaching problems in higher rainfall areas. Top dressing nitrogen fertiliser onto furrow sown cereals after heavy rain will also help correct the effects of leaching.
Partial germination by lower rainfall:
Little can be done to control this risk. Reseeding may be necessary. More complex furrow sowing designs
Firming by presswheels:
Firming the sand will increase water holding and improve the uptake of soil moisture by the seed and nutrients by the plant. Results from Agriculture Western Australia trials in the West Midlands of Western Australia show an 11 per cent additional benefit over furrows for lupins in a false break of season. Press wheel pressures greater than 2 kg/cm width should be used caution on the more fertile (yellow) sands if lupins are being sown without a deep working point.
Banded surfactants:
Surfactants come in many formulations. Research on early formulations emphasised the need for longevity to increase the cost benefits. They have already been proven for the establishment of pastures, crops and fodder shrubs.
Statewide trials of the more persistent formulations have revealed a very unpredictable crop yield response to banded surfactant, despite improved crop establishment. New formulations are now being tested which are less persistent, or counteract poor water retention.
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