Drainage

Waterlogged pastures in winter and spring are a common occurrence on many farms in Southern Victoria. Some soils can not drain excess rainfall and become saturated. Waterlogging reduces soil strength, making it vulnerable to pugging and other damage when grazed and plants suffer due to lack of oxygen for root respiration.

There are two basic ways to overcome wet soils:
i). improve soil drainage so the removal of excess water is expedited (e.g. surface and sub-surface drainage).
ii). avoid intensive grazing of paddocks when wet as pugging damage is more likely to occur – try on-off grazing, feedpads, stand off areas.

The best time for farmers to assess the cause of their waterlogging problem, and therefore which drainage system is likely to be the most suitable, is when the soil is saturated (i.e. winter – early spring).

Depending on the severity of wet soils (i.e. length of time affected, % of farm affected, and how often it occurs) will help decide what option is needed on the farm. If it is a significant problem then consider drainage improvement together with on-off grazing.

Even if prolonged wet periods do not occur, it is still highly desirable to adopt an on-off grazing strategy to reduce pasture damage caused by pugging compaction during wet periods.

A trial showed that by November sub-surface drained pastures had much better pasture composition and density. Drained plots had double perennial ryegrass content, eight times the white clover content and a third of winter grass content than the comparable undrained treatment.

Many landholders may be unsure of their soil types and what type of drainage method is best suited to their soil. It is important to select the most appropriate method based on soil characteristics. Mole drains, for example, will not be effective on permeable soils. Mole drains are designed for impermeable (i.e. high clay content) soils. Pipe drains will work on both permeable and impermeable soils, but on impermeable soils they need to be spaced so close together that they may become uneconomical.

Mole drains are unlined channels formed in a clay subsoil using a ripper blade with a cylindrical foot. They are used when natural drainage needs improving due to lack of slope or when a heavy clay subsoil prevents downward drainage.

Humps and hollows are useful in areas or on soil types that are not suitable for tile or mole drainage. They are also useful where the lack of suitable outfalls prohibit the use of tile drains, usually due to insufficient depth or fall.

An experienced drainage contractor can be most helpful in developing a plan of works. It should ideally cover 3-5 years of works.

‘Hump and hollowing’ is the practice of forming the ground surface into parallel convex surfaces separated by hollows. The humped shape sheds excess moisture relatively quickly while the hollows act as shallow surface drains.

Landcare Notes on the DPI website

Farm drainage

Managing wet soils: case study of stand-off areas

Managing wet soils: case study of subsurface drainage (2)

Managing wet soils: feedpads and stand-off areas

Managing wet soils: grazing techniques

Managing wet soils: mole drainage

Managing wet soils: On-off grazing

Managing wet soils: Subsurface pipe drainage

Managing wet soils: renovation of damaged pastures and soils

Managing wet soils: surface drainage

Managing wet soils: what are your best options?

Managing wet soils: what off-paddock system?

Managing wet soils: Types of subsurface drainage?

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