What are mobile nutrients
Mobile nutrients, like Nitrogen, are able to move from one portion of the plant to another as needed.
You can notice this for example when you have a Nitrogen deficiency. The lower plant will take the nutrients from lower leaves in order to reuse the nitrogen in the upper levels of the plant.
You can see this with the leaf turning yellow, which indicates that the nitrogen is being sucked out of it.
How to know if the nutrient is mobile?
If the deficiency impacts the older leaves first then odds are that the nutrients is mobile since this shows that the plant is extracting the nutrient in order to re-allocate it where it’s needed.