Tractors have gained weight faster than they have gained horsepower. A 400-horsepower machine that weighed 14 tonnes two decades ago now commonly arrives at 18 or 19 tonnes before ballast, and three long-running European and North American trials have started to put a number on what that costs underground.
The trials, which tracked wheeled traffic on silt loam and clay soils for between four and nine seasons, found yield reductions of 3% to 9% in compacted zones below 30 centimetres. Unlike surface compaction, the subsoil layer does not break up over winter; two of the three sites still measured elevated bulk density six years after the traffic event.
Weight is not the only variable
The more useful finding is that axle load and inflation pressure matter more than total machine weight. A heavy tractor on correctly ballasted axles running 0.8 bar in wide radials did measurably less damage than a lighter machine on overinflated tyres carrying a mounted implement on one end.
“Nobody is going back to 200 horsepower. What they can do is stop running road pressures in the field, which is free.”
That has pushed central tyre inflation systems from a curiosity to a specification line. Several manufacturers now offer factory-fit systems, and the trials' authors argue the payback case should be made on yield rather than on tyre life, where it has traditionally and unconvincingly been pitched.
The researchers also recommend controlled traffic patterns where machinery width allows, noting that confining wheel paths to 30% of a field's area removes the question for the other 70%.