A top-of-the-line farm tractor is taller than an African elephant; A combined harvester loaded with grain weighs 36 tons, as much as a small herd of elephants.
When these mechanical "behemoths" work in fields, their extreme weight slowly crushes the soil, making it difficult for plant roots to grow.
A new study, published recently in the Proceedings of the National Academy of Sciences, suggests that the impact could reduce the yield of the world's cropland by 20 per cent over the next few decades.
"We shouldn't overlook heavy machinery, which can really cause damage." Rattan Lal, a soil scientist at Ohio State University.
Since the 1960s, tractors have gotten bigger and bigger, and the largest tractors today weigh almost 10 times as much as they did then, heavier than some sauropod dinosaurs. This dinosaur was the largest living thing that ever lived on land.
While bigger machines are more efficient, all that extra weight comes at a cost.
To find out how farm tractors are changing, and how they might be affecting soils, Thomas Keller of the Swedish University of Agricultural Sciences and the Desert Research Institute in the United States collected industry data going back to 1958. They then simulated the forces exerted by tractor tires on soil at different depths.
In mechanized agriculture, the compaction of tires exists in the topsoil for a long time to a depth of less than 50 cm. And many farms plow topsoil or till each season in preparation for planting, making this compaction virtually non-issue.
The researchers point out that serious problems now exist in soils that are less than 50 centimeters deep, where compaction exceeds safe thresholds. This squeezing breaks down the tiny Spaces between soil particles, causing less water and air to enter the deeper soil. These changes can reduce crop yields by 10 to 20 percent, and the effects can last a long time because it takes earthworms and other organisms decades to soften the deep soil.
Not only combine harvesters, but also other agricultural equipment used for farming and fertilization, as well as vehicles used for logging, are getting heavier. About 20% of the world's agricultural land is at risk of reduced yields due to deep soil compaction, such as in the world's important grain-producing regions, the Brazilian savanna and southeastern Australia.
Thomas Way, an agricultural engineer with the U.S. Department of Agriculture, says there are several ways to minimize soil compaction problems. One is not to drive on wet land, which can make the soil more fragile. The other is that on sunny days, farmers can use GPS to drive the same route each time to reduce the total area of farmland subjected to compaction.
The study also raises a "prehistoric paradox," which is that if modern agricultural equipment would reduce plant productivity to that extent, what would have happened when sauropod dinosaurs roamed the Earth?
Perhaps they stuck to the same route or waded along the shoreline, the researchers speculate. Whatever the answer, it's clear that modern agricultural machinery should be designed with soil strength in mind, or they risk going the way of the dinosaurs.