Rejuvenation of compacted soil
Natural recovery from damage
Recovery of a pasture and reappearance of damaged and buried tillers can be relatively rapid, and new tillers are initiated to replace those destroyed by trampling. Perennial ryegrass and white clover commonly used in pastures are among the forage species more tolerant to trampling and poor aeration. This is one reason why they are so persistent in grazed pastures.
Recovery of soil physical conditions after pugging is rather slow and compacted conditions may persist for six or more months. The more important natural regenerative processes include wetting and drying cycles, which cause shrinkage and swelling and cracking of the soil, growth and death of roots causing root channels and the burrowing action of the large earthworm population (e.g. commonly 500 to 800/m2 under pasture).
Use of aerators (subsoiling)
Where compaction has been caused by intensive grazing over a number of years, the effects can be cumulative. In such instances, natural rejuvenation of structure can be very slow and the use of aerators can be beneficial.
The presence of a compacted layer is determined in the same way as that for arable soils (see page 7). That is, several holes are dug in the paddock to a depth of about 50 cm and the profile is examined carefully. Compacted soil may be indicated by an accumulation of roots or water just above or within the compacted zone. It may also be apparent by a lack of large pores, by the existence of "greyed soil" (a bluish-grey colour caused by long-term waterlogging), or by the absence of roots and earthworm activity within the compacted soil and/or high density of that soil. Density of the soil can be assessed by prodding the soil with a knife.
Aerating (also called ripping, subtillage or subsoiling) is the loosening by rigid tine equipment of compacted subsoil. The principles of its use were outlined previously on pages 13 and 14. Where waterlogging of pasture soils is a persistent problem, drainage of the whole profile is the first priority. This should be done prior to considering aeration. Aerating soils with poor drainage is not recommended for wet districts as it may simply move the zone of waterlogging deeper into the soil and result in a soil that is difficult to manage and prone to further compaction.
Extent of the problem
Some degree of treading damage will occur on any soil when it is wet and grazing animals are left in the paddock. The degree of damage is usually greater with cattle than sheep. In many cases, natural regenerative processes will restore soil structure to a satisfactory state. Soils derived from volcanic parent material (volcanic soils) have been found to be more resistant to treading damage than many other soils. The strong binding ability of volcanic clay minerals and associated soil organic matter gives these soils high strength.
Soils with poor drainage are particularly susceptible to compaction because they remain wet for a long time after rainfall. The Yellow Grey earths, for example, have a naturally compact subsoil that is slow to drain. As a result, treading damage on these soils is relatively common, particularly where mob-stocking over winter is carried out.
Impact on farming
As discussed in previous sections, the detrimental effects of treading are most prevalent under wet conditions and include:
- damage to pasture stems and leaves,
- burial of plants in the mud,
- remoulding and compaction of surface soil, and
- subsequent waterlogging of soil.
The ensuing loss of pasture production is an obvious economic loss to the farmer. In particular, farmers who have a large proportion of soils that are potentially susceptible to structural need to consider whether treading is a problem on their property. If so, the measures outlined on page 23 to limit treading damage need to be instituted..
Research in the Manawatu has quantified production losses. Using high stocking rates of 15-44 sheep per hectare, with treading being applied in summer or winter, significant reductions in pasture growth were measured over the following two to three months. These declines averaged:
- about 22% (up to 26%) in pasture yield when treading occurred on dry soil,
- about 27% (up to 60%) when treading occurred on moist soils,
- about 40% (up to 90%) when treading occurred on very wet soils.
In general, short-term losses in the Manawatu seem to typically range from 8 to 23% in summer, and 7 to 45% in winter. Pasture growth is more restricted following treading with cattle than sheep. Nevertheless, the figures for both sheep and cattle suggest that, given a typical feed utilisation ratio of around 0.7, moderately compacted paddocks force a farmer to budget extra feed equivalent to 5-16% of his stock requirements in a dry summer, or 5-32% in a wet winter.
Provided it remains in pasture, soil compacted by animal treading recovers its structure much faster than cropped soil compacted by cultivation machinery. On moderately compacted soils, pasture growth can recover completely within six months. However, severe compaction and associated pugging usually slow regrowth for much longer. Annual yield losses ranging between 15 and 30% have been reported from severely compacted soils in the Manawatu, coastal Otago and Southland, Northland and the Waikato. At a feed utilisation ratio of 0.7, these figures suggest that severe compaction reduces the number of stock a farm can carry by 10 to 20%.
In pastoral systems where compaction is a problem, significant benefits can accrue from subsoiling. Some research in North Otago showed that loosening compacted subsoil increased pasture rooting depth from 43 to 69 cm and total root length by 73%. This greatly increased the amount of soil water in the subsoil that was available to the pasture and decreased its susceptibility to drought. On dryland pastures, where compaction is a problem, significant increases in pasture production can accrue from subsoiling. For example, experiments on subsoiling some dryland pastures in Canterbury showed an increase in pasture production from 6 t/ha on control plots to 7.2 t/ha on subsoil plots.
Contact for Enquiries
MAF Information Services
Pastoral House
25 The Terrace
PO Box 2526
Wellington, NEW ZEALAND
Fax: +64 4 894 0721
Contact this person
