During flowering, plants need more magnesium and calcium because of bud production. Calcium and Magnesium are two of the most important secondary nutrients. They are a little less important than the primary nutrients – N, P, K (Nitrogen, Carbohydrates, Water).
It is not easy to detect Cal-Mag deficiency in the early stages of a plant. It is important to act as soon as you detect the first sign. First signs appear in the lower leaves of the plant. The tips of the leaves curl . . .
Get something that consists of cal and mag only. Be sure to get a supplement that does not contain other nutrients. Supplements that contain nitrogen and other nutrients bring a risk of over-feeding. This is dangerous to your plants!