Gypsum (calcium sulfate) does two things that matter a lot in East Portland: it supplies calcium quickly, and it helps break up clay.

For tomatoes: calcium deficiency causes blossom end rot — the dark sunken patch on developing fruit that ruins a lot of Portland tomatoes. Gypsum is the right in-season calcium source because it works quickly and doesn't raise soil pH. Apply at planting and water in well. Note: consistent watering matters as much as calcium supply — uneven irrigation is the most common cause of blossom end rot even in calcium-adequate soil.

For clay soil: calcium from gypsum helps clay particles aggregate into the crumb structure that allows water, air, and roots to move through soil. East Portland clay is notoriously compacted and poorly draining — gypsum applied consistently over time works in the right direction. It won't fix clay overnight, but it's one of the better tools for improving it.

Does not raise soil pH. If you need to raise pH as well as add calcium, use oyster shell or lime instead.

Microna Pro Gypsum, granular, OMRI listed organic.

Available in 1 lb and 5 lb bags.