Wheels and soil – the mechanics of compaction

Dr Marc Dresser is a Research Engineer working at Landcare Research, in Hamilton.

When queried about compaction, Wikipedia, that fountain of all knowledge states that…

“This topic is complicated, because it involves the response of the plant to the soil structure and the availability of water. Thus, it requires knowledge about the stress distribution in the soil below the applied load, and knowledge about the resulting soil deformation and shearing.” I hope to cover a number of these in this paper.

Often, we can see a compaction problem on the surface. We can see the standing water and the ruts. We can see performance issues in the following crop (if the compaction is localised…What if it occurs over the whole paddock….maybe then we blame the variety, or the weather ?!?!)  Sometimes, its more evident than others… and we understand that harvest for some crops is time critical…whatever the weather…though surely we must have a plan in place to fix it?

But even in good conditions, the soil surface may look good, in fact it may look great, but do we actually know what’s happened at depth?

How often do we look at a paddock and it appears to be in pretty good shape, so we do nothing?
What I suggest, and this has been advocated by many in the past, is that a profile pit, some observations and a little bit of investigation could improve subsequent crop performance, or at least not hinder it, could maximise water use efficiency, minimise runoff, and improve overall soil health.

Initially, let’s go back to basics before we get too carried away with ideology and preaching to the converted – you guys are reading this– it’s those that aren’t, that we should be reaching out to….  Soil strength comes from the 2 main areas.  Firstly cohesion amongst soil particles, from forces that attract each other, and from the direct friction between individual interlocking soil particles as they rub shoulders with their immediate neighbours in the soil matrix. And because of these two characteristics, for any given soil, an increase in bulk density will give an increase in soil strength, and a decrease in moisture content will also increase soil strength.

The soil is a relatively stable, living breathing micro system, normally in an open, friable, extremely productive state…and along we come with a big heavy tractor, loaded in excess of 10 tonnes, a highly inflated narrow width tyre, pulling a poorly set up rotary hoe…

So what happens?

The normal stress forces the soil particles together, consolidating the soil from the top down, while the tractive force requirements, applied by the tyre lugs on the soil, causes slip, which gives an additional shear stress perpendicular to the normal stress which in turn forces the particles together sideways.

In addition the vibration of a machine on the surface has the effect of shaking any loose fine particles into any available lower spaces and causing an even more consolidated mass, that now has minimal air spaces, minimal pores available for water movement and an increased restriction to root penetration.

Oops! …We won’t mention the poorly set up rotary hoe smearing the entire seed bed … that’s for another paper!
Work undertaken in the early 70’s by Davies et al. showed the change in infiltration rate under different tractor loadings and wheel slip treatments. Solely a compaction treatment (no slip) resulted in the infiltration rate reducing from 820mm/h to 19mm/h for a single pass of a MF135 loaded to 2 tonne. Adding some wheel slip (approx 30%) a reduction of infiltration rate to virtually 0 was achieved from the same tractor.

Its worth pointing out that under high draught operations, if you can see a tractor wheel slipping, its already experiencing between 10 and 15% slip.

Total weight of a Massey 135 is about 2 tonnes, and a Ford 5000 about 4 tonne.
Not very much in comparison to modern tractors! – a medium sized JD 180hp tractor (perhaps a 7900 series) weighing in at over 8 tonne unballasted…..

So its clear that we can, in one pass of a tractor, reduce the soil from having an infiltration rate of over 700 mm/hr to a virtually sealed soil structure.  And produce a perfect example of a compacted soil in the process!

So what is the effect of that compaction?

Well, increasing the soil density has a profound effect on growth.
Work from the early 80’s by Negi et al. shows quite clearly that it doesn’t take a great increase in soil density to have a huge effect on yield of forage maize in this instance. They show that 10 – 11 t/ha dry mater yield was recorded in the 1.3 – 1.45 t/m3 soil dry density range, and this dropped to less than 6 t/ha dry matter yield at 1.55 t/m3 soil dry density.

The soil provides support for animals, tractors and other field going equipment.
It is a medium for plant growth and water retention, and it’s a playground for all the recreational tillers and 4wd’ers. All of these interactions we know can cause compaction. And if, therefore, we know we are causing it, how do we identify compacted areas so we can do something about it?

Surface compaction is relatively easy to identify. Its that hard blocky structured layer that often jars your wrist as you try to dig a profile pit. It is often restricted to the top 300 mm, making it easy to spot, easy to target and even easier to remove with a wide, relatively shallow, cost effective tillage operation.
But what about deeper compaction? The kind of compaction that extends beyond 300mm?

This is the compaction that we can’t easily see, is often hard to find and often missed. This compaction takes a long time and a lot of energy to remove, assuming we can remove it at all…  Determining where this compaction is, quantifying it, and then devising methods to minimise it in the first instance, or if we can’t, then to remove it, is a hard task.

