Here is a quote from a nematode control link
http://www.attra.org/attra-pub/nematode.html#Managing

Most nematode species can be significantly reduced by tilling in chitinous
materials such as crushed shells of crustaceans (shrimp, crab, etc.). This is
effective because several species of fungi that "feed" on chitin also attack
chitin-containing nematode eggs and nematodes. Increasing the amount of chitin
in the soil will also increase the population of these fungi. A
shrimp-shell-based fertilizer called Eco Poly 21™ Micro shrimp fertilizer is
available from Peaceful Valley Farm Supply (13). At 2002 catalog prices, it
would cost between $87 and $216 to treat an acre with this product (the
suggested application rate is 20 to 50 lbs. per acre). Clandosan™, a
nematicide made of crab shells and agricultural-grade urea, can be used as a
pre-plant treatment (it should not be used on plants because the amount of urea
in it can "burn" or kill them) (14).

I bet eggshells contain chitin-like material.

Ed