Soil Compaction Threatens Yields

Researchers warn that compaction caused by heavy agricultural machinery threatens around 20 percent of arable land worldwide.

In many places, the increasing use of heavy agricultural machinery is a permanent threat to the productivity of the soil – and thus also to future yields. Researchers in the scientific journal PNAS warn that around 20 percent of arable land worldwide, including Central Europe in particular, is at risk. Such damage in deeper soil layers is hardly reversible and also promotes other problems such as erosion and flooding. An independent German expert confirms the problem, but the authors overestimate its extent.

The mechanization of agriculture has made a significant contribution to increasing yields, write the soil scientists Thomas Keller from the Swedish University of Uppsala and Dani Or from the Swiss Federal Institute of Technology in Zurich (ETH) in their essay. However, larger vehicles increased the risks of compaction, especially of the lower soil layer. The duo writes that this threatens the long-term productivity of the soil.

The load on the upper soil layer has remained almost constant

From 1958 to 2020, the weight of loaded harvesters increased almost tenfold – from 4,000 to around 36,000 kilograms. Modern beet harvesters could even reach up to 60 tons when fully loaded. In order to protect the upper layers of soil, to prevent the machines from sinking too deeply and also to save fuel, the tires have become larger and the tire pressure is also minimized. Due to the larger contact area, the load on the upper soil layer remained almost constant.

However, this does not apply to the subsoil – i.e. the layer from a depth of roughly 45 centimeters that can no longer be reached directly by machines such as ploughs. With a higher wheel load, larger and flat tires can adjust the contact surface pressure on the upper soil, but not the depth effect.

The compacted subsoil impairs root and thus plant growth as well as water, nutrient and gas transport. Damage to the subsoil is, of course, difficult to see from the outside, write Keller and Or. However, it is evident, among other things, in falling yields and in the fact that water seeps into the ground more poorly and accumulates more or flows off above ground – with the corresponding flood risks. It is particularly tricky that damage to this soil layer is difficult to reverse and the consequences can last for decades.

The main problem is the combination of heavy machinery and wet floors

According to the researchers, this threatens almost 20 percent of arable land worldwide. In addition to Europe, parts of North America, South America and Australia are affected – areas that are central to the global food supply.

Joachim Brunotte from the Johann Heinrich von Th√ľnen Institute (TI) in Braunschweig is also in favor of avoiding compaction of the subsoil in any case. However, he disagrees with the authors’ estimate that subsoil compaction threatens 20 percent of arable land worldwide. “I was surprised by this number,” says Brunotte, according to whose account this number is based on a number of theoretical assumptions. Even in Germany, according to Keller and Or a hotspot of subsoil compaction, a maximum of five percent of the areas are at risk, says Brunotte and refers to measurements.

The main problem is the combination of harvesting bulk crops such as corn, potatoes and sugar beet with heavy machinery on wet soil. Above all, the tramlines of the machines in the fields and the headlands are at risk – i.e. those field areas on which vehicles such as harvesters or tractors turn. Especially on the headland, there is often a drop in yield compared to other arable land, says Brunotte.