Been all over the web and to many forums, the consensus is cal/mag for soil,coco, peat based mediums. I had problems with my first closet grow and was told to add cal/mag to feed. Bought MEGACROP and haven't had a problem since with cal/mag issues. I grow in soil or peat based mediums... Just saying that people with way more experience told me and I listened to them. Worked for me.