20 August 1999


Heavier tractors, heavier

implements and two wet

springs. Sounds like

its time to check those

mole drains

IF there is little but a trickle of water flowing from piped field drains following a decent bout of rainfall, it is easy to assume a fouled up drainage system is the cause.

But on heavy soils, through which water finds it difficult to filter from the surface, it may be that the piped drains are fine and are only starved of water because of collapsed or distorted mole channels.

These are an integral part of piped drainage systems on heavy soils, intercepting water as it seeps from the surface and providing an easy route to the drains themselves. By speeding up the removal of excess water in impermeable soils, a combination of mole channels and pipe drains will return fields to workable condition more quickly and save crops from extended periods with wet feet.

Although mole channels can remain in good working order for some years, they do not last a life-time. Water erodes the bore and stability suffers when there are extremes of dry and wet conditions. As water flow is impeded, performance of the entire drainage system tails off.

The only remedy is to re-mole the site.

"Judging by the amount of ponding in fields this spring, Id say there are plenty of drainage systems that would benefit from a fresh set of mole channels," suggests Darren Hayes of Warks contractor DW Clark Drainage.

ADAS adviser Chris Stansfield agrees. "Weve had two years with very wet winters and early springs," he says. "That must have led to a good deal of soil damage – compaction and smearing – and probably some further deterioration of mole drains."

With harvest well underway and fields that produced ponds overnight a distant memory, drainage is probably the last thing on growers minds. But post-harvest is the time when most moling is performed, even if it is not the ideal time from the point of view of soil characteristics.

"Ideally, moling needs to be carried out when soil is moist but drying," advises Mr Stansfield. "A dry surface gives plenty of traction, and relatively dry upper soil layers will crack more readily as the mole blade passes through. These fissures allow water to filter more quickly through the soil profile into the mole channels and on to the drains."

Plastic soil

At moling depth, however, the soil needs to be plastic so it deforms as the mole bullet and expander pass through to form a stable channel.

"Moling in a drying cycle also allows the channel walls to harden and stabilise," Mr Stansfield says. "If they have to carry too much water too early, their useful life can be significantly reduced."

These ideal conditions usually occur in spring rather than late summer, but with growers reluctant to see heavy machinery pass through growing crops, this work tends to be a post-harvest/pre-cultivations operation.

Not all heavy soils are suitable for mole draining. Old field drainage plans provide the best indicator because, in most instances, moling will have been carried out as part of a drainage scheme and be indicated on the plans.

"Mole channels can only be made in soil types with the right moulding characteristics because they are lined with nothing more than compressed soil," says Mr Hayes.

"A good guide is to take a bit of soil from moling depth, spit on it and roll it between your fingers. If it remains sticky or breaks up, it is probably not a good moling soil. But if it produces a smooth and shiny ball, mole draining should be an option."

Traditionally, mole drainage has been used on heavy clay soils in East Anglia and the Midlands. But ADAS has long pointed out that the technique can be used successfully elsewhere.

"The main criteria is that the soil must have at least 30% clay content," says Mr Stansfield. "But there is little point moling over a drainage system that was not designed for it in the first place."

That is because pipes and mole channels must be connected by a band of permeable backfill – usually stone – laid on top of the drains. This also dictates the depth to which any new moling must be carried out.

Permeable fill

"We typically put in land drains at 28in to 30in depth, with a 15in band of permeable fill on top," says Chris Cook, operations manager at DW Clark Drainage. "By moling at 20in to 22in, the channel goes straight through the gravel."

Going to a decent depth is also important from the point of view of getting good mole channel formation – it has to be deep enough for the soil to deform and mould itself around the expander rather than heaving and cracking – and to ensure surface cultivation implements do not come close enough to damage the network.

"Its important for all these reasons that any grower carrying out his own mole drainage checks the working depth," says Mr Stansfield. "Hearing the blade crunching through gravel backfill is a good indicator. But there is nothing to beat digging a hole to check that a decent, long-lasting mole channel is being formed."

Farms will often have sufficient power to go moling themselves rather than using a contractor these days – particularly those operating tracked machines. &#42

Above: Ron Hankins makes double beam mole ploughs for contracting, hire, sale and to maintain drainage efficiency on 243ha (600 acres) of heavy arable land at Scotland Wood Farm, Kelmarsh, Northants.

Right: Twin beam arrangements of the Maidwell Moler is now a familiar layout for tractor-mounted mole ploughs. Its a stable layout with high cross beams for trash clearance and a flexible top link coupling for articulation over humps and hollows.

Mole drains provide a "fast-track" route for excess water to reach pipe drains in impermeable soils.


Disc coulter ahead of the depth-adjustable leg limits surface heave, screw adjuster allows fine adjustment of leg pitch to balance draft and down-force needed

for effective penetration.

Spadework is essential to check

soil type and condition, and

the depth of moling.

The end result – water trickling from the mole channel. At Scotland Wood Farm, moling is carried out every five years on average. But the channels can remain in good working order a lot longer.

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