No drainage work should be carried out before the drainage characteristics of the soil are established by a site and soil test pit investigation, Teagasc’s James O’Loughlin warned at a recent Aurivo Farm Profitability Programme farm walk.

“The worst thing you can do is simply ask someone to drain a field for you. You have to identify the cause of the problem and come up with the most suitable solution. You can spend as much on a bad drainage system as a good one,” he explained.

Drainage system type

Two types of drainage system exist: a groundwater drainage system; and a shallow drainage system. The design of the system depends entirely on the drainage characteristics of the soil.

Distinguishing between the two types of drainage systems, he said, essentially it comes down to whether or not a permeable layer is present that will allow the flow of water with relative ease.

If such a layer is evident, a piped drain system at that depth is likely to be effective. If no such layer is found during soil test pit investigations, it will be necessary to improve the drainage capacity of the soil.

“This involves a disruption technique such as moling, gravel moling or subsoiling in tandem with collector drains,” he said.

Drains are not effective, he said, unless they are placed in a free-draining soil or complimentary measures (mole draining, subsoiling) are used to improve soil drainage capacity. If water is not moving through the soil in one or other of these two ways, the water table will not be lowered.

James O’Loughlin speaking at the Aurivo Farm Profitability Programme farm walk on Padraig Staunton’s farm

The costs

Before undertaking a drainage project, O’Loughlin stressed that farmers must calculate the potential return on investment from the associated works.

Farmers need to ask: ‘What will I get from the field once it’s drained or how much extra grass will it grow?

“Drainage really needs to be a part of the business plan and farmers should aim to maximise production from other paddocks before even considering drainage work.

“I would suggest that drainage should be the last option; focus on the paddocks that need the least amount of work first and grow your business that way. You can then look at drainage, if the costs allow. It’s important that you put a value on it,” he said.

When it comes to the costs involved, O’Loughlin said that this can vary between farms and even between fields. Depending on what is required, the cost of drainage can vary from €4,000/ha up to €7,000/ha. The latter encompasses the costs of gravel moles.

Key steps for successful drainage:
  • Identify the problem and the solution to address it;
  • Remember, a shallow draining system fitted where a groundwater drainage system is needed will not work and vice versa;
  • When using pipes, avoid Ts or intersections as they are prime candidates for blockages;
  • Use the correct size drainage stone. Most of the stone being used for land drainage is too big and only clean aggregate in the 10-40mm grading band should be used;
  • Subsoiling is not effective unless a shallow, impermeable layer is being broken or field drains have been installed prior to the operation;
  • Maintain your land drainage system. Open drains should be clean and as deep as possible. The field drains feeding into them should be regularly jetted or rodded.