The use of a soil tank and integral soil preparation machine that can remove all the soil in that tank, and layer it back in, 50mm layer at a time, such as the one at Cranfield University in the UK is invaluable in this kind of research. Each 50 mm soil layer can be prepared to any density and moisture content you require…time after time after time. Repeatable tests, with no in field variability, means small changes in machine setup give repeatable differences in results.

Using special techniques, it is possible to measure resultant forces, subsequent soil deformation at depth and examine methods to minimise the formation of compaction.  This can be done for both non powered implements and tyres, as well as powered implements, tyres and track units.  Couple to this the ability to electronically record the pressure at various depths in the soil in real time. The sensitivity at 0.25m depth showing quite clearly that detection of the front sprocket, the two idlers and the main drive sprocket as the track unit passed over the sensor was possible. In fact detection of a furrow slice being lifted off by a plough was also detectable, as was the increased load as the next furrow slice was turned over back onto the sensor location!

So with this recording equipment, why not test various implements such as mouldboard plough, discs, tines, drill openers, subsoilers, harvester shares etc, operating at their standard depths. What effect do they each have on pressure transmission to depth? Add to that an ex front row forward, lumbering up the soil bin as a comparison.
And then, let’s also add some tyre and track loading and pressure combinations – including a road going truck style wheel, such as those commonly used for collecting harvested product either directly alongside the harvester, or sometime on the paddock headland.   It became evident that most implements are a non issue with regard to pressure transfer at depth, as was the author, in comparison to the tyres and tracks!

Its not surprising that the road going truck tyre, inflated to 7 bar and rock solid with no carcase flex exerts more than twice the peak pressure @ 250mm depth than anything else tested.
The performance of the track, even at 12t load, is amazing in comparison.
This agrees with other results, from the 1960’s that show the difference in normal stress in the soil profile of a tyre and a track.

Now we have proven that we can easily cause an issue, and we can, in some cases minimise it, thats all well and good, but how can we, as field managers who want to do something about it, measure this effectively in the field?

A penetrometer is as good a device as any. Its transportable, it’s robust and relatively cheap. It must be noted that penetration resistance is moisture dependant – the more moisture in the soil the more lubrication for the tip to pass through the soil – so care must be taken when operating and interpreting the results.  By operating the penetrometer in the field under a few of the tyre combinations mentioned before, we can determine how good it is at detecting possible compaction.

In the field, a pea viner equipped with 700mm wide tyres and standard recommended inflation pressure (32psi) showed a huge increase in the penetration resistance caused by the machine passing over the soil. In addition, the large increase extended beyond the maximum recorded depth of 500mm, and at this depth, it is becoming uneconomical to remove.

For conventional tractor tyres, it becomes evident that increasing the inflation pressure increases the depth to which the compaction is evident.

Although in this instance, by adding duals, there was no major change in penetration resistance. This is because the tyres are already operating at the lower end of the inflation pressure spectrum at 1 bar / 14psi, and carcase stiffness becomes a major component.

The rule of thumb is that inflation pressure has the greatest influence on the degree or severity of compaction, while the load influences the depth to which that compaction extends.
On top of this there are repetitive loading situations.
Research shows that over 75% of the total compaction is done in the first pass.

So in answer to the question – should we be driving in the same wheel marks or next to the previous wheel marks?

The answer should now be quite obvious….
Stick to one set of wheel marks, and then the problem is relatively localised. Subsoiling one set of wheel marks 3 times in order to obtain the correct depth is easier and cheaper than subsoiling the whole paddock!
It now becomes evident that one of the major factors we can alter is field going traffic and it is here that I think we need to concentrate. The ultimate, is of course Controlled traffic farming, but maybe a full CT system is untenable, for reasons of machine configuration, or other factors?

Research into driver behaviour and has yielded a 12 hour gps log for one tractor and trailer of a pair from a 3-viner pea harvest operation. By calculating total wheel marks in the field it was evident that over 70% of the field would have been wheeled during that operation. And as stated earlier, 75% of the total compaction occurs in the first pass.

So what we need to do, if we are to responsibly manage our paddocks, is to minimise the compaction that occurs in the first pass.

This can be done by:-
Lower tyre inflation pressure, (remembering to sue safe working pressures at rated loads and speeds)
Minimise machine weight if at all possible and minimise slip.
Better tyre selection for the task – maybe wider section tyres or correctly spec’d duals are suitable
Control traffic when in the field – driver education is essential!
- and do you need to be in the field in your UTE or truck?
Do get your spade out and dig to investigate if there is a problem.
And if there is, target the problem with appropriate remedial techniques