the diatomaceous eath is ground up sea ahells

Diatomaceous earth (also known as Kieselguhr) is the 'fossilised' skeletons of diatoms. The skellies are extremely fine silica, whereas the shells of molluscs (the usual form of seashell) are mostly calcium carbonate so the diatomaceus earth is siliceous rather than carbonaceous. The carbonate would rarely be ground as finely as the diatoms and would affect the pH of the soil whereas the silica won't.

Cheers, Rowan