Sand and dust may exist on almost any asteroid or planet _ a few may even have water beaches. But only life can create soil, a mixture of sand, dust, and organic matter.

Oh, I know science fiction blandly declares that we could grow food in the “soil” of Mars or the moon, but that is simply a substrate _ like hydroponic water _ until it crawls with fungi, bacteria, and the residues of plant decay.

Soil is amazing stuff, truly a world in a thimbleful. And a lot more rare than we like to believe _ three quarters of the earth’s surface is water after all, and chunks of the rest are solid ice or bone-dry desert. Even where it exists, soil forms a very thin veneer on the local geology.

Soil is necessary for most human crops, and even remote Islanders surviving on products of the sea like to grow a few things. Soil is more an indication that life on land exists and how healthy it is, rather than a requirement.

Like everything else, soil is easily contaminated by the products of our industrial civilization. Once its organic properties are killed off, it returns to dust and sand. Possibly never to become soil again, depending on what the pollutants were.

We ignore soil at our peril. Easy to take for granted, dirt underfoot, but the core of many of our experienced ecologies.

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