Drainage back on agenda

Published 18 June 15

As an industry we have started to concentrate more in recent years on soils and soil condition but probably have not spoken enough about drainage, which of course is closely related to soil health. It is apparent that a large number of situations where cows could graze earlier and later are hampered by wet ground conditions. Good cow tracks are part of the story, but drainage has been overlooked for a long time and is well overdue for consideration.

Drainage has been overlooked and needs to climb back up the agenda, argues Rob Burtonshaw from Farm Services, who spoke at the AHDB Dairy, AHDB Beef and Lamb, and  BGS Seminars at Grassland UK Shepton Mallet, on 7 May

If you have walked across a field in England, you have almost certainly walked over a land drain. Our heavy soils and wet weather can combine to make working on the land difficult, it has always been the case and people have always tried to do something about it. British fields are covered by a network of drains, slowly moving water, under the power of gravity. Working whenever it rains, twenty-four hours day, seven days a week, this hidden infrastructure is the most underappreciated tool on the farm.

Ever since we started farming we have moved water to and from our crops, and often at great effort. The reason for this effort: the effect on yield. The supply and oversupply of water is a critical limiting factor in yields but that does not stop drainage being overlooked, and many fields would benefit from further investment. Mud, ruts and more mud is not a glamorous combination. It is easy to overlook something which is buried a metre underground, totally out of sight, but drainage is a proven answer to an age old problem.

In 1865 Thomas Scragg invented a method of mass producing clay tiles, and so began the boom in drainage. Of course drainage was nothing new, the practice had been going on for millennia, but the cost of pipe had held back those desperate to increase production. The late nineteenth century saw a massive surge in drainage and many of the drains installed then are still working today.

Drainage was considered worthy of government grant aid after the war, but in the early nineteen eighties these grants were removed, and the number of hectares drained each year dropped from over 100,000 to less than 10,000, where it remains today. For the last thirty years new installation has been at an historic low, growers have been relying on old schemes and often there is a price to pay.

The yield increases with drainage are significant; data from Ontario, Canada, collected over a twenty year period shows a yield increase of 38% in winter wheat, in drained areas. This yield increase does vary greatly from field to field, year to year, but modern drainage schemes are one of the few techniques that can completely transform a poorly performing field. In addition this improvement is not just for one year but many - drainage schemes installed to a high standard of workmanship can last for generations. Such long term capital investment is not insignificant, a comprehensive drainage scheme with lateral drains spaced and with permeable backfill, at 20 metres, will probably cost at least £2,250 per hectare. However, it might not be necessary to install a new system to get good results.

Great attention should be given to maintenance and care of old schemes, sometimes new installations can be avoided by simple actions which do not take long – although they might involve getting wet.

Drainage

Image 1 Don't overlook grazing

If a new scheme is required, one option might be to use a trenchless drainage plough to install the pipework rather than a chain trench. Normally, trenchless installation is quicker and uses less permeable fill, meaning that the price is cheaper. If a recycled aggregate is available locally, it can be a good alternative which can result in savings. Most important is a good design, a clever proposal which minimises the need for main drains is vital.

A new scheme should be devised from a level survey and an assessment of both the soil and the demands on the land. A network of pipes will then be installed around a metre below the ground. The pipe diameters will vary from 60mm for relatively short lateral drains to 100mm and 160mm for main drains, and larger still if the area to be drained is very large or incorporates piping ditches.

On heavier clay soils the use of permeable fill is vital as this provides a pathway for the water to find the pipe from the more porous top layers of soil. On very light soils then it is possible to consider not using a permeable backfill and reduce the costs. Drains should be installed using a specialist drainage machine and the grade maintained either by GPS or laser control.     

Drainage is an investment for the long term, and one which has a proven track record. Almost all farmers or growers need the soil, yet often it is taken for granted. Soil needs to be cared for and invested in like any other resource and a major part of this is balancing the water contained in the soil. 

Ditch maintenance is vital. If outlets are covered by sediment, water cannot escape and the whole scheme backs up. If an outlet is not running during the wet winter months, further examination is required, push some rods up the drain and see if there is a blockage. Tree roots can find drains and block them, remove the roots and often the scheme begins to work again. Dig out the old completion plans and see if you can find the outlet, if it appears to be missing, use a scale rule to work out where it should be, and dig. The old plans tend to be very accurate and, more often than not, a pipe can be found, it might be blocked, but maybe it can be unblocked or a segment replaced and the scheme revitalised. Just make sure that it is a completion plan you are following, not a proposal, completion plans are usually signed and often have annotation.     

Mole ploughing is perhaps the most cost-effective method of improving your drainage, however, successful mole ploughing relies on the right conditions. The soil must be damp and easily moulded at depth, yet dry on the surface. Without these conditions the mole void will not form and cracks will not develop, both vital if water is going to be moved. It is difficult to know if conditions are right and the only way to know if the moles are being produced correctly is to dig down and check. Without using the spade it is impossible to know if what you are doing is working.