Most commercially available legume seeds, and some grass seeds, are coated. Seed coatings may be calcium carbonate, a polymer or a type of clay. Reasons for coating the seed may be to increase seed size, facilitate adherence of a pesticide, provide a minor element or (for legume seed) to add and protect Rhizobium bacteria.
The diameter of coated seed is larger than for uncoated seed, but it flows faster, which requires calibration of the planter when switching from one to the other. Coated seed has fewer seeds per pound but generally has a higher level of seedling survival.
From Forage-Livestock Quotes and Concepts, D.M. Ball, et. al,
PHOTO: Photo by Mike Dixon