There is no one-size-fits-all solution available to farmers when it comes to management of the soil, visitors to CAFRE Greenmount were told last week.

The underlying theme across all presentations at the sustainable soil management event was that issues with soils need to be investigated before action is taken.

This could be digging a hole with a spade to 30-50cm to see if there is a compaction issue, or undertaking a soil analysis before deciding on a fertiliser plan.

According to CAFRE adviser Darryl Boyd, if you undertake remedial work for soil compaction on land where it is not required, it can actually be counterproductive and create new problems with soil compaction and drainage.

“Where soil is compacted, the depth of the problem tells you what machine you need to use. Make sure you are breaking up soil one or two inches below the compaction,” Boyd said.

Work should be done when soils are dry and able to be lifted and broken into smaller aggregates. Carrying out sub-soiling when conditions are wet can add to problems.

Boyd said that re-compaction is most likely to occur shortly after soils are broken up, so autumn is the ideal time to carry out remedial work, as there will be no traffic on the ground over the winter. This gives the soil time to settle naturally, and if there is a hard frost it will help to crack the soil even more.

Breaking up compacted soil will allow water to penetrate through soils quicker. However, if drainage systems are not working properly or not present, water will not get away and will still accumulate in soils.

“Subsoiling is not a substitute for failed drainage systems. The only thing that will fix that is pipes, stones and money,” Boyd said.

He advised that spikers and aerators work from the surface downwards and generally break up compaction in the top 10cm of soils.

This type of compaction is often caused by grazing livestock, particularly sheep, in the winter months. Adjusting implements so that the spikes run at angles to the surface of the soil, and driving at a faster forward speed, are the main ways of increasing disturbance. Grass growth can be checked for a short period as roots are initially damaged.

Deeper compaction

When it comes to alleviating deeper compaction, sward-lifters and subsoilers are an option as they work to disrupt soils below the surface.

“Tines are narrower and closer together in a sward-lifter and they do not reach the same depths as subsoilers. Most compaction issues in NI are in the top 35cm and a sward-lifter can rectify this,” Boyd said.

Winged teeth are available for the end of tines on sward-lifters and create more disturbance than conventional teeth. He recommended spacing tines with winged teeth at a distance equal to twice the operating depth. Tines fitted with conventional teeth should be spaced at one and half times the depth.

The topsoil on the left is severely compacted, but the topsoil on the right has no compaction and has plenty of space between soil aggregates.

All tines should be fitted with the same type of teeth so that swards are lifted evenly, and sward-lifters and subsoilers should ideally move at 45-degree angles to collector drains to create a path for water to flow into them advised Boyd.

However, care needs to be taken not to damage existing drains. These machines are also not suitable on stony land.

The other option to alleviate compaction is to plough.

“If there is an old sward that needs reseeded anyway, ploughing could be your best option, as long as you are able to plough to the depth of the compaction problem,” Boyd said.

Machinery pressure

After a soil compaction issue has been addressed, farmers should think about what caused the problem in the first place, so that it is not repeated. Robin Bolton from CAFRE said that a cause of compaction on many NI farms can be machinery travelling over land when ground conditions are marginal. A common example is slurry spreading in the spring. The increasing size of machinery is a significant factor, with larger slurry tankers particularly likely to cause compaction. Bolton highlighted two scenarios, the first a 115-horse power tractor and 1,300-gallon tanker. This example had a weight of 1.6 tonne (t) on the front axle of the tractor, 4.5t on the back and 7.5t on the slurry tanker’s axle. This compared to a 140-horse power tractor and 2,000-gallon tanker with weights of 3.2t, 6.8t and 11.8t on the front, back and tanker axles respectively. A four-tonne load per tyre can cause pressure up to 1m below the surface. He recommended aiming for axles loads below six tonnes, or three tonnes per wheel. The pressure and spec of tyres also influences the amount pressure being put on soils. Good practice could be to use ferry tankers to supply to a dedicated machine in the field that spreads a lighter load, has broader wheels and lower tyre pressures.

Role for a mole plough

There are probably a lot of local farms with a mole plough sitting in the back of a shed, which in recent years has gone unused. However, according to CAFRE adviser Trevor Alcorn, a mole plough could potentially be a relatively cheap and simple alternative when looking to correct compaction and improve land drainage. Speaking at the Enniskillen event, Alcorn pointed out that many soils, particularly in the west, have a stable clay base suited to mole ploughing. When undertaking work, ground should be dry on top, to allow efficient traction and cracking of the soil, but not too dry, as the success of the operation relies on the clay being plastic enough to allow a stable channel to be created. In many situations, conditions are near ideal at present. If done correctly, a channel should last at least five to ten years. Where open field drains exist, pull the mole plough from that point, out to a maximum of 20 to 30 metres into the field, advised Alcorn. The channels should be spaced every couple of metres.

On more substantial drainage jobs, mole ploughing can be completed over the top of conventional pipe and stone drains. In this situation, the collector drains should be placed every 20 to 30 metres, and the mole plough drawn across at right angles.

Improving grass swards

The lowest cost method of grass sward improvement is sward renovation or overseeding, with CAFRE figures indicating a typical cost of around £161/acre.

Dr Norman Weatherup from CAFRE told visitors at last week’s event that the main savings come from no cultivation or spraying costs. Grass seed should be sown at 8 to 10kg/acre, and tetraploid or hybrid grass varieties should be used as they have a bigger seed than a diploid variety, and will establish quicker among the existing sward.

Other reseeding alternatives include using minimum cultivation techniques after burning off the existing sward. While it costs up to £215/acre, which is only £23/acre cheaper than a full conventional reseed, the main benefit is that fertile soil is not being ploughed down (or stones ploughed up). According to Brian Hanthorn from CAFRE, granular lime should be spread after a min-till reseed to neutralise acid that is produced in the seedbed as the old sward decays.

Hanthorn said that a conventional reseed is generally recommended in dense swards as the existing grass is ploughed down. Land is out of production for a longer period with a conventional reseed, however it is more reliable for establishing new grass, Hanthorn said.