dirt.

命を支える土を 私たちが「無価値なもの」とみなすのは

Maybe that's because, most of the time, soil takes care of itself; decomposing plants and

放っておいても 自然に生まれるから。

animals and degrading bedrock produce new soil at roughly the same pace that wind and

動植物の死体や、岩の分解で

water erode it away

風や雨で流れ去ったぶんの土が 補充されるからです。

But this balance between soil formation and erosion is easily tipped.

でも、このバランスは たやすく崩壊してしまいます。

For millennia, the tiny Pacific island of Mangaia was covered in a thin layer of fertile

2,000年前、太平洋はマンガイア島の 豊かな薄い土壌を

soil, but after humans arrived around 2000 years ago, their slash-and-burn agriculture

やってきた人間の焼畑農業が 風雨にさらしました。

exposed the soil to the elements.

続く数世紀で、島の斜面は 栄養豊富だった表土を失い

Over several centuries, rain and wind swept virtually all the nutrient-rich topsoil from

土の残された、わずかな谷間をめぐって 人々は争います。

Mangaia's hillsides and concentrated it in just a few arable valleys, which people

作物がとれないので 彼らはネズミを食べ

viciously fought over.

共食いも起きました。

With less land to grow crops, people resorted to alternative food sources like rats—and

私たちも、同じことをしています。

even each other.

農業で、土の流出を 引き起こしているんです。

We're doing the same thing throughout the world today - not the part where we eat each

私たちの農業は 深く根を張る木を取り去り

other, but the part where we farm the soil away.

土を耕して、ゆるめてしまうので

That's because we make for agriculture by clearing away deep-rooted native vegetation

土は、雨や風に 弱くなってしまいます。

and use hoes, plows, and tractors to loosen the soil, making it easier for wind and rain

根の浅い作物に 土を留める力はありませんし

to sweep away.

収穫後の土は はだかの状態で残されます。

And we grow mostly shallow-rooted crops that are no good at holding onto soil, and that

このため世界の耕作地では 生成される50倍の速度で、土が失われます。

get stripped away during harvest, leaving fields bare for much of the year and at the

小型トラックが、年に80億回 畑の土を運び出し

mercy of the elements

川やダムの底に 捨てているようなもの。

As a result, the world's farmlands lose soil 50 times faster than new soil can form.

カリフォルニアの1/10サイズの耕作地を 毎年無駄にしているのと同じことです。

That extra erosion adds up to about 8 billion pickup trucks of soil moved annually from

土の生成と流出の バランスをとることは、可能です。

fields to places like the bottoms of rivers and behind dams - which are less convenient

耕す回数を減らしたり

for farming.

刈り取った後の茎や葉、または 「被覆作物」で、土を覆えばいいんです。

That soil loss reduces global crop yields by as much as if we took a California-sized

畑に木を植えて 土を守ってもらうこともできます。

swath of farmland out of production every decade.

これらの取り組みは 土の流出を95%も減らし

Fortunately, we know how to bring erosion back into balance with the rate at which soil

持続的な農業の 助けになります。

forms: plow less often, and after harvest, leave plant parts behind or plant so-called

ただ、畑の面積を割くこの方法は 短期的な生産量は落ちるので

cover crops to, well, cover the soil as protection against water and wind.

十分に実施されていないのが現状です。

We can also incorporate trees or native plants that keep soil in place year-round.

畑の土が減っていっても 人類がすぐに絶滅するわけではありません。

Putting strategies like these in place can cut erosion by as much as 95 percent, helping

でも、土をめぐる争いは 起きるかもしれません。

keep crop yields high in the long run.

マンガイア島より ずっと大きなスケールで。

But in the present, they can hurt yields, because adding other vegetation to farm fields

means less room for crops.

As a result, we've been slow to make these soil-saving strategies the norm.

If we can't keep the farmable soil on our farms, human civilization won't immediately

implode, but we might end up fighting over the patches of land where that soil ends up,

like the Mangaians, but on much, much bigger islands.

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