A useful step in identifying nutrient deficiency symptoms is to determine whether the deficiency is the result of a mobile or immobile nutrient based on where the symptoms first appear in the plant. Mobile nutrients include nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg). Mobile nutrients are able to move out of older leaves to younger plant parts when supplies are inadequate. Because of their mobility inside the plant, visual deficiencies will first occur in the older or lower leaves (see photo). If the deficiency continues, then the symptoms will spread throughout the plant.
In contrast, the relatively immobile nutrients do not readily move within the plant. As such, their deficiency symptoms first appear in new growth or young leaves and can be localized. Some of the common immobile nutrients include manganese (Mn), sulfur (S), copper (Cu), iron (Fe), calcium (Ca) and boron (B).
Macro nutrients such as N, P, K and Mg are needed in large quantities, so they are best applied to the soil. The micronutrients are needed in small qualities and can be soil and foliar applied.
(Source – http://www.farms.com/news/identifying-nutrient-deficiency-symptoms-in-field-crops-108725.aspx)