Neptune’s Harvest Organic Crab Shell is an excellent dry organic source of NPK, calcium and magnesium. It will also help with nematode and fungus problems. Here’s why: Crab shell is high in chitin, which promotes the growth of chitin-eating bacteria in the soil. The exoskeletons of fungus and nematode eggs are high in chitin. When added to the soil, crab shell helps to create a hostile environment for the fungus and nematodes by feeding the biological life that eats chitin and chitin-based organisms.