"Used as a soil amendment and fertilizer, Greensand (0-0-3) is mined from deposits of minerals that were originally part of the ocean floor. Contains about 3% total potash, along with iron, magnesium, silica and as many as 30 other trace minerals. May also be used to loosen heavy, clay soils. Improves plant health. Ideal for use in organic gardening"
It's a very very slow release form of potassium but is used more as a trace mineral supplement.