A Story about Saltpeter and the Peterman

Compost piles built to extract saltpeter for the production of gunpowder.

Compost piles built to extract saltpeter for the production of gunpowder.

If you come visit us at Whisper Famers, you’ll likely see a few bags labeled “saltpeter” lying around. What exactly is this strangely named substance, where does it come from, and why should every backyard gardener know about it?

Saltpeter is another name for the chemical compound potassium nitrate (KNO3). Saltpeter was first generated on farms, underneath horse/cow stables where urine and feces would drop below the floor. Farmers would create a subterranean room underneath the stable and build a nitre-bed, which was kind of like a compost pile made of straw, mulch, ashes, or a mixture of these.

When organic materials break down/decompose, they turn into chemicals, minerals, or inorganic ions. So when wood was burned for cooking, it would leave behind pot ashes or “potash” aka potassium - (K+). Urine/feces from animals would fall onto the mulch/ash mixture and bacteria would consume the animal waste and release nitrates into the pile.

The nitrate would then form a bond with the potassium in the ash to create potassium nitrate, or saltpeter. Eventually, the levels of saltpeter were so great that white crystals would start to grow out of the pile, so that a “peterman” could climb down there to collect them. The saltpeter would then be mixed with sulfur and charcoal to create gunpowder.

But it turns out that those same chemicals used for making gunpowder can also be used to fertilize and grow new plants, and this cycle can repeat endlessly. For example, the roots of some watercress could take in some potassium nitrate to grow, and chicken might eat the watercress. The chicken would have that potassium nitrate in its manure, which could then be composted by a farmer. As the compost became richer (and with additions of wood ash) white crystals of potassium nitrate would eventually form. These crystals could be collected and added back into a water garden to grow more watercress.

Alice Lin