Manage traffic on wet soils to limit soil compaction

 Dick Wolkowski, Extension Soil Scientist

The forecast shows another week of rainy weather and farmers are probably getting nervous about finishing up harvesting and conducting other fall field operations such as manure application. Soils are at or near their field capacity water content, which is the point were maximum compaction is possible. The easy, armchair quarterback advice is to stay off these wet soils, but in reality that may not happen. So how much compaction really may occur when heavy equipment is driven on wet soils? Does it make sense to confine needed traffic to the same tracks? A recent evaluation was conducted at the Arlington Agricultural Research Station on a Plano silt loam soil to examine these questions. 

The figures below show the results of a study last month where penetration resistance was measured after driving a large combine in multiple passes on a wet soil that was unplowed or recently chisel plowed. The field was in winter wheat stubble that had been no-till planted in the fall of 2006 into corn silage residue. Each point represents the average of six measurements that were taken in the center of the combine wheeltrack. The results are predictable. The soil that was plowed showed increased penetration resistance with succeeding passes. The soil that was unplowed appeared to handle one pass, but multiple passes increased resistance. These data show that up to 70 – 80% of the compaction occurred in the first pass over the field.  Clearly important decisions need to be made where and when to drive on a field if compaction is to be avoided.

The best advice continues to be the admonition to stay off wet soils. Even a few hours of sun and wind will cause some drying that can substantially increase the bearing strength of the soil. Since the majority of the compaction occurs in the first pass attempt to maintain traffic in established tracks whenever possible.

Effect of the number of passes on the penetration resistance of a 14.5 ton combine, Arlington, Wis.