Crab shells for chitin, a carbohydrate that is a natural bug repellent

4 million pounds of shells get thrown out every year in just one coastal county, you can see little bits of crab shell in the soil

By Gregory Jackson


Merwin’s Wharf zero organic waste week gave us a treat:  25 pounds of crab shells, which we composted at Forward Church’s Willow Community Garden.  This week also brought challenges in residential leaf donations, leaves were buried under a half a foot of snow, but we managed to pickup leaf donations and mix it with crab shells.

The average coastal county will waste about 4 million pounds per year of crab shell waste, when Patrick Condon of Chesapeake Bay discovered crab waste is a great natural fertilizer and saves space in landfills, and founded New Earth Services 21 years ago.  At Wal-Mart Stores (WMT) they sell crab shell as a natural fertilizer for $16.06 for a 4-pound bag.

What is the benefit of composting shells? 

Groundz has been composting egg shells for the past year, allowing them to crunch and pulverize as other food waste material is added.  Crab shells contain chitin, a natural bug repellant and great source for nitrogen, phosphorous, calcium, and magnesium.  Jeff Holden, owner of Portland Shellfish (in Maine), created a composting business about 14 years ago, using his company’s own shell waste.  He used to donate the shells to a potato farmer, but the farmer was not storing the shells properly, according to Maine’s Department of Environmental Protection.

Composting shells like crab strengthen plant’s defenses against insect (thus a natural “insecticide”) when added to the soil, by mimicking the presence of chitin creatures like insects.  The plant produces small quantities of chitosan, an enzyme that breaks down chitin; it’s like a natural “immunization” for the plants natural defenses in the event an insect or exoskeleton of fungus, or nematode eggs are made of chitin.  As chitin, in the form of crab shells or added to the soil through compost, the plant with build up defenses.  However, not all fungus or nematode are bad for plants, in fact fungal, bacterial, and nematode diversity are good for soil ecology, as Groundz learned in our most recent compost sample tested at earthfort lab.

The other component of adding shells (which contain minerals like calcium) is soil integrity.  Try to get past the idea of “perfect looking compost”.  We tried for months to sift our compost of material not looking like soil, for instance shells, sticks, leaves.  If there is no smell and you cannot tell where your food waste went, in a complete assortment of composting materials, then we would say just go ahead and add compost to your beds, as is.  The reason is because material like shells will naturally take longer to break down because of minerals, which are meant to take longer in soil ecology.  This brings two benefits.  First, imagine a piece of shell or leaf mixed into your soil; soils compact over time and the shell can serve as a “roof” underground, which improve drainage, a place for future roots to grow, and aeration.  Secondly, mineral richness in soil contributes to clay (which despite popular belief of getting rid of clay soils is good), actually serves as a type of “backbone” for humus to adhere to.  Without clay in your beds, humus you create with compost will only last a couple years in the soil from rain and rinsing, but when it binds to clay, humus will last decades in a much needed bio-accumulation of nutrients and minerals.


2 responses to “Crab shells for chitin, a carbohydrate that is a natural bug repellent

  1. Pingback: Rising Star Coffee Roaster, Little Italy zero coffee waste | GroundZ·

  2. Pingback: Use iodized salt homemade mix instead of rock salt to melt icy driveways and pathways | GroundZ·

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