Plant nutrients are either Plant mobile or plant immobile. Understanding these two characteristics is important because it helps gardeners interpret deficiency symptoms more accurately.
Plant mobile nutrients are those that are capable of being translated within the plant is deficient of these elements the nutrients that is already within the plant will transported to where it is needed most the young tissues. Deficiency symptoms of plantmobile elements are observed on the older leaves first. One example of a plant nutrients is nitrogen. In nitrogen is deficient in a plant older leaves would turn the nutrients where it is needed to prolong the life of the stressed plant. Examples of plantmobile nutrients are nitrogen, phosphorus, and potassium. Manganese and sulfur are modernly mobile.
Plant immobile nutrients cannot be translocated from older tissue to a new one due to the nature of the elements and sometimes other conditions. In other words they are stuck where ever they landed the first time. They have reached their destination. Deficiency symptoms for these elements are observed in the young plant parts. Calcium is an example of plant immobile elements. It plays an important role in cell expansion. When calcium is deficient, the young shoots and flowers buds exhibit the devastating effects. If the condition is not corrected the shoots and buds get aborted eventually. Examples of plant immobile nutrients are, iron, calcium, manganese, zinc, copper, and boron.
This is the reason that I believe foliage spray does a lot of help due to nutrients that cannot be mobile the leaves can absorb them helping fight deficiencies