Calphos soft rock phosphate is comprised of prehistoric skeletal remains present in oceanic sediment and contains a minimum total of 20% P2O5 and 20% calcium. 50 lb bag Available in powder or granular form. Use 5 lb per 100 sq. ft. on phosphate deficient soils and less on fertile soil. Use 1 - 10 lbs for trees. Supplies Phosphorous and calcium up to 5 years.