Maintenance of Ground Cover
Maintaining ground cover is critical for reducing the effects of raindrop impact and aggregate disintegration from slaking. Ground cover prevents raindrops from directly striking the soil surface and allow rainfall to slowly penetrate the soil surface. A number of options exist for maintaining vegetation or residue cover during and between cropping operations. These include sowing a cover crop, retaining cereal stubble and direct drilling into a stubble mulch.
Cover Crops
A cover crop can be sown between cropping operations or as a companion crop during the early stages of crop development. An example of a cover crop system used in Tasmania is sowing oats with onions. The oats are either sowed before the onions and are burnt off with non selective herbicide just prior to onion emergence, or the oats and onions are sown together and burnt off about 3 weeks after onion emergence with a selective herbicide. Difficulties with the use of cover crops include competition for soil moisture and sunlight with the cash crop.
- Sown between cropping operations or as a companion crop during the early stages of crop development
- Short term 'solution' to ground cover, ie sowing oats with onions (use of selective herbicide).
Stubble Retention
Retaining a cereal stubble provides ground cover and organic matter during fallow periods and successive crops. Advantages of cereal stubble include, no competition with the crop and reduced herbicide use. Difficulties with tillage and sowing through the cereal stubble may be overcome by finely chopping the stubble before sowing and modifying direct drilling equipment.
- Provides ground cover and organic matter
- Weed control and reduces herbicide use
- Requires development of stubble management techniques and equipment.
Direct Drilling
Direct drilling into crop stubble, reduces tillage and cultivation costs, increases organic matter inputs, increases ground cover, and reduces time and labour. Direct drilling is appropriate for all soil types provided it is well managed with respect to soil moisture, stubble preparation and appropriate use of machinery. The success of direct drilling is largely determined by the amount of organic matter which can be incorporated into surface soils. Under good management the crumb structure or natural tilth of the surface layers should improve with the increased organic matter levels.
- Increases ground cover and organic matter.
- Reduces tillage and cultivation costs
- Time and labour
- Erosion
- Organic matter loss.
- Requires good stubble management
- Correct soil moisture
- Good initial soil structure
- More seed.
Organic Matter
Soil organic matter is important for good crop production and preventing erosion. Organic matter and micro organisms cement individual soil particles into large stable aggregates which resist erosion. Soils with stable aggregates are able to withstand the forces of raindrop impact and slaking and have more and better connected pore spaces which transmit water from the soil surface reducing runoff.
Erosion may be prevented by adopting cropping systems which either prevent the loss of organic matter, or return organic inputs back into the soil. Organic matter is lost from the soil by oxidation during tillage and cultivation operations. Means to minimise organic matter loss include; direct drilling, reduced use of powered tillage implements, minimal cultivation, appropriate herbicide use and chemical fallows.
- Organic matter is the 'glue' that holds soil aggregates together
- Enables soils to resist erosion
- Lost by oxidation during cultivation
- Maintained by reducing fallows, incorporating green manures and minimising tillage.
Green Manure Crops
Organic matter may be returned to the soil by incorporating green manure crops into cropping rotations. Green manure crops such as ryegrass, oats, cereals, or lupins are turned back into the soil to be broken down into organic matter. As well as returning organic matter to the soil, green manure crops provide cover against erosion, biological ripping to improve soil structure, suppress weeds and fix atmospheric nitrogen (legumes).
The amount of organic matter returned to the soil depends on the amount of vegetative material, soil biological activity and climate factors. Typically a green manure crop of oats would return 6 t/ha dry matter to the soil, of which about 25% would be converted to organic matter, resulting in a return of 1.5t/ha organic matter or 0.075% increase in organic matter to a 20cm thick topsoil.
- Green manure crops increase soil organic carbon
- Provide cover against erosion
- Improving organic carbon levels with green manure crops is a slow process.
Preventing and Controlling Runoff
A number of means exist to control or prevent runoff from eroding cropping lands including
- cutoff drains to prevent run-on from entering cropping areas
- contour drains and mulched rip lines to stop runoff within paddocks
- grassed waterways to safely deliver water off paddocks.
Cutoff drains divert water from coming onto the paddock, and safely carry water around the cropped area. Cutoff drains are usually a deep U-shape, the size and width dependent on the amount of expected runoff. Drains usually run along fencelines, headlands or across slope. They are usually constructed with an excavator and trenching bucket. Drains should be installed in summer and where necessary hardened against scouring with either rock riprap or modified to have a grassed flat bottom.
- Stop runoff entering a paddock
- U-shaped open drains
- Follow fencelines and paddock boundaries.
Grassed Waterways
Grassed waterways can be used to control runoff within or adjacent to cropped paddocks. Grassed waterways are wide, shallow grassed channels that carry a large volume of water quickly down a steep slope. Grassed waterways are constructed to spread water evenly over the grassed surface and allow mechanical access for slashing or regular maintenance, they are usually at least 2.5 meters wide, with a minimum depth of 100-200mm.
Depending on the side slope, grassed irrigator runs can be installed using a grader, ridged moulder or spin drainer. Irrigator runs can also be grassed and used as a waterway. Grassed irrigator runs have a modified profile to allow waterflow down the centre of the drain between the wheel marks to prevent the irrigator bogging on wet soils. Grassed waterways and irrigator runs should be sown immediately with perennial ryegrass, fescue and white clover. Seeds should be applied at twice the normal rate and fertilized.
- Carry a large volume of water quickly down a steep slope
- At least 2.5 meters wide, a minimum depth of 100-200mm
- Follow natural drainage lines
- Designed to spread water evenly over the grassed surface and allow mechanical access for slashing or regular maintenance
- Installed using a grader, ridged moulder or spin drainer
- Should be sown immediately with perennial ryegrass, fescue and white clover.
Contour Drains
Contour drains are temporary features which collect runoff from within the crop and divert it into grassed irrigator runs or waterways. Contour drains are installed immediately after planting. Contour drains should be no longer than 50m, 15-30 cm deep and installed on a slight grade to prevent silting. In stable krasnozem soils, contour drains are installed at 5-6% slope and 0.5-2% on erodable sandy soils. Contour drains however present difficulties with other cropping operations such as spraying and harvesting.
- Temporary, installed seasonally after planting
- Divert runoff into grassed irrigator runs or waterways
- No longer than 50m, 15-30 cm deep on a slight grade to prevent silting
- In stable krasnozem soils ~ 5-6% slope and 0.5-2% on erodable sandy soils
- Contour drains present difficulties with trafficking during other cropping operations such as spraying.
Mulched Rip Lines
Development of mulched rip lines has replaced the use of contour drains. Unlike contour drains the mulched rip lines do not require the construction of a ditch. Instead the 'ripper mulcher' places two shallow rip lines and a trail of straw mulch on the soil surface. Contour drains and mulched rip lines work by breaking slope length and preventing the runoff from developing enough speed to scour and form rills. The spacing between contour drains and mulched rip lines is generally between 20-40 meters depending on slope, soil type, crop to be grown, organic matter levels, and likelihood of a heavy rain event.
- Consists of two shallow rip lines and a trail of straw mulch on the soil surface
- Replaces the use of contour drains
- No difficulties with trafficking ~ no ditch
- Works by breaking slope length, slowing water flow and promoting rapid infiltration
- 20-40 meters spacing between lines depending on slope, soil type, crop to be grown, organic matter levels, and likelihood of a heavy rain event
- Requires 3-4 square bales of straw per hectare, 2ha /hr
- Installed on the contour.