Heavy machinery is highly used in today's agriculture, and although the machines in many ways make fieldwork easier, they also pose a risk for compaction of the soil. Tillage can effectively loosen the compaction of the topsoil, but it often has a very limited effect on the subsoil further down. Researchers from Aarhus University have, in an international collaboration, investigated whether using tyres with certain properties can reduce the risk of compaction of the soil.

"The properties of the tyres play an important role in relation to the risk of soil compaction, and we have investigated if it is affected by the construction of the tyres or by inflation pressure," explains PhD student Loraine ten Damme from the Department of Agroecology at Aarhus university.

Soil stress measure by a probe

In order to measure how different tyres, affect the soil, the researchers have measured the soil stress by using a simple probe with a rubber membrane head, also known as Bolling probes. 

“We have used a so-called balloon probe, which is a kind of balloon that is attached to the end of a long PVC pipe. The pipe is placed in a drilled hole in the ground and later filled with an incompressible fluid like water. When a tractor drives over the probe, the balloon compresses, and we can record the pressure and calculate the stress on the probe,” explains Loraine ten Damme.

According to the researchers, it is an easy way to detect soil stress even though the method does not show the direction of the stress. In other words, it is a measurement of the mean normal stress.

Tyre construction does not affect soil stress

Loraine ten Damme and her colleagues have studied the evolution of tractor tyres and how they can reduce soil stress as much as possible. During the last 50 years, the tyres have evolved from small and narrow tyres to the large wide tyres that we see on tractors today. 

“It came as no surprise to us, when we found that soil stress is significantly lower with the modern large tyres. However, in our study we also found that soil stress was exactly the same for tyres of the same dimension, but with different construction. This means that we can conclude that the construction of the tyre has no impact on soil stress. On the other hand, it is the construction of the tyres that allow the tyre to have certain inflation pressures,” says Loraine ten Damme.

Low-inflation pressure tyres can reduce soil stress

When the construction does not play a major role in relation to soil stress, the researchers had to look at other aspects such as tyre inflation pressure. It became very clear that the larger the contact surface and the lower the inflation pressure of the tyre, the better.

“A low-inflation pressure tyre plays a role in regards to soil stress. When the inflation pressure in the tyre is low, the contact surface will be larger and the pressure will be distributed over a larger area,” explains Loraine ten Damme. 

According to the researchers, this means that in terms of reducing soil stress, it is not necessarily the best solution to buy the most expensive tyres on the market, instead it is important to look for tyres that can be used with a relatively low inflation pressure. 

"If you have a high-inflation tyre, then you will concentrate stress on smaller area. It is better to take advantage of the flexibility of modern tyres and switch between low inflation pressure when driving in the field and a higher, when driving on the road, even if it takes some time to inflate or deflate,” says Loraine ten Damme.

There are benefits to changing tyre pressures

It takes extra time to inflate when you as a farmer have to drive on the roads and deflate again when you go out into the field. But, there are, according to researchers, other good reasons to do so, even if it seems burdensome.

“It can seem a bit troublesome to have to adjust the inflation pressure during fieldwork. When you e.g. are spreading manure, your load will decrease and you will need to deflate and during harvest it is the other way around. However, there are benefits beyond a less compacted soil. When you have the right air pressure in the tyres on the tractor, you can also complete the field work faster and thus save both fuel and time” concludes Loraine ten Damme.