In the Dr. Seeds Professional Hydroponics kit, calcium & magnesium are packaged as a separate product from all the other 14 essential minerals because calcium should not be dissolved with water at the same time as phosphorus and sulphur. If calcium, phosphorus and sulphur are all mixed at the same time they bind together, forming unsoluble clumps. To avoid the clumping issue, dissolve calcium and magnesium fertilizer with water first, then pour it into a solution containing your other fertilizers.
Calcium and magnesium dosage must be adjusted to suit the starting state of the irrigation water. If using tap water, consider that there is calcium and magnesium already present in your water and therefore less calcium and magnesium fertilizer is needed to be added to it. On the other hand, water that contains little or no calcium and magnesium, such as rain water or R/O water*, needs more calcium and magnesium added to it.
In all cases, the starting state of water before adding any other minerals should be approximately 0.3 EC (300 TDS) of mostly calcium and magnesium.
The average volume of calcium and magnesium in tap water is 0.15 EC (150 TDS), although this varies significantly from region to region. Check your municipalities website to find out how much calcium & magnesium is in your water, or use commercially available testing strips, or use an EC metre to measure your water’s mineral content.
There are two Dr. Seeds kits available, one for R/O water*, and one for tap water. The kit for R/O water* contains a full supplement of calcium and magnesium. The kit for tap water contains half the calcium and magnesium as the kit for R/O water* because average tap water contains native calcium and magnesium already and doesn’t need as much extra added by fertilizer.
*R/O water is water that has had all its mineral content removed. It is sometimes called inert water.