When deciding to add the remnants of soap to your compost pile, there are certain types of biodegradable soap that can add to the natural break-down process of composting. The key ingredients are derived from glycerin mixed with natural oils. Examples of these include hemp seed oil, beeswax, avocado oil, and babassu oil. Biodegradable soaps such as these can act as surfactants in compost piles that include wood and leaves, making all materials in them more resistant to mildew. Keep in mind that composting only applies to solid (bar) soaps, since liquids do not go through the same decaying process.
What makes soap biodegradable is that its oils can be broken down by the same bacteria that break down the rest of the compost materials. This process normally takes about 6 months, and 90 percent of the oxygen, carbon dioxide, and biomass in the soap should be composted by this time. Soil is a required addition to this process for soap, and it is important to keep any remnants at least 200 feet away from any water source. This type of composting is becoming increasingly common for municipal districts that collect leaves, wood scraps, and other yard trimmings to be converted into nutrient-rich additions to fertilizer.
If flies in your compost pile become a problem, biodegradable soap can help cut down on the number of them. Flies are a sign of anaerobic decomposition, so your compost pile may need more ventilation. It is a well-known practice among composters to use red worms to speed up the process, and soaps made with the above-mentioned natural oils have been found to have no negative effects on these worms.
A popular new form of biodegradable soap is what is known as soap nuts (sometimes called soap berries). These are actually small fruit from the sapindus tree found in Nepal, Northern India, and other parts of Southeast Asia. These soap nuts come in a small cloth bag that can be added to laundry loads, and they have shown to be just as effective as conventional detergent. It is common practice to break the shells into halves first; this increases the amount of suds created. Each of them can be used for 4 to 6 loads, and then simply add the shells to the compost pile.
First-time buyers of soap nuts/berries are advised to buy them by their weight, not by the number of loads of laundry the advertisers profess they will do. It is also important to be sure the soap nuts are de-seeded. Reputable sellers will always inspect them to ensure this is thoroughly done. The reason for this is that seeds can often stain laundry. Remember to ask the seller if the seeds will be removed before making any decision to purchase.
Whether you choose to go with traditional natural-oil bar soap or experiment with soap nuts for your laundry, both can add benefits to your compost pile and help with its proper decomposition to give you the nutrient-rich soil addition you have in mind.