Next year's crop is then planted directly into the mat, retaining more nutrients in the soil.

ECONOMIST: Brazilian agriculture

That means more nutrients in the soil, where it feeds food crops, and less in lakes and streams, where it feeds algae that sucks up oxygen and kills fish.

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Their modular hydroponic system -- where plants sit in a soil-free set up with nutrients delivered through irrigation channels -- can be retrofitted to just about any wall.

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In the hills, torrents of water washed soil away, or left it but rinsed the nutrients out of it.

ECONOMIST: Honduras and Nicaragua