Oyster shell is a long-term soil investment, not a quick fix. Coarse-ground oyster shell breaks down slowly over multiple seasons, gradually releasing calcium and gently raising soil pH. Apply it with next year's garden in mind, not this one.
 
For asparagus: asparagus prefers a pH of 6.5–7.5. Portland soils tend to be acidic due to our wet winters. Work oyster shell into the bed at planting and reapply every few years. My own asparagus bed has had oyster shell worked into it since 2002  and it's still producing.
 
For clay soil structure: calcium from oyster shell helps clay particles aggregate over time, improving drainage and workability. This is slow work, but it's permanent improvement.
For general soil building: a light annual application maintains calcium levels in soils that lose it to leaching over winter.
 
A note on blossom end rot: coarse oyster shell will not prevent blossom end rot in the current season, as it breaks down too slowly. Use gypsum for that. Oyster shell is building your soil for the long term.
 
Available in 1 lb and 5 lb bags.