17 August 2001

DRAINAGE CHOICES

With water-logged fields just a distant memory for most,

now could be a good time to consider the value of sub-

soiling or mole draining. Geoff Ashcroft sought some

timely advice from ADAS Chris Stansfield

Having just suffered one of the wettest seasons for some time could serve as a useful reminder for many, that the mole plough and sub-soiler still need to be considered as valuable tools in the cultivation armoury.

Before the cultivation process gets in full swing, there are a few simple steps that need to be taken to establish what course of remedial action needs to be carried out, to ensure in-field drainage remains effective and subsequent yields can be maximised.

"The humble garden spade should prove a suitable starting point for many," explains ADAS head of environment Chris Stansfield. "In every situation, it is worth digging exploratory pits to view the soil structure and get an idea of what is going on below the surface. Many farms will have had to force crops into the ground last autumn – and even in the spring – so compaction issues, once again, fall under the spotlight. But you wont get a clear picture unless you dig."

It is also worth considering that digging just one pit in each field will only provide an isolated picture of what is going on. Mr Stansfield reckons several digs will reveal a broader picture across each field, particularly where wet spots are already known, which could lead to better management of compaction and drainage issues.

"Good drainage provides two key functions – it lets excessive amounts of water out and also enables roots to grow down to find moisture," says Mr Stansfield.

Hand digging

While many will be tempted to reach for the keys to the digger at this point, hand digging will help to reveal more about the condition of sub-soils.

"If you find the ground hard to dig using a spade, for example, just imagine how hard it is for plants to put down tap roots," he adds. "It is reasonable to assume that any firmness or solidity felt with the spade could be panning.

"When youve dug a pit, have a good look through the profile of the soil to establish what is going on. Look for layers of crop residues from previous years cultivation work – a visible seam of residue could mean that an anaerobic layer is forming and if the ground remains wet, then air cant get down to it, and bugs wont break down the residue.

"If the soil structure is good, root growth can easily go down 60cm or more," he says. "Any horizontal root growth almost certainly means there is a pan that needs breaking. Once it has been established that the sub-soil needs attention, growers have two possible courses they can follow: sub-soiling or mole draining.

"Soils with a permanent drainage system or those with a clay content of 30% or higher, are better suited to mole draining, while those with less clay or fields lacking a permanent drainage system should be sub-soiled," he says.

"When moling over permanent in-field drains, it is essential to establish a temporary connection with the gravel back fill thats laid over field drains. It is no good making mole channels if the water cannot get down through them to the existing field drains."

Mr Stansfield reckons its worth checking drainage plans before setting off the wrong way across the field with the mole plough.

Right angles

"Always work at right angles to the run of the drains – its a very easy mistake to make. Also, walking behind the mole when in work will allow you to hear when the moles expander passes through gravel back fill – then you know youre doing the job right and youre deep enough.

Spacings between moles should be between 3m and 5m, he reckons.

"A good mole will remain effective for three to five years," he says. "But to get maximum benefit from the mole, it needs to be done when the ground is still drying out and not left until September or October, when heavy rainfall and excessive soil moisture can weaken the effect created by the expander."

In soils where permanent drains are not installed, or the clay content is low, Mr Stansfield recommends sub-soiling to create a deep shattering of the soil to ensure water gets away.

"Spacing between sub-soiler legs should be as close as you can get away with. And using shallow leading tines provides a loosening effect ahead of the winged shares, which create a full width shatter," he says. "You need to disturb as much soil as possible, to allow air and water to penetrate the root zones."

The drier the land, the better the shattering effect and carrying out sub-soiling on stubbles means maximum traction is available at the surface.

"Once the job is complete, try to avoid running about on the land with excessive machine weight which could undo the effectiveness of the sub-soiling or moling," he says. "Dual wheels should be essential on wheeled tractors."

Whether moling or sub-soiling, it is recommended that the occasional dig is carried out behind the machinery, to check the effectiveness of the job.

"You need to look below the surface to see if what youre doing is having an effect," he adds. "If its not working, you may need to refine your strategy before you go in with the drill. Waiting until the following summer could prove to be a season too late." &#42