Renovating damaged pastures and soils
Grazing wet paddocks usually causes varying degrees of damage to pasture and soil. Some form of renovation is often required to return the pasture and soil to its full productive state.
Damage caused by pugging
Stock treading on wet soils causes pasture and soil damage in the following ways.
- crushes and bruises plants, which leads to a reduction in plant density and yield — research in south-west Victoria has indicated that a single pugging event can reduce ryegrass tiller density by as much as 52%
- buries and fouls herbage, reducing pasture utilisation
- creating bare space, which can be invaded by weeds
- makes the soil surface rough, resulting in management problems
- destroys soil structure and pore space, which reduces infiltration rates and further compounds the poor drainage of the soil
- kills earthworms
- increases the risk of erosion and nutrient run-off
- causes soil compaction (increasing soil bulk density), which further reduces plant and root growth and the water-holding capacity of the soil. New Zealand research has shown infiltration rates to be 10 times slower in compacted soils. The connection system between soil pores is damaged, which results in reduced aeration and water storage.
The aim of renovation is to do one or more of the following:
- reduce soil surface roughness
- increase plant density of improved species back to acceptable levels
- repair soils by correcting compaction and restoring structure of pore spaces
Possible pasture and soil renovation methods include:
- natural regeneration (this depends on soil type and conditions)
- smudging or harrowing
- cultivation and resowing
- direct drilling in seed
- sub-soiling or ripping
Pugging by cattle can severely damage pasture and soil structure, but some regeneration of pasture and soil physical conditions does occur naturally. This natural regeneration is a result of a number of processes, including the:
- wetting and drying cycles that cause the shrinking, swelling and cracking of a soil (especially in clay and clayloam soils).
- growth and decay of plant roots leaving root channels, especially by grasses.
- burrowing action of earthworms promote root channels.
Impact of soil type
The rate of natural regeneration can vary considerably depending on the soil type and conditions.
New Zealand research in the Hauraki plains clay soils has indicated that soil structure had not completely regenerated 14 months after a single pugging event. Experience suggests that this process may take as long or longer on some Australian soils, especially if a number of pugging incidents have occurred.
On soil types that regenerate quickly or where minimal pugging occurs, natural regeneration might be enough to repair soil. On some other soil types the natural processes might be too slow to counter the effects of a serious or repeated pugging events, so some of the following renovation methods can be used.
Smudging or harrowing
A common method of levelling and reducing the soil surface roughness caused by pugging is by smudging or heavy harrowing. These methods break off mounds of soil and help fill in the pug marks.
Smudgers are usually constructed out of heavy railway line welded together and dragged behind the tractor. A variety of other implements such as chain, diamond or pasture harrows can be used to level the soil surface.
Smudging and harrowing should be done in spring when the soil is dry enough for sufficient shattering of the soil mounds to occur and to fill the pug marks. If soil is too wet, smearing and compaction of the soil can result, causing further damage. These procedures will not improve the soil structure.
Seed the treated area
Harrowing usually rips out some plants from the pasture, further reducing plant density. Spreading pasture seed on the treated area has been found to be beneficial. A New Zealand example shows that broadcasting ryegrass seed after harrowing resulted in a 16% dry matter production increase the following spring, compared to treatments that was harrowing only.
Rolling is another method used to level a pugged soil surface. It involves towing a heavy roller over the paddock to flatten out the pugged marks caused by cows treading on wet soil.
Rolling is often more effective when the soil is still moist enough for the mounds to be pushed back into the soil. But take care, as rolling when the soil is still moist can cause further compaction of an already damaged soil. This compaction can cause more problems, such as reduced water infiltration, root growth and aeration. These cause reduced future pasture growth, as a result.
Rolling usually results in less damage to pasture species. As with smudging and harrowing, applying pasture seed by drilling or broadcasting will help restore adequate plant density.
Cultivation and resowing
A full cultivation of the soil, together with re-sowing the pasture, will:
- level the soil surface
- increase the density of productive plants
- to some extent improve soil structure damaged by pugging
This is the most time-consuming and expensive option but the most reliable option to return pasture to a productive state. This option would generally be selected in the case of severe damage.
When to resow
A paddock can sometimes be sown straight back to pasture following winter pugging by completely resowing in spring if conditions allow. The resowing would be done once the soil has dried enough for cultivation. Another option is to sow a summer fodder crop and resow the pasture in autumn. Surface drainage works should also be repaired at the time of cultivation.
Direct drilling in seed
Introducing seed into a damaged pasture by direct drilling can improve damaged pastures and increase the density of the productive species. New Zealand research has shown pastures that had been oversown following winter pugging produced 16% more dry matter over a 2-year period than pasture that had not been resown.
It can be difficult to oversow by direct drilling into pugged ground that has a rough surface. The uneven surface makes is difficult to achieve good seed placement and coverage. Direct drilling is often done after harrowing to level the paddock and will improve the consistency of depth with which the seed is sown at. Direct drilling can be particularly effective when there is a large amount of bare ground for seedlings to establish.
Best results are often achieved in autumn when soils are wetting up after the dry summer conditions.
Sub-soiling or ripping
In addition to causing a rough soil surface, pugging can also cause a compacted layer below the pugging depth. This compacted zone can impede water infiltration and restrict subsequent pasture growth. In some situations it has been beneficial to treat the compacted layer by ripping or subsoiling. To remedy sub-soil compaction the soil must be loosened at the level of the compacted layers.
Investigating the sub-soil
To identify where a compacted layer exists in the soil, dig several holes to 50cm depth. Examine them for indications of a compacted layer, including:
- an accumulation of roots or water at a particular level in the soil profile
- soil that appears to be bluish-grey in colour (result of long term waterlogging), or
- poor root or earthworm activity
Scratching suspect levels with a knife can indicate dense compacted layers.
Before you start
A range of sub-soiling equipment is available, most being heavy tyne or 'deep-ripper' types.
Sub-soiling is a relatively expensive and time consuming operation and should not be undertaken lightly. Results can be quite variable and local experience on the particular soil type can be the best guide.