I'm trying to figure out if it is possible to configure a small tractor so that no soil compaction occurs below the depth of tillage.
We have clay soil and no significant freeze-thaw cycle.
Leading contender for tiller is a reciprocating spader. Falc, Celli, Gramegna, ... (BTW as far as I can tell, reciprocating spaders are only made in Italy, does anyone know why?) We would want a unit 1.2 meters or narrower. Depth of tillage varys greatly with model.
The lightest machine would be a BCS or similar single axle "walking tractor". I get the impression that the largest BCS is just barely large enough to run the smallest spader made. Would this machine be beefy enough to operate smoothly in clay, or would it be a nightmare to control similar to walk-behind rototillers? Are there other walking tractors sold in the US? (I know there are other brands in Europe.)
Next step up are the small Italian tractors such as Ferrari, Antonio Carrerra, etc. We've only started looking at these. Are the Italian farm tractors more reliable than Italian cars? Pros and cons of these? Any particular brand we should look into? Are there very low pressure tires available for these?
I have found a few graphs showing pressure below the surface, such as:
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'd like to find more of these, with a wider range of weight and inflation pressures, especially on the light weight low pressure side. Most of the info I've found is intended for megafarms, and talks about 15 ton axle loads. Everyone says lighter causes less compaction, but how light is light enough? I don't expect to eliminate compaction at the surface, even raindrops cause surface compaction. Is it realistic to expect to prevent compaction below the depth of tillage?
How many psi does it take to create compaction?
Soil is naturally uncompacted by freeze-thaw cycles, roots, earthworms, moles/gophers. Are there other natural uncompacting forces?
*generally* speaking, the concensus I've read is anything larger than a small Bobcat type vehicle can ruin tilth and aeration, even with just one pass. Your specific soil makeup would play a large role in it's 'compression strength'.
Basically you need to place a load on the soil below tillage depth that is less than the elastic-plastic limit. Its as simple as that.
The size of this force depends on the soiltype and the soil conditions. Typically it is quite low for wet soils and high for dry ones. It is lower for clay soils than rocky/gravelly soils, particularly when wet.
So to take great care of your soils do not travel on them when they are wet (and wet here means at the depth you are concerned about).
The other factor is the load you are applying. A heavy tractor with small wheels will cause a lot more compaction (a *lot*) than a light tractor with very big tyres.
Now lets look at some examples. A 200lb person standing on one heel (say 3"x3" = 9sq") is applying about
20lb/sq". A 1400lb cow standing on eight claws each a 5"x3" triangle (about 60sq") applies about 23lbs/sq". A person standing on both feet (say 2x12"x3" = 72 sq") applies about
3lbs/sq". A person wearing snowshoes each of 150sq" applies about 2/3 lb/sq".
That's the surface pressure. Obviously the force spreads out with depth. Roughly you can say the area affected by the pressure is effectively spread outwards the same distance as the depth. This makes a lot of difference for small footprints. At 8" the person on one heel, the cow and the person on two feet will have their weight spread over an area in excess of 64 sq" and will all be pretty similar.
OK now lets look at tyres. Many people say the ground pressure of a tyre on soil is just the inflation pressure. This is *wrong*. Its true for a tyre on a hard surface that does not give, but its NOT true on a soil, which is elastic. Look at a tyre travelling on dry(ish) soil and it sinks in a bit, with the soil coming back to its original position. Look at a railway train going past and you can see the sleepers gow down under the weight and then rebound to their original position, and thats on gravel you wouldn't imagine would be elastic, but it is. Soil is elastic up to the elastic-plastic limit I mentioned earlier.
Now farmers have known from experience over decades that big diameter tyres cause a lot smaller ruts, or don't make ruts when smaller ones do, than small diameter tyres. That's because when a small diameter tyre sinks 1" (max) the *extra* area in contact with the ground is a LOT less than a big diameter tyre.
The formula is [pi x r x arc-cos(1-1/r)]/90 (in degrees)
For a 1500lb small tractor with matched front wheels and a 1/3-2/3 weight distribution, all in inches and degrees. If you don't believe me, do a scale drawing!
wheel length soil-load soil-load climb-out diameter on soil 12" tyre 8" tyre angle
OK. Notice that I haven't considered the tyre inflation pressure so these are really for smooth steel wheels! Well, that's quite a fair approximation because the tyre pressure is likely to be about 12-15psi, which is much more than the pressure between the tyre and the soil.
The climb-out angle is important because the tractor has sunk slightly in the soil so to go forwards it has to climb out of the little (temporary) depression so its climbing uphill. That puts a shear force on the soil, and these are absolutely the worst causing surface slipping and smearing. In fact for making ruts, particularly for trailers, the climbout angle is by far the most important.
So the moral is, put the biggest diameter tyres on the tractor that you can get, and make them as wide as possible.
AND never, never, never travel on soil when (within reason) you are leaving a small rut behind you. For this you may need to exclude the very soft friable soil on the very surface.
Bummer!
It doesn't matter what tillage tool you use IF the soil profile you are cultivating is not plastic. That is if you take a small pellet from the wettest depth of soil and roll it about in your fingers it crumbles (safe) rather than making a little ball (plastic=bad). Whatever (in general) tillage tool you use will damage the soil structure when its plastic.
So if the soil is in the right state to cultivate, it will withstand pretty well any tyre you put on the tractor. So tractor and tyres really don't matter that much. Cultivations batter a soil about much more than any reasonable tractor could even dream about.
The worst of all is the rotary tiller that has a *horizontal* shaft with blades on. The blades shear the soil at the *bottom* of the soil profile, which is almost always the wettest, and you get an instant impermeable pan that will likely last forever.
We can discuss this further if you like.
The small jap ones are pretty good. Many of the other small tractors are rebadged japanese (eg kubota). We had a much abused kubota for years, and it always started first time (even after having been left in a shed for 5 years).
A small tractor, 25-35HP, with big tyres, will go on land that you couldn't even walk over. However the fact that you can get on the land does NOT NOT mean you won't cause compaction. See above.
Absolutely. Farmers can, do and should do it all the time.
About as long as a piece of string.
I think you have it all there pretty much. Remember food for roots and for worms and many other soil dwelling organisms including bacteria, which are also important soil ameliorating organisms, will help. That means manures, and even more importantly high yielding crops, because high yielding crops have high yielding roots and the vast majority of the roots are left in the soil after cropping.
I remember an article I read in a U.S. farm magazine. They tested several different deep chisels to see which was most effective at loosening the soil. The implements were pretty close as far as effectiveness. One thing in the article sticks in my mind. I think they were doing the test in the fall. Tear the ground up and then stay off. The soil packed back down pretty quickly if they drove across it after loosening it.
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