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Vehicle Weight Causing Subsoil Compaction
by LuAnn Rolling, District Conservationist
“I really think we are wrecking the soil quicker than 30-40 years ago,” Anthony Bly, SDSU Extension Soils Field Specialist, tells Civil Eats in a June 6, 2022 article. He is concerned that the enormity of modern equipment causes tillage-based soil degradation. He said “The total weight of combine harvesters has increased nearly 10-fold, from around 8800 pounds in 1958 to about 80,000 pounds in 2020. At a time when farmers are desperate to hold onto soil moisture, the subsequent compaction caused by all this weight can limit water infiltration, which is particularly troublesome when high intensity rains can’t be captured and stored in the soil.”

According to research conducted by Thomas Keller from the Swedish University of Agricultural Sciences and Dani Or from the Department of Environmental System Science in Zurich Switzerland, mechanization has greatly expanded food production capabilities, however the higher vehicle weights increase the risks of subsoil compaction.  They say the alarming trend of chronic subsoil compaction poses a risk of potential productivity loss to over 20% of the arable land in the world. They say soils can only withstand so much pressure before they lose the pores and pathways which allow air to circulate and water to reach plant roots and other organisms.

Keller and Or’s research, printed in the Proceedings of the National Academy of Sciences of the United States of America, PNAS, Volume 119, shows that trends of increasing weights of agricultural machinery suggest that the focus of agricultural vehicle design on increasing efficiency, floatation, and traction may have ignored intrinsic soil limits exceeded with deeper subsoil stress. They say that subsoil stresses induced by today’s agricultural vehicles have reached or crossed critical levels for ecological functioning of subsoil root zones, with adverse consequences for land productivity.

Andrew Klopfenstein, an Ohio State ag engineer, studied the yield impact from running a 2,000-bushel grain cart in the field. The cart weighed around 200,000 pounds when fully loaded with corn, with an approximate weight of 30 tons per axle. Running this fully loaded grain cart across a field resulted in compaction as deep as 34 inches under both normal and wet soil conditions. He says that these serious compaction concerns could lead to yield losses of over 50% for the following year’s corn crop. He notes that deep subsoil compaction can take up to 10 years to eliminate, and normally can’t be done with deep tillage.

A recent trend toward using rubber track undercarriages on heavy agricultural machinery reduces contact stresses and may alleviate vertical stress in upper subsoil but does not significantly reduce the risk of subsoil root zone compaction.

The costs of soil compaction are likely to escalate with climate change. The projected increase in frequency and severity of droughts and the projected increase in rainfall intensity in combination with reduced water infiltration and storage capacity due to compaction is likely to increase the incidence and severity of floods. This highlights a need for better knowledge of recovery of compacted soil and methods for mitigation of compaction effects, as well as a need for a paradigm shift in the development of agricultural machinery to change the trend towards heavier vehicles.

How do we reverse this trend? One option would be to break up vast, monoculture landscapes and use smaller machines on smaller parcels. Producers can also avoid trafficking wet soil, keep axle loads below 10 tons, reduce the number of trips, increase soil organic matter and soil life and switch to no-till.