Using shallow drainage systems can assist with soil infiltration problems

Mary Kinston

A shallow drainage system is used where soils are heavy and infiltration is impeded at all depths.

The aim of shallow drainage is to change the nature of the subsoil to improve its permeability.

There are three main methods which are mole drainage, gravel mole drainage, and subsoiling/ pan busting. While closely related, each deals with a particular soil type.

Mole drainage suits soils of high clay content, and forms a stable channel.

Being relatively cheap (€50/acre) the success of mole drainage depends on the formation of cracks in the soil. The ideal time to carry out this work is during summer to maximise cracking and prevent wheel spin.

Mole drains should be spaced 1m to 1.5m apart. The depth is often 0.4-0.5m but is dictated by the soil profile.

The ideal set-up has the leg of the mole plough located in a brittle soil and the mole foot in a clay layer below.

The mole plough should be drawn uphill at a consistent gradient and away from open drains.

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Collector drains should be dug to a depth of 0.75m below ground and filled with stone to within 250mm of the surface to ensure connection with the mole drains.

These should be installed across the slope and this should be done before mole draining is used.

These cost between €450 and €1,400/ac depending on space between each collector drain.

Gravel moles are more expensive (€600/ac). They employ the same principles as mole drainage but are filled with gravel from a hopper to support the channel walls in soils of lower clay content.

Subsoiling or pan busting are effective where an impermeable soil has resulted because drainage is impeded by an impermeable layer such as an iron pan or cemented layer.

The cracking or loosening of the soil and breaking impermeable layer may be sufficient to relieve excess water. This is a low cost technique (€50/acre) but will need to be repeated over time.

If you are seeking more information, Teagasc are hoping to release a booklet on drainage options at the Moorepark Open Day in July.

Irish Independent

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