A top-of-the-line farm tractor stands taller than an African elephant. And fully loaded with grain, a combine harvester weighs up to 36 tons—as much as a small herd of pachyderms. As these mechanical beasts lumber across fields, their extreme heft can slowly crush the soil and make it harder for plant roots to grow. Such effects, a new study suggests, could diminish harvests across 20% of global cropland in the coming decades.
“Heavy machinery is something we should not ignore,” says Rattan Lal, a soil scientist at Ohio State University, Columbus. “It can really cause damage.”
Tractors have been getting ever bigger since the 1960s, and the largest now weigh almost 10 times what they did then. That’s heavier than some sauropod dinosaurs, the largest creatures ever to walk on land. Although the machines’ size makes them more efficient, all that extra weight comes at a cost.
To find out how agricultural vehicles have changed—and how they might affect the soil—Thomas Keller of the Swedish University of Agricultural Sciences and Dani Or of the Desert Research Institute assembled published industry data going back to 1958. Then, they modeled the forces exerted by the tires on the soil at various depths.
In mechanized farming, compaction has long occurred in the upper soil, in layers shallower than 50 centimeters. On many farms, this topsoil is plowed or tilled each season to prepare the ground for planting, making such pressures largely a nonissue. The problem now is deeper, researchers say, because compaction in the layers below 50 centimeters often exceeds safe limits.
This squeezing can collapse the tiny spaces between soil particles, letting less water and air reach the deep soil. All told, these changes could decrease crop yields by 10% to 20%. And the effects would likely be long-lived: It could take decades for earthworms and other organisms to loosen the deep soil.
It’s not just combine harvesters. Other farm equipment, used for ploughing and spreading fertilizer, is also getting heavier, as are vehicles used for logging. Roughly 20% of agricultural land around the world is at high risk of lower yields from deep soil compaction, the authors report today in the Proceedings of the National Academy of Sciences. These areas, like the savanna of Brazil and southeastern Australia, have vulnerable soil and heavy equipment—and they are the bread baskets of the world.
Thomas Way, an agricultural engineer with the U.S. Department of Agriculture, says there are ways to minimize soil compaction. One is to not drive on fields when they are wet, which makes soil more vulnerable. And during dry weather, GPS can help farmers drive the same routes each time to lessen the total area that is under pressure.
The new study also raises what the paper authors call a “prehistoric paradox.” If modern farm equipment depresses plant productivity to such a degree, what happened when sauropod dinosaurs roamed Earth? Perhaps they stuck to well-worn paths, the researchers speculate, or maybe they waded in water along shorelines. Whatever the answer, it’s clear that modern farm machines should be designed with an eye toward soil strength; otherwise they risk going the way of the dinosaurs.
Source: Science Mag