Crab meal is added to soil in the author’s garden with a shallow tiller called a “tilther.” (Barbara Damrosch)

The longer I live, the more I appreciate food that is simple and knowable. It might seem easier to make soup from an envelope of dehydrated mix. But filling the pot with real potatoes, leeks and thyme is easy, too, and results in a tastier soup.

I want the same clarity when feeding the soil in my garden or, more precisely, the microorganisms that live in it. Their consumption of organic matter makes the soil fertile, by making nutrients available to my plants’ roots. So I feed them what I know: manure, kitchen scraps, plant parts — either broken down in the compost pile or incorporated directly into the soil.

The same impulse led my husband to a crab processing plant many years ago. The owners were happy to give him crab shells they’d otherwise have hauled away for incineration. The shells enriched his soil and helped him grow healthy, productive plants. Since then, he’s sought any crustacean shells he could find, tilling them into our gardens and fields.

Here’s how they work their magic. The exoskeletons of crabs, lobsters, shrimp, crayfish and countless insects contain a substance called chitin (pronounced KYE-tin), a slow-release source of the nitrogen that plants need. Because it doesn’t leach out of the soil, it doesn’t pollute waterways the way soluble nitrogen fertilizers do.

But that’s not all. When crustacean shells are added to the soil, they stimulate and increase populations of chitin-devouring bacteria and fungi. Once these have decomposed the shells, they go on to devour certain chitinous pests, most notably root-knot nematodes, which can lead to poor yields in a number of crops.

The shells also contain calcium carbonate, the key ingredient in garden lime. Calcium is another essential element for plant health, and it raises the pH of the soil in areas that are too acidic. The shells of mollusks such as oysters, mussels and clams are also rich in calcium. Those don’t break down as rapidly as crustaceans do, but that’s not a defect. We see the white clamshell fragments in our dark earth as time-release calcium pills at work.

Gardeners who don’t live near a coast where shellfish waste is plentiful might make a deal with seafood restaurants, offering to take it away as an even exchange. But they can also buy dried, ground crab or shrimp shells in bags. This is also a good option if neighbors object to fresh seafood waste. (Look for pure meal without additives, such as urea.) If local stores don’t carry them, check online sources such as Peaceful Valley Farm Supply (groworganic.com).

Shellfish meal can be added, raked in or tilled just before planting, but whole crustacean shells are best chopped up, turned under and allowed to mellow in the soil for a month or more. Leaving them on the soil surface, as we once did with some crab shells when the ground was frozen, is a bad idea. They attracted flocks of sea gulls, whose habit it is to pick up shells and break them by dropping them on shoreline rocks — or in this case our neighbors’ cars.

A better idea: Invite the neighbors to a big lobster or crab feast, and after dinner put all of the shells on the compost pile. Burying them with a garden fork will not only hide them from birds but also set the heap to “bake,” cooking up the best compost you’ve ever made.

Damrosch is the author of “The Four Season Farm Gardener’s Cookbook.”