What is the logic to French using either just "de" or otherwise "du, de la or de l'" in front of a word? I understand the differences between "du", "de la" and "de l'", but not how it is decided to use one of those in stead of just "de".
As examples, I do not understand what the choice was based on in the following phrases:
une compression de personnel
les richesses du sous-sol
l'armée de l'air
l'armée de terre
l'avocat de la défense
l'âge de la retraite