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New results on soil compaction

Tyre width and tyre pressure should be optimised in order to bear the weight of modern, very heavy agricultural machinery.

The crush is on when modern, weighty agricultural machines drive in the field. The result can be damage due to compaction of both the plough layer and the soil underneath. Compaction can reduce yield and can also, not least, affect the soil’s other characteristics, such as its ability to drain off surplus rainwater.

Researchers from the Danish Institute of Agricultural Sciences have investigated if it is possible to reduce the problems with soil compaction.

- Even though there has been talk of soil compaction for decades, there are surprisingly few measurements of the forces that are transmitted downwards in the soil profile from traffic on the field. This is in part due to the great difficulty that lies in placing sensitive instruments in undisturbed soil. We were determined to obtain some data, says senior scientist Per Schjønning from the Danish Institute of Agricultural Sciences (DIAS).

Nowadays, much larger and better tyres are supplied for agricultural machines and wagons than was previously the case. That means that damage to the uppermost soil layer even at heavy loads on the wheels is often minimal and not worse than years ago, when poor, small tyres were used but with a lesser wheel load. The question is if traffic on modern farms has been able to avoid increasing the damage to the soil layer below a depth of 50 cm.

Tests were conducted with different tyre sizes and different wheel loads in an undisturbed stubble field and on newly ploughed soil in the spring. Overall, the results show that the stress level in the deeper layers increases proportionally with the wheel load. It is, therefore, not possible to compensate for heavy wheel loads by upping the tyres, as Per Schjønning points out. However, the studies did show that stress in the deeper layers is actually affected by tyre pressure; the effect is minimal, though, compared to the dominant effect of wheel load.

- With regard to tilling of the soil, theories have been tossed around that a loose plough layer would diminish stress transmission. Our studies show that traffic on newly-tilled soil gives the same stress in the soil profile as on untilled soil. This result is important for harrowing and sowing on newly-ploughed areas, says the senior scientist.

- We can conclude that using good, wide, low-pressure tyres is important for reducing compaction in the upper soil layer, while the only way of significantly reducing it in the layers below is a reduction of wheel load.

Together with advisers from the Danish Agricultural Advisory Service the scientists from DIAS have developed a decision support system that from the spring of next year will make it possible to study the effects of tyre pressure and wheel load on soil compaction via the Internet.

For more information please contact:Senior scientist Per Schjønning, Department of Agroecology, telephone: +45 8999 1766, e-mail: [email protected]

Tuesday 19 December 2006 | Communication Unit