In the upcoming film Interstellar, Earth’s soil has become so degraded that only corn will grow, driving humans to travel through a wormhole in search of a planet with land fertile enough for other crops. In the real world things aren’t quite so dire, but degraded soil is a big problem—and one that could be getting worse. According to a new estimate, one factor, the buildup of salt in soil, causes some $27.3 billion annually in lost crop production.

“This trend is expected to continue unless concrete measures are planned and implemented to reverse such land degradation,” says lead author Manzoor Qadir, assistant director of water and human development at the United Nations University Institute for Water, Environment and Health. Qadir and his colleagues published their findings October 28 in Natural Resources Forum.

Irrigation makes it possible to grow crops in regions where there is too little rainfall to meet the plants’ water needs. But applying too much water can lead to salinization. That’s because irrigated water contains dissolved salts that are left behind when water evaporates. Over time, concentrations of those salts can reach levels that make it more difficult for plants to take up water from the soil. Higher concentrations may become toxic, killing the crops.

Qadir and his colleagues estimated the cost of crop losses from salinization by reviewing more than 20 studies from Australia, India, Pakistan, Spain, Central Asia and the United States, published over the last two decades. They found that about 7.7 square miles of land in arid and semi-arid parts of the world is lost to salinization every day. Today some 240,000 square miles—an area about the size of France—have become degraded by salt. In some areas, salinization can affect half or more of irrigated farm fields.

Crop production is hit hard on these lands. In the Indus Valley of Pakistan, for instance, salinization causes an average decline in rice production of 48 percent, compared to normal soils in the same region. For wheat, that figure is 32 percent. Salty soils also cause losses of around $750 million annually in the Colorado River basin, an arid region of the U.S. Southwest.

“In addition to economic cost from crop yield losses, there are other cost implications,” Qadir says. These include employment losses, increases in human and animal health problems and losses in property values of farms with degraded land. There could be associated environmental costs as well, because degraded soils don’t store as much atmospheric carbon dioxide, leaving more of the greenhouse gas to contribute to climate change. The total cost of salt degradation, therefore, could be quite a bit higher than the most recent estimate.

Salt damage can be reversed through measures such as tree planting, crop rotation using salt-tolerant plants and implementing drainage around fields. Such activities can be expensive and take years, but the cost of doing nothing and letting lands continue to degrade is worse, the researchers argue. “With the need to provide more food, feed, and fiber to an expanding population, and little new productive land available, there will be a need for productivity enhancement of salt-affected lands in irrigated areas,” they write.

On a cautiously hopeful note, Qadir adds that the issue is reaching the ears of policy makers: “Amid food security concerns, scarcity of new productive land close to irrigated areas and continued salt-induced land degradation have put productivity enhancement of salt-affected lands back on the political agenda,” he says. “These lands are a valuable resource that cannot be neglected.”