Green Planet Pro Cal is a valuable source of supplemental calcium and magnesium. Why is Pro-Cal so great for your plants? It is an excellent supplemental source of Calcium, Magnesium and Iron, which plants need to grow strong and healthy. Plants engaged in rapid growth can often come up short on these essential elements, so supplementation is a great idea. The savvy grower makes Pro-Cal a must have in their arsenal of plant growing boosters.