How to Extract Potassium Nitrate From Soil
Potassium nitrate, or salt peter (also known as saltpetre), as it was historically called, has been extracted from the earth for hundreds of years. It has been and is most often used as an oxidant for fuses and fireworks, but it has garden and horticultural uses as well. It can provide nitrogen and potassium to plants, which are two of the three required macronutrients, and it is also the most common elements of most commercial stump removers. Purchasing potassium nitrate is far easier and probably more cost-effective than making it yourself, but it can be done. It is extracted from manure as opposed to soil.
Preparing the Pile
Put sheets of plywood down where the pile of manure is to go. The plywood will prevent the liquid that you pour over the pile from going back into the soil.
Mound the pile of manure over the top of the plywood. The pile can be as high as 6 or 7 feet.
Mix green materials -- such as leaves and grass clippings -- and straw into the pile to increase its porosity and oxygen flow. Up to half of the pile can be green materials.
Build a protective structure around the pile. Stack rows of cinder blocks in a staggered pattern on either side of the pile to form the side walls. Spread a tarp over the top to act as a roof. Place an additional row of cinder blocks to hold the tarp down. The structure will protect the pile from wind and rain.
Pour cows' urine -- or the urine of other farm animals -- over the pile. You will need to add urine at least once per week for up to four months to extract the potassium nitrate.
Extracting the Potassium Nitrate
Scrape the yellow crust off of the top of the pile with a shovel. The crust is the potassium nitrate crystals forming on the pile's surface. The crystals will form after months of pouring urine over the top of the pile.
Place alternating layers of wood ash and cloth onto the bottom of a bucket with holes in the bottom. Start with a layer of cloth. Add a thin layer of ash over the cloth, and then add another layer of cloth. The cloth and wood ash will act as a filter.
Put the crusted material into the bucket with a shovel. Gloves should be worn. Fill about three-quarters full. Place the bucket onto two cinder block set apart the width of the bucket so that the liquid that runs through can be collected underneath.
Boil 2 gallons of water.
Place collection pan under bucket. Pour boiling water over the top of the material in bucket.
Place the collected liquid into black plastic or metal buckets so that the excess water can evaporate. The resulting salts and residue left behind will be potassium nitrate.
Due to potassium nitrate's main use as an accelerant in explosive materials, always handle it carefully. Prior to manufacturing, check local laws to make sure it is legal to collect it where you live. If you plan to use potassium nitrate as a fertilizer, do so sparingly, as the potassium content is extremely high and can potentially burn plant roots if used in too high a concentration.
Chris Bond has been writing about gardening, sustainable agriculture and local history since 2005. He has been published in "The Plain Dealer," "The Repository" and online. Bond holds an A.A.S. from the State University of New York at the Finger Lakes campus in Canandaigua, N.Y.