Tractors, combine harvesters and beet harvesters: farmers have increasingly larger and heavier machines at their disposal. The idea is that this will allow them to grow and harvest more. But in reality, they may be destroying the soil, researchers warn.

Much has changed in agriculture in the past century. Small-scale farms have given way to large farms that have many hectares of land on which to grow crops on a large scale. With the growth of farms, the agricultural vehicles that farmers use to work, sow and harvest the land have also become larger and heavier. “The total weight of modern harvesters is now approaching that of the largest animals to ever walk the earth: the sauropods,” researchers write in the journal. Proceedings of the National Academy of Sciences Agricultural Sciences† For example, a fully loaded, modern beet harvester weighing about 60,000 kilograms can easily compete with the heaviest herbivorous dinosaurs that roamed the earth millions of years ago and could weigh between 60,000 and 80,000 kilograms.

Reverse

Those modern, large and heavy agricultural machines are impressive. And also developed with the best intentions; they enable farmers to work large tracts of land efficiently and thus increase yields. But there is a flip side to the story, the researchers say. Because as those heavy machines drive across the land, they compress the ground beneath. For the top layer of soil this is not much of a problem; that is regularly plowed again. But what the researchers find in their study is that the heavy agricultural vehicles are increasingly also compressing the deeper layers of soil – which a plow leaves untouched. And that is cause for concern. “Field research has shown that the compaction of the subsoil (i.e. those deeper layers of soil, ed.) is difficult to restore and can affect the functioning of the soil for years or even decades,” the researchers write.

Structure

“Compressing the subsoil destroys the structure of the soil,” explains soil scientist Wei Hu, who is not involved in the study. “It has a negative impact on the functions of the soil. Think of the infiltration capacity (i.e. how much water can penetrate into the soil, ed.), the amount of water that is available for plants, the airiness of the soil and the extent to which roots can penetrate into that soil. And that in turn has a negative impact on the harvest and the environment.”

Trends

In their study, the researchers look at trends in the weight of agricultural vehicles and also look at the wheels that are mounted underneath. On the one hand, it shows that agricultural vehicles have become considerably heavier, especially since the second half of the last century. But on the other hand, the pressure on the surface appears to have hardly increased in recent decades; by choosing larger wheels, the pressure on the surface is fairly distributed. That is a conscious choice; after all, this prevents the agricultural vehicles from sinking into the ground under their own weight and getting stuck. But although the pressure on the surface – while agricultural vehicles became heavier – has therefore remained fairly constant, the pressure on deeper layers of soil appears to have increased. “At greater depths, the pressure experienced by the bottom is purely dependent on the wheel load and less determined by the pressure on the surface,” the researchers explain. “The pressure on the underlying soil, i.e. in the root zone (the zone in which plants take root, ed.) appears to have increased gradually.” And the agricultural vehicles also appear to be compressing the soil to an ever greater depth. “While a few decades ago, compaction of the soil was limited to superficial soil layers that fall within the tillage depth (and are therefore plowed every year, ed.), we now find the compacted soil layers at a greater depth, where they may not be plowed annually. (and therefore the compression cannot be undone, ed.).”

Global problem

The compaction of the deeper layers of soil is a worldwide problem, the researchers say. In their study, they estimate that about 20 percent of global farmland is at risk of being compacted at greater depths. The lion’s share, of course, is in areas where large-scale agriculture is practiced (think of Europe, North America, South America and Australia). But in the near future, the researchers warn, India and China – where farms are still typically small – and Sub-Saharan Africa – where agriculture is not yet so mechanized – could also face this challenge.

The irony

It’s an insidious and largely overlooked threat, the researchers say, that their study aims to highlight. They hope it will lead to a critical rethink of agricultural vehicle design. Because there is a lot at stake. “Ironically, highly efficient tractors and harvesters can hinder progress – towards growing food production for a growing population – due to the unintended risk of soil compaction.”

The Paradox

Funnily enough, though, the research doesn’t just have implications for how we farm; it may also provide more insight into how sauropods got their wet and dry. Because if these dinosaurs were as heavy as our modern agricultural machines, they must have compacted deeper layers of soil quite a bit and thus limited the growth of their food. Perhaps even more so than our agricultural vehicles, which move on at least four wheels, because the sauropods only distributed their weight over three legs while walking. But if the sauropods compacted the soil and thus limited the growth of their food, how could they get enough food to sustain that enormous body – with that enormous weight? It’s a good question. All the more so because you can expect that the rich vegetation that the sauropoda ate required a lot of moisture and the soil on which the sauropoda roamed must have been quite moist (and could therefore be compacted quite easily). Maybe the sauropods traveled around a lot and just moved on after destroying the bottom? The researchers think not. “Probably the foraging area in which these giants were active was quite small, because it must have been a huge challenge for the sauropods to move over the wet ground.” The scientists therefore come up with another hypothesis: the sauropods created a street. A tamped-down path that they stamped on en masse, eating out of the verge with the help of their long necks, leaving the bottom below undisturbed.

Whether that image is correct has not yet been proven. But if it actually happened, the herbivores had a pretty sustainable foraging policy from which we humans could clearly learn something.