Importance of Good Drainage Highlighted at Forth Monitor Farm Meeting

Drainage was one of the main topics at the recent Forth monitor farm meeting, hosted by Duncan McEwen and his son, also Duncan, of Arnprior Farm, just west of Stirling.

The 815 acre (330 has) Arnprior Farm is one of the network of Quality Meat Scotland (QMS) monitor farms throughout Scotland, and has a variety of soil types, each presenting a different drainage challenge.

The farm is dissected by the A811, with the land to the south of the Stirling to Loch Lomond road, running up to a peak of 590 feet (180 m). The Carse land on the northern side is mainly heavy, blue clay, at just 30 feet (10 m) above sea level, and has been particularly challenging in light of the heavy rainfall of the past two years.

Duncan McEwen senior explained how important good drainage is at Arnprior: “The farm just doesn’t work without drainage. Even with drains of at least 3ft 6 inches deep, at times on our Carse land during the last two years the ground has been too wet to travel on. Some of the land ploughed after harvest has been so water-logged we’ve been unable to cultivate it, so our options on this land for this year are limited. We’re considering either planting oats or re-seeding with a silage mix, once we can get onto the ground!”

Seamus Donnelly, Senior Consultant and Area Manager, SAC Consulting (a division of SRUC, Scotland’s Rural College), who holds a BSc in Soil Science, used the McEwen’s Carse land to illustrate to the community group some of the points to consider when attempting to successfully drain land.

Mr Donnelly listed some of the main benefits of well drained land, including: improved root growth – water logged soil restricts supply of oxygen to roots; better crop and grass yields; reduced risk of liver fluke in livestock; less surface run-off; less soil damage; longer utilisation of fields; and reduced production of Greenhouse gas - Nitrous Oxide (N20).

Mr Donnelly told the group: “Nothing beats walking your land to assess its drainage ability and identify problem areas.

“If you suspect a problem, dig a number of test holes and examine the subsoil. In poorly drained soils, roots are brown and shallow. Also mottled colours in the subsoil, a sour smell and/or a layer of un-rotted manure or crop residue, show that insufficient oxygen is getting into the soil.”

Once problems areas are identified, Mr Donnelly recommended the following course of action: “Start by checking the main ditch or drain outfall to ensure these are clear, with water running.

“Dig holes to check existing drains are not blocked with silt, roots or iron ochre build-up. Also establish whether or not they are dry or under water.

“Then, if further investigation is required, if possible speak to previous generations who farmed this land, and consult old maps to establish drainage work done in the past.

“Once the cause of the poor drainage has been established, decide on the course of action – maintenance, partial repair or new.

“New drainage costs between £1,500 to £2,000 per acre, so if there is no previous drainage system, avoid the area – there will be a reason why this land has not been previously drained.”

Mr Donnelly then outlined the major points to consider before starting work:  “Investigate the old systems – there may be more than one – and establish which are running water. Link any new system into best functioning old system and design from outfall back into the site.

“When designing a drainage system, allow for further expansion and, where possible, install ditches on boundaries. Isolate the site with a gravel catchment drain and minimise requirements for culverts, to reduce potential for future blockages.  Install correctly sized pipes.

“To intercept water, drains must be laid across the slope and lead directly into either a ditch – if iron ochre is likely to be a problem this will enable drains to be rodded/jetted easily – or large leader drain. Space drains at 15 to 20 metres, but closer if the soil is of low permeability or reduced gradient.

“The majority of Scottish soils would benefit from gravel backfill which provides an easy route to move water from the base of the topsoil through the less permeable subsoil to the drains. There are other benefits from backfilling with gravel – it allows the option of secondary treatment such as subsoiling and is also a good way to connect in the old system,” Mr Donnelly added. “It is expensive though, so it makes sense to assess your soil first, dig a few holes and see what soil type you have. If gravel is required, it must be taken to the base of the topsoil layer, but you may get away with the drain depth at 60-70cm if it is a surface water problem and when combined with a narrow trench or trenchless be able to reduce the quantity required and the overall cost.”

The next Forth monitor farm meeting will be towards the end of May.

For further information, please contact either of the joint facilitators:

Colin MacPhail, email: colin.macphail@sac.co.uk or Stephen Whiteford, email: stephen.whiteford@sac.co.uk.Telephone for both – 01786 450964

For general information on monitor farms, plus detailed reports of meetings, please visit: www.qmscotland.co.uk/monitorfarms

Caption: Seamus Donnelly, Senior Consultant and Area Manager, SAC Consulting (a division of SRUC, Scotland’s Rural College) examining a clod of soil with some of the members from the community group and Duncan McEwen Senior (far right).