Today's giant farm vehicles threaten 20% of the world's cropland

In 1958, a combine carrying a full load of freshly harvested crops might weigh 8,800 pounds (4000 kg). Today, a fully loaded combine can clock in at 80,000 pounds (36,000 kg).

The story of increasingly large farm vehicles isn't necessarily bad. The invention of these huge machines — along with advances like new fertilizers and genetically modified crops — mean that today's farmers can grow far more food than ever before. But there's reason to worry that equipment manufacturers have begun pushing the envelope too far.

In a paper published Monday in the peer-reviewed journal PNAS, researchers show that farm equipment has grown so large that its heft can damage the soil that lies more than 20 inches (0.5 m) below the surface.  

"Ironically, highly efficient tractors and harvesters may hamper progress toward increasing food production... due to the unintended risk of subsoil compaction," the authors write.

Protecting soil isn't a new concern, but the largest machines bring fresh threats

Engineers who design agricultural machinery are well aware that soil is delicate. While dirt might seem simple and uninteresting, healthy soil is teeming with life and complexity. "Soils are ecosystems containing fragile structures – pores and pathways which allow air to circulate and water to reach plant roots and other organisms," according to sustainability researcher Jess Davies and soil scientist John Quinton. 

It's been obvious for a long time that the weight of farm vehicles driving over fields causes the upper layers of soil to compact. Engineers have mitigated this by putting progressively bigger tires on heavier farm vehicles. They've also used more flexible materials that make it possible to inflate the tires to lower pressure. Those changes increase the amount of surface area contact between the vehicle and the ground. These measures have enabled engineers to build larger and larger vehicles without increasing the amount of contact stress on the upper layers of soil.

Heavy equipment can compress the subsoil, causing problems in the future

It's not just the upper layers of soil that farmers need to worry about. In their analysis, the researchers found that "subsoil stresses under farm vehicles have affected progressively deeper soil layers over the past six decades." In the 1940s, '50s, and '60s, farm vehicles weren't heavy enough to compress soil below the level that's tilled each year. But that's no longer the case. Pressure from tractors, combines, and other pieces of equipment "has now penetrated deeper into the subsoil, thus potentially affecting untilled crop root zones," the authors write. 

"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 propagation," according to the researchers. 

Those layers of subsoil may be hidden from view, but they play an important role in what happens at the surface. The researchers say the consequences can combine to result in "a persistent decline in crop yields." That's especially concerning because this isn't a niche problem. "The fraction of arable land that is presently at high risk of subsoil compaction is about 20% of global cropland area, concentrated in mechanized regions in Europe, North America, South America, and Australia," according to the researchers. 

Unlike so many problems, this one has a relatively straightforward solution. The researchers say the issue could be addressed if "future agricultural vehicles [are] designed with intrinsic soil mechanical limits in mind to avoid chronic soil compaction."