字幕列表 影片播放 列印英文字幕 Dry, drought-prone regions of the world produce nearly a third of the global food supply, and many of the crops in these regions are in danger of going thirsty – not only because the soil they grow in contains too little water, but because it contains too much salt. Salt is a natural part of soils everywhere it forms from minerals weathered out of rock – but in wet climates, most of it gets dissolved by percolating rain and carried down to the groundwater below. In dry climates, on the other hand, percolating rainwater rarely makes it that far most of it gets soaked up by the deep roots of native plants, causing its salts to precipitate out and gradually accumulate in the soil below. This salty layer isn't a problem as long as both the plants and the water table stay where they are, but when native vegetation gets swapped out for shallow-rooted crops, more rainwater makes it all the way to the groundwater, causing the water table to rise. On its way up, it dissolves the salt deposit, bringing salty water to crops' roots. And here's where the hydration problem comes in: individual molecules of salt are a lot bigger than molecules of water, so they get stuck in narrow junctures in plants' plumbing and cut off their water supply. At best, a plant that can't hydrate properly grows slowly; at worst, it dies. And irrigation just exacerbates things: irrigated water comes from rivers and lakes and is slightly saltier than rain, so it adds salt directly to the soil while speeding the water table’s rise. We humans have run into this problem before historians believe salty soils contributed to the fall of ancient Mesopotamia. We’re seeing some effects today, too: as much as one quarter of all irrigated dry farmland on Earth experiences reduced yields due to salt. But even without additional water, groundwater comes up fast: for example, when forests in southwestern Australia were converted into non-irrigated farmland, it took just 12 years for the water table to rise 18 meters to the surface. One way farmers in dry regions deal with this problem is by periodically flushing their soils with enough freshwater to remove the salt. This works – temporarily – but requires a lot of water, sometimes more than is used on crops over an entire growing season. A better option is to plant thirsty, deep-rooted trees and shrubs, which can soak up most of the percolating water and reverse the rising water table. In Australia, native trees planted amongst conventional crops slurped up so much water that the water table fell 3 meters in a decade, taking its load of dissolved salts with it. And farmers in Uzbekistan achieved similar results by switching back and forth between crops and native shrubs every few years. But regardless of whether we alternate them spatially or temporally, we will most likely need to rely on drought-adapted native plants to help preserve the large swaths of dry, increasingly-salty farmland that produce a third of the world’s food. Because in this case, if we salt our food before we taste it, we might not get to taste it at all. Hey, this is Emily. I just want to put out a huge thanks to Leesa.com for sponsoring MinuteEarth! Leesa.com is an online mattress company that offers reasonably priced luxury 3-layer foam mattresses. They'll deliver your mattress right to your door, and if you're not and if you're not perfectly happy with it 100 days later, you can return it for a full refund. Leesa also donates one mattress to a homeless shelter for every ten they sell. AND you can get yours at a $75 dollar discount – just go to leesa.com/earth and use the offer code "EARTH75" to let them know you're a MinuteEarth viewer. That's leesa.com/earth with the offer code "EARTH75" Thanks Leesa!