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.
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