Nitrogen applications are very powerful. They are like stepping on the garden growth accelerator. You want to be sure everything is in tune, and all the other minerals are present and available before ramping up nitrogen applications. In an ecosystem, it is the scarcest resource that limits growth. If your plants lack water, or other minerals, don’t apply lots of nitrogen.
You, the garden-farmer, are the best judge of when to apply nitrogen. You can factor in your knowledge about your growing conditions, your observations and your intuition. Decