Rotary spading combines a degree of soil inversion with soil mixing and has been used to incorporate surface spread clay rich subsoil for ameliorating soil water repellence in South Australia and on the south-coast and northern agricultural regions of Western Australia.
Spading achieves partial inversion unlike the mouldboard plough where full inversion is achieved. Although to a lesser extent than mouldboard ploughing, one-off rotary spading of sand plain soils has been shown to reduce water repellence aiding water infiltration, reduce compaction and change the distribution of organic matter and nutrients all of which usually impact positively on productivity and yield.
The spades on a rotary spader lift seams of subsoil to the surface creating an increased number of preferred pathways for water entry, improving the wetting-up of the soil. Additional mixing or homogenisation of these soils may destroy these preferred pathways and needs to be avoided so the benefits aren’t lost.
While complete soil inversion with a mouldboard plough is better at controlling weeds and more thoroughly reduces repellence, the rotary spader is more successful when it comes to incorporating clay and/or lime into the soil because the mouldboard plough completely buries these amendments rather than mixing them throughout the working depth. Rotary spaders have been one of the few tools able to effectively incorporate high rates of clay rich subsoil of 250 tonnes per hectare or more.
Overall, approximately two thirds of the topsoil is buried through spading, with the remaining third being mixed through the topsoil. Growers using the spading option need to take care not to bury the clay subsoil so deep that the effect of the clay in ameliorating top soil water repellence is lost.
Like complete soil inversion, one of the greatest risks of rotary spading is erosion as a result of complete removal of stubble cover. This risk can be minimised by only spading 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.
Rotary spade versus mouldboard 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.
Spaded soil (left) compared to untreated soil (right) after 52mm of rain. (Source: DAFWA)
The rotary spader simultaneously cultivates (up to 40 cm), incorporates and prepares the seed bed. (Source: DAFWA)