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Do You Need to Sterilize Vermicompost?

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Question: Do You Need to Sterilize Vermicompost?
Is unsterilized vermicompost safe to use around edible crops? What about the bacteria in worm compost--can it make us sick? When it seems as if we're hearing about food-borne illnesses more often, thoughts inevitably turn to the substances we use in our home vegetable gardens. Happily, vermicompost is one of the safest fertilizers you can use, and here's why.
Answer: The short (and adamant) answer is no, you should not worry about sterilizing your vermicompost. In the end, attempting to do so causes more harm than good.

Here's the deal. Vermicompost is produced when the worms and other microbes in your worm bin consume the food scraps and bedding material. What's left is composted organic matter and worm/microbe excrement. As disgusting as this may sound to the neatniks among us, the fact that vermicompost contains worm excrement is exactly what makes it so safe to use around food crops.

The scientific explanation goes something like this. In a normal compost pile, four basic processes take place to rid the pile of harmful bacteria: heat, harmful bacteria being eaten by another organism, harmful bacteria loses the battle for food or space, and anaerobic conditions suffocating the bacteria. Together, these four processes help keep harmful bacteria in a normal compost pile at very low levels. Heat is the most important of these processes, which is why you often see recommendations to keep turning your pile so it heats up. Heat helps things break down more quickly, and it kills pathogens in the pile.

But in vermicompost, heat doesn't really happen. So what keeps vermicompost for being a haven for baddies? The answer is, the worms. As the worms eat the organic matter in the bin, they consume the bacteria, as well as harmful nematodes, weed seeds, and pathogenic fungi. The worm's digestive system breaks down the majority of bacteria and other harmful substances. What doesn't get broken down in the digestive tract is excreted, where it is quickly consumed by other microorganisms present in the bin, thanks to all of the worm excrement. It's an amazing system, and one that needs no help from us to ensure that the vermicompost is safe to use around edible crops.

One of the most common ways of sterilizing compost and vermicompost is to subject it to heat, either in an oven, or, less commonly a microwave oven. This is one of the worst things you can do to vermicompost. According to Dr. Elaine Ingram, soil scientist and owner of Soil FoodWeb Inc., pasteurizing or sterilizing vermicompost under high heat causes two conditions that cause your vermicompost to do more harm than good in the garden. First, pasteurizing vermicompost results in there being a food source available for disease organisms, but no competing organisms left to fight them off. Second, temps of 160 to 165 results in the death of mid-temperature organisms, and heat loving microorganisms die as the compost cools down. This leaves the vermicompost ripe for pathogens to be blown in or brought in by birds and animals, resulting in a disease-ridden vermicompost.

So, no. Don't sterilize or pasteurize your vermicompost. The whole point of any compost is to encourage beneficial microorganisms to inhabit your soil. Sterilizing it destroys these beneficials. Leave your vermicompost alone. It does just fine all on its own.

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