The mobility of nutrients in the plants themselves determines how to read signs of nutrient deficiency in leaves. A deficiency of immobile nutrients can be seen in yellowing new leaves, whereas a deficiency in mobile nutrients can be seen in yellowing old leaves.
This is because movable nutrients travel from old leaves to suffuse new growth, whereas immobile nutrients cannot be transferred between new and old growth, so deficiency symptoms will show up in the new growth.
Nutrients that are mobile in plants include the basic N-P-K primary macronutrients; nutrients that are motionless in plants include the secondary macronutrient, like calcium, magnesium and most of the micronutrients.
Managing nutrients responsibly involves not only the understanding of the quantities needed, but also how they move in soil and within the plant, and knowledge to identify the deficiency or toxicity symptoms.