Calcium and Magnesium are essential compounds your plants will use throughout their entire grow cycle. Often considered the next most important minerals after N-P-K, calcium and magnesium deficiencies will show rapidly in your plants, stunting growth and development and leading to lower yields. This is less common in outdoor gardens and quality soils, and much more common in low quality soils, coco coir and hydroponics. Municipal (Tap) water is also a good source of calcium and magnesium as these are the most common minerals present in tap water. Grower's using RO/Distilled water will most likely need to supplement calcium and magnesium to keep their plants happy and healthy.