PRO-CAL is a highly beneficial calcium, magnesium and iron plant nutrient supplement. This recommended application rate normally is sufficient to make up for any deficiencies in most soil or hydroponic formulas. Note - in hydroponics, iron falls out of the solution relatively fast due to use of magnetic driven pumps without grounding probes. Other contributing factors can be high levels of dissolved oxygen from air stones and pumps. - More concentrated than competitive products. - Helps increase overall density in your plants. - A must for growers who use coco. - Helps plants resist disease. - Remedies yellowing plants.