Many of my citrus trees are planted directly in the ground, however, I do have some container plants which are mostly kumquat varieties and pineapples. All of the container plants are watered with rain water, therefore I fertilize them once at 4 to 6 weeks intervals with Calcium Nitrate (CaNO3) to provide them with a source of calcium. In between I fertilize with a 25-5-15 fertilizer. In citrus trees, calcium is the most abundant mineral, of the 17 minerals required by citrus, of which none exists in rain water. An inadequate calcium supply can reduce plant growth and fruit yield LONG BEFORE deficiency symptoms become evident. If leaf calcium is less than the optimum level, calcium nitrate may also be applied as a foliar spray.