Soil inversion (mouldboard ploughing)
For the past five years researchers and growers in the northern agricultural region of Western Australia have been assessing the role of soil inversion using mouldboard ploughs.
The mouldboard plough has shown to play an important role in overcoming non-wetting soils, burying herbicide resistant weeds, removing compaction and if required bury lime into acid subsoils. It is used as a one-off soil renovation tool (approximately once every 10 years or more) on sand plain soil in a stubble retention and minimum tillage system.
Mouldboard ploughing overcomes water repellence by burying the repellent topsoil and bringing to the surface a layer of subsoil which is not repellent. Water can readily enter the soil and after sufficient rainfall the buried topsoil fully wets-up and becomes inhabited with crop roots and can stay wetter for longer where it is not subject to evaporation.
The pros and cons of mouldboard ploughing
Bury 'em deep - or not?: Are you considering the use of mouldboard ploughing to reduce the weed seed bank or combat non-wetting soils? Watch this video first.
In a loamy sand soil at Binnu in which the clay content increases with depth in the profile mouldboard ploughing buried the water-repellent topsoil which had a clay content of 3.5 per cent and replaced it with inverted subsoil with an average clay content of 5.5 per cent. Bringing the deeper higher clay content soil to the surface was equivalent to spreading and incorporating 125 tonnes per hectare of clay rich (26 per cent clay) subsoil.
Trial results in the northern agricultural region of WA show substantial productivity benefits from inversion ploughing using mouldboard ploughs. Average cereal grain yield increased by more than 600 kilograms per hectare in the first year and 300kg/ha in following years.
While the primary benefits of mouldboard ploughing is the removal of compaction and increased water infiltration, other agronomic advantages including nearly complete weed control, increased nitrogen mineralisation and improved nutrient access has also been achieved.
Complete soil inversion is required to completely bury weed seeds and water repellent soil and achieve optimum benefits. Good plough set-up and use of a skimmer blade which scalps off the topsoil and turns it into the furrow ahead of the mouldboard (which then folds the subsoil on top) is critical for success.
One of the greatest risks with mouldboard ploughing is the erosion that can result in the year the technique is applied. While there is no way to completely avoid this risk it can be minimised by only mouldboard ploughing the soil when it is wet and by seeding it immediately with a cereal cover crop.
Lupins, canola or other broadleaf crops should not be used as a cover crop because of their sensitivity to sand blasting. On mouldboard ploughed paddocks the stubble of the first year’s cover crop should not be grazed so that sufficient stubble is retained to protect the following year’s crop.
In a trial which had poor soil inversion with a mouldboard plough resulting in a concentration of dry topsoil in the seed zone, the addition of banded-applied wetting agent improved yield by 270 kg/ha over mouldboard ploughing without the use of wetting agent. It should also be noted that this occurred despite above average rainfall at the site — 252mm growing season and 177mm out of season.
Rotary spade versus moudlboard plough - which impliment to choose?
Deciding which of the implements is best to use comes down to each individual growers priorities and their purpose for using the tool.
A mouldboard plough is the best tool if weed control is a high priority. It is the cheaper and faster option, but can require more technical skill to get the plough set-up correctly. The inverted soil is very soft and will need to be rolled in a separate operation. Mouldboard ploughs can invert sandy gravel soils with quite high gravel content provided the gravel is loose and not cemented. The rotary spader on the other hand may not be able to work effectively in these soil types.
Research has shown the rotary spader can control 60-70 per cent of broadleaf weeds, but the cultivation can stimulate germination of grass weeds and increase their numbers compared with the mouldboard plough which controls >90 per cent of both grass and broadleaf weeds with successful inversion.
The rotary spader is the better tool for incorporating amendments, including clay rich subsoil (claying) or lime into acid subsoils.
Spading leaves some water repellent topsoil near the surface so emerging crops do have access to some soil nutrients, including phosphorous in the surface soil, but it may also mean that water repellence can redevelop more rapidly on spaded soils compared with those inverted by the plough.
Many growers and contractors prefer to deep-rip the soil prior to spading to decrease the soil strength improving the speed and depth to which the spader can work, and remove rocks or stumps, but this is an additional cost.
Badgingarra wheat mouldboard ploughed (left) and untreated control (right).
Thirteen-furrow reversible mouldboard plough with hydraulic breakout.