Fox Farm Cultivation Nation Cal-Mag, 5 GallonCultivation Nation Cal Mag A supplement for use during fruiting and flowering to address nutrient deficiency problems that can occur in hydroponic, soil and both small and large scale growing environments. Mix 1 Tbsp (15 mL) per gallon (4 L) of water. Begin use just before bud development. Continue use as needed, not more than once per week. Maintain a pH range of 5. 6 6. 8. A pH outside this range can affect the availability of calcium and magnesium.