Relationship tractor tire and soil compression?

Question

On clay soil, particularly when moisture is adequate or greater, tractors will cause collapse of clay crystals that reduces productivity for many years,

Question... How does the size of tires, number of tires, pressure in the tires and weight of total tractor influence extent of soil compaction on clay.

Is interior pressure of the tire a major determinant?

Answer 1

wildturkey sure made that sound good, but he confused even me!

Soil compaction is dependent upon several things including the soil type, (sand, silt, clay) and soil moisture.  For example, when dry, my clay soils are as hard and strong as concrete. When wet, things sink in up to the hubs just setting there for a length of time.  The more sand and gravel a soil has, the less easily compacted it is.  With any soil, the wetter it is, the more easily compacted.

For a given weight of an object, the more tire or track surface you have on the ground, the less weight per square inch is applied.  For example, I'm sure you know that cows are much heavier than a sheep, but when hoof area is taken into account, sheep apply more pounds per square inch than do cows.  About the only thing that applies more ground pressure is a woman in high heels, so if ground compaction is a concern, make sure you have your next tea party in the house rather than in the yard.

Yes, the interior pressure of a tire does effect soil compaction.  A properly inflated tire will apply its load more evenly side to side.  An overinflated tire is going to have a more rounded tread surface instead of a flatter one.  The rounded surface applies more pressure to the soil in the center of the tire and less at the edges than of a properly inflated tire.  The result is much more soil compaction in the center of the tire mark than at the edges.

When more tires or wider tires are used on the same weight machine, the few pounds per square inch of weight is applied to the contact area.  At first you'd think this is the only way to go, but not always.  Sometimes you're better off applying 18 lbs/sq/in with a single tire to a single 18 inch wide track than to have duals and apply 9 lbs to a 36 inch wide tire track.  In other words, when considering the yield reduction caused by soil compaction, you're better off keeping soil compaction confined to the same small area than to have it spread equally across the entire surface.

wildturkey is certainly correct when mentioning the load being pulled.  The weight of a tractor may compress the soil particles, but the stress of a tire under load smears the soil, doing the same thing.  A more heavily loaded tire slipping less can very easily cause less compaction than a lightly loaded tire thats spinning a lot.  However, the lighter tire will leave most of it's compaction at a more shallow depth than the heavily loaded tire.

In the northern US, compaction isn't as much of a problem as in central Missouri.  Up north, the very cold winters cause deep freezing and thawing of the soils which breaks up a lot of the previous year's compaction.  Down here, our soils don't freeze nearly as deep or as often and can't be depended upon to break up compaction.  

Incidently, a 60-80,000 pound bulldozer has enough track surface that they may exert only 5-6 psi soil pressure while a much smaller, lighter rubber tired farm tractor can exert 18-25 psi.  I've seen a dozer track over a rabbits nest and never hurt the bunnies.

Answer 2

There are two factors that determine the pressure exerted by the tire on the soil.

First you have the weight of the tractor as a force towards the center of the earth (gravity)

Then you have the resistance (reaction) of the load to the force of the tractor.

So to find the total force exerted on the ground first draw a vector straight down, the length scaled to equal the weight of the tractor.

Then draw a horizontal vector scaled to equal the force required to pull the plow.

Complete a rectangle by drawing lines parallel to the first two vectors, then measure the diagonal line from one corner to the other.  Measure that distance and that is the total force applied to the ground by the tractor while pulling the plow.

There is also compaction by the plow, resulting in "plow pan" which is a layer of compacted soil that makes it difficult for roots or water to penetrate deeply.

Compaction is one of the big reasons "minimum tillage" farming is so successful.