Calphos 50lb Calphos is guaranteed to contain a minimum of 3% ( a minimum of 20% total ) available phosphoric acid, minimum of 20% calcium. Calphos spells lush, greener growth. It also stimulates strong root formation, hastens crop maturing and encourages earthworms and soil bacteria. 400 LB per acre or 10-15 pounds per 1,000 sq. feet.