The next generation of sustainable fertilizer might just come from sewage.

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Phosphorus is an important part of the biochemistry of most plants and animals. We need it to live, and it can compromise our health when there isn’t enough in our diets. Plants also need it to grow well enough to harvest in modern agriculture. It’s naturally occurring, but for a long time we’ve been using commercially produced fertilizer in order to help crops grow stronger and faster, which has in turn increased crop yields.

However, phosphate rocks, the source of that phosphorus, are a non-renewable resource. Eventually we’re going to run out.

A scientist in Madagascar has figured out a source of renewable phosphorus, though: thermally treated sewage sludge. A lot of phosphorus gets passed into the sewage system after it passes through us, and when properly treated, that sewage can be used as a fertilizer for plants. Beyond the phosphorus, it contains other nutrients—and best of all it’s almost endlessly renewable. In other words, it could be a sustainable fertilizer.

Sewage is not as effective as the triple superphosphate fertilizer sometimes used today, and against which the sewage was compared. In laboratory tests, while the sewage-treated plants grew larger than unfertilized plants, they didn’t match up to the plants that were given commercial fertilizer. That’s to be expected, since there’s so much more phosphorus in that fertilizer and because the other nutrients in the sewage result in competition with microbes, so the plants get less of it. But the phosphorous that isn’t absorbed by the plants remains in the soil and can be accessed by plants later, effectively reducing the amount of sewage needed to fertilize a given field.

Of course, there is a lot of sewage out there, and it’s not going away any time soon, so the answer could always be using more of it as well. In any event, there are a lot of benefits to using thermally conditioned sewage sludge as a sustainable fertilizer, and it sounds like something that we should seriously consider as a renewable source of phosphorous.