17 May 2002


Tractors and trailers running

on narrow tyres may have

sufficient traction for most

tasks on livestock farms.

But what of the damage

they cause to soil structure

and grass productivity?

Peter Hill examines the

problem and solutions

THE trouble with soil compaction is, from the surface at least, it is invisible. The symptoms are also subtle – ground that remains wet long after the rain has stopped because of restricted drainage and grass reluctant to green up in the spring because of inadequate root development.

In consequence, soils produce poorer forage yields and sward quality and poaching increases. All of which hits the productivity of valuable grass leys.

Teagasc, Irelands agricultural advisory service, conducted a three-year experiment comparing grass yields from fields harvested with machinery running on standard tyres and then larger tyres that could be operated at no more than 0.8bar (12psi) to give a significant reduction in soil compaction.

On a relatively dry site at the organisations Oak Park research centre, near Carlow, the LGP system produced 8% more grass. On a wetter, poorly draining site at Kilmaley, the result was a 16% increase in yield.

Researcher Dermot Forristal notes that in what were three relatively dry years, neither system caused visible surface damage such as rutting.

So even without these symptoms, it is clear that compacted soil limits forage production. The only way to find out is to dig down into the soil profile and inspect the structure and depth of root growth.

On susceptible soils, compaction may well be found deeper than expected since the effect of a heavy slurry tanker, for example, goes far beyond the depth of any wheelings or ruts left on the surface, and certainly well beyond typical ploughing depths.

Taking steps to minimise compaction in the first place requires a two-part strategy – keep off the land when soils are susceptible to compaction (ie when wet) and use tyres of a generous size and specification on tractors, trailers and other heavy field machinery.

Arable farmers have increasingly taken both aspects of this strategy to heart in recent years with positive results, and grassland farms would do well to do the same.

Not necessarily by going to the extreme of fitting huge balloon-type tyres, which are expensive and often impractical, but by at least moving to modern tyre designs (preferably radial) of a size that allows lower inflation and, therefore, ground pressures.

Principal role

The principal role of any tyre is to support the weight of the vehicle on which it is fitted. Its volume and the pressure of air inside the tyre, together with differences in construction, determine the weight a particular tyre can carry.

In general, the larger the volume of air contained in the tyre, the lower is the pressure needed to carry a given weight.

Inflation pressure is important because it broadly equates to the load or pressure imposed on the soil. There is some logic in this given that a tyre stiffens and the contact patch width and length get smaller as it is pumped up.

As inflation pressure is reduced, the tyre carcass can flex, spread its tread across the ground with a longer and wider contact patch, and so reduce the ground bearing pressure of the vehicle.

Reducing ruts

That will reduce the extent and depth of compaction and reduce surface rut depth. It will also make heavy trailers and tankers easier to pull because of a reduction in rolling resistance, which is largely generated by the tyre compressing the soil as it moves forward.

The aim should be to fit tyres that carry the weight of the vehicle at the lowest possible inflation pressure. The latter will be determined by the weight each tyre has to carry, but will be influenced by how far and how fast the tyre will be used on the road.

If a tyre is run too squashy for too long and too fast, it will deteriorate and eventually fail.

The only answer is one of two compromises. Either road speed must be limited so that the tyre can be inflated to its optimum pressure for use in the field, or some field performance must be lost by using an inflation pressure best suited to speedy or extended road travel. &#42

In assessing different tyres for tractors, trailers and other grassland machinery, consider the following characteristics.

Size Width and diameter contribute to the size of the tyre:soil contact patch. Larger diameter tyres have a longer contact patch and less rolling resistance.

Strength Can the tyre support the required load at a relatively low inflation pressure?

Flexibility At low pressures, can the tyre flex sufficiently to maximise the width and length of the tyre/soil contact patch?

Practicality Can the tyres be fitted without making the trailer or implement too wide or too high? Are tandem axles positioned far enough apart to allow large diameter tyres to be fitted?

Options When buying a new trailer, slurry tanker of other field implements of significant size and weight, check out the options list for the largest available tyres and seek advice from a tyre specialist on appropriate sizes and inflation pressures.

Big tyres on heavy harvesting machinery can reduce soil compaction and yield loss on silage leys.

Right: Crossplies such as the Alliance I-328 (pictured) and Vredestein Flotation+ may not have the suppleness of a radial but in generous sizes can still reduce surface damage and sub-surface compaction from heavily laden silage trailers.

Radial technology, as illustrated by the new Trelleborg TWIN Radial (left) and Nokian ELS (below), is becoming more widely applied to trailer tyres, increasing their load capacity, flexibility, and ability to run at lower inflation pressures.