All You Need To Know About Soil Compaction

When I was visiting my grandparents a couple of weeks ago, I told them that I was going to go on a field trip with my program and we would visit a tractor company. To which my grandma – who grew up on a farm – responded: “make sure to ask them about soil compaction!” So I did.

Beyond the fact that I believe my grandparents are thrilled their grand-daughter is going ‘back to the roots’ and studying agriculture, if in a slightly removed form, it was interesting to me that this topic had stayed on my grandma’s mind way beyond the time she was last in touch with farming (in fact, she was a biology teacher her entire career). Turns out – the issue remains as important as ever, and has only increased in relevance in recent years.

So what is soil compaction, and what is there to worry about?

• In its essence, soil compaction relates to the compression of soil particles through external pressure, which reduces the pore space between them and makes the soil more compact. According to the University of Missouri, a healthy soil for crop production contains about 25 percent water and 25 percent air by volume. Soil compaction radically changes this ideal 50:50 ratio between pore space and soil particles.

• This means that both water infiltration and drainage are reduced (since there is less physical space for the water to trickle through), especially in soils that are already saturated (and thus, it is especially a problem in wet conditions). Runoff effects can lead to erosion and soil and water loss.

• In addition, there are also less possibilities for the exchange of gas and aeration, which can affect the uptake of certain nutrients – for example, plants exert energy to take up potassium, and therefore need an active root metabolism to do that. With reduced aeration, metabolism is reduced as well (in my mind, the roots can’t ‘breathe’ as well).

• Furthermore, compact soils also make it harder for plant roots to grow and explore a wider area of land, since they need more force to push through the soil. Thus, they have less access to nutrients and water due to their limited reach. Finally, there can also be increased risk of crop decrease.

• Researchers first started worrying about the issue with the increased mechanization of agriculture, especially with the introduction of the tractor: the “single most important mechanical innovation in agriculture”, as one of my papers describes it.

• Using tractors and other heavy machinery can affect soil compaction in two ways: through ground contact pressure (affecting mainly topsoil compaction) and axle load (affecting the lower subsoil), which is related to the total weight of the equipment that the axle of the vehicle has to support.

• According to research from Penn State University, while topsoil compaction can be partially counteracted through tillage (where you break open the topsoil layer again with plows and similar equipment), subsoil compaction is basically permanent. It is not even affected through deep frost penetration, which was originally thought to reverse any short-term damage done through compaction.

• An international soil compaction project found that compaction due to axle loads of 10–12 tons reduced yields approximately 15 percent in the first year, decreasing to 3–5 percent 10 years after compaction. 10 percent of the yield loss in the first year was believed to be due to compaction in the topsoil and upper part of the subsoil. The effects of topsoil and upper subsoil compaction disappeared in approximately 5 and 10 years, respectively. The final three to five percent yield loss were thus believed to be due to deep subsoil compaction, which did not disappear during the period (up to 12 years) in which measurements were taken.

• Thus, the researchers’ main advice is to avoid any compaction in the subsoil and reduce topsoil compaction as much as possible to preserve the quality of agricultural soils in the future. To achieve that, axle loads should be no more than 6 to 10 tons per axle at the upmost limit. Another issue is that while broader tires can reduce ground contact pressure, they make no difference in the subsoil compaction whatsoever – thus, the only option is to decrease the weight of the machinery (or the load of, say, the manure spreader or grain cart) or to add additional axles.

• However, the trend among farmers goes into the absolute opposite direction: while twenty years ago, a typical box-type manure spreader in Pennsylvania would weigh 2.5-tons, today liquid manure spreaders may have a weight of 20 or 30 tons. Similarly, the weight of tractors has increased from less than 3 tons in the 1940’s to approximately 20 tons today for big four-wheel-drive units.

• Further, current farming methods can even accelerate soil compaction, for example when spring planting is done early when the soil is still wet (and thus cannot yet support heavy farming equipment), or when monocropping eliminates crop rotations, which would decrease compaction effects through the diverse rooting systems.

• When I asked the tractor company representatives (we were at Fendt) on how their company responds to the issue, they said that their tractors were on average up to 2 tons lighter than those of their competitors (John Deere – the most common brand worldwide). In addition, they said their wheel system enabled farmers to distribute the weight of their equipment better.

• Still, the future tractor buying trends seems to point into a continuation of past developments: this site reports “growing demand for increased horsepower in both row-crop and 4-wd tractors. The additional horsepower is needed to pull larger farm implements such as planters, air seeders, nutrient applicators and tillage equipment“. This makes sense if you see farming as an industrial activity where economies of scale are exploited – the more your machine can do, the less time you spend per acre and the more acres a single farmer can ‘manage’… Whether that is enlightened and sustainable soil management is, however, a whole different question.

Have you heard of the problem of soil compaction before? Any farmers out there with insights on how you manage it?