Soil compaction is a worldwide problem, with estimates of crop loss due to this issue putting the total at somewhere between 6-15%, depending on soil type and local climatic conditions.

The problem is widely recognised and a great deal of effort is expended in trying to counter the effects – these normally revolve around spreading the contact pressure of a tyre.

Limited effect

These attempts to counteract soil damage usually consider the effect on the topsoil only where running wider tyres at a lower pressure does indeed have some benefit.

Yet, studies into how the subsoil is affected by traffic indicate that the practice of using low-ground-pressure tyres does not make as great a difference at depth as may be thought.

At 30cm or below, it has been noted that it is the axle weight rather than ground pressure which will determine the degree of stress placed upon the soil.

40 years ago it was proposed that an axle weight of 6t should generally be considered a maximum to avoid subsoil compaction. It was very much a rule-of-thumb figure and it has been challenged many times since.

Assessing compaction potential

Linkoping University in Sweden has been giving some thought to the problem and it proposes a simple weighing device by which farmers can measure the weight of individual tractor axles as opposed to an overall weight, which will be given by a weighbridge.

The mechanism consists of an air-driven hydraulic jack that has been adapted to measure the oil pressure and display it in kilogrammes rather than kilopascals.

jack sedish university compaction
A weighing jack for assessing axles weights as proposed by Linkoping University

However, operating it and relying upon the displayed result may not be as easy or as decisive as the developers might wish.

The jack lifts at a single point in the centre of the tractor axle, a practice which is somewhat haphazard and unstable, while access to the rear may well be obstructed by mounted implements.

Work still needed on soil compaction

There is also the fact that raising either end of a tractor will cause the transfer of weight to the wheels that remain on the ground, resulting in a reading that is lighter than the true weight.

The further the axle is raised the greater the weight transfer, which is a particular problem at the front as the axle can swing down to either side, requiring more lift to ensure it clears the ground.

While it is important to draw attention to subsoil compaction and the idea that it is dependent on axle weight rather than ground pressure, this method of assessing the potential of a tractor to cause damage may benefit from a little revision to ensure it is both safe and accurate.