By David Jones, CROPNUTS Independent Agronomist

We all know soil compaction is bad and the limitations it causes to plant growth and crop yield. But how often do we ask specifically which farm machines are doing the damage, so that we can work out how to avoid it?

We all make assumptions about where the fault lays in the farming system, but work in recent years at Penn State University has highlighted soil compaction problems we face as farm machinery gets larger.

Firstly, a disclaimer; I am an agronomist, not a farm machinery specialist. But in the ever continuing drive to larger, heavier farm machinery, I feel it is important to raise awareness of the soil compaction damages that arise, as well as to your wallet when purchasing these new high horsepower tractors and combines.

Run the right tyre pressures

Top soil compaction is governed by contact pressure; lower your tyre pressures appropriately and you will reduce top soil damage.

Below 30cm depth, and axle load starts to become the dominant factor affecting soil compaction damage, as this picture from Penn State highlights:

How different soil layers are affected by contact pressure and axle loads

How different soil layers are affected by contact pressure and axle loads


Essentially, no matter how well you have balanced your axle loads and tyre pressures, heavy axle loads will still cause soil compaction damage to the subsoil – damage that is not obvious but is still costing you in crop yield.

The crop yield effects of upper sub soil compaction last for 5-10 years, assuming no further damage is sustained to the soil.

Yield loss due to sub soil compaction

Crop yield loss due to sub soil compaction

It is worth remembering that crop yield effects are even more acute in dry years like we are experiencing at the moment; if the soil cannot hold water and roots cannot explore, crop yields suffer even more from sub soil compaction.

Measure axle load to protect your subsoil

Calculating axle load is very important. Combine front axle loads commonly exceed 15 tons with a full tank of grain. Even when running floatation tyres at 0.8 Bar, Penn State Researchers found that you can still get a peak soil-stress force of the same amount at 50cm deep.

Calculating axle load is important

Calculating axle load is important


Top Tip

A client of mine recently changed his sprayer from a self propelled farm machine to a mounted Kuhn on a 130hp John Deere. He opted for a 34” rim rather than the conventional 38”, which gives the tyre tread a 20% longer footprint to spread the pressure over a great area without going to wider tyres and tramlines.

The tractor is also used for fertilising, so he keeps the front tank on as ballast to avoid having to change tyre pressures between operations and to reduce downtime when changing from the sprayer to broadcaster.

Soil pressures exerted by different machinery

Soil pressures exerted by different farm machinery

Penn State researchers found that AVERAGE pressure is lower on tracked farm machines, but at just 15cm depth there are still peaks of high soil stress/pressure which are not visible on the surface, but affect the upper and lower subsoil (see the graph below each tractor).

HOWEVER, farmers always remind me that a tracked farm machine puts its power down better in practice, allowing you to get away with a lower horse power, lighter farm machine to do the same job:

Less axle load = less subsoil compaction!

About David


David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.


0 replies

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply

Your e-mail address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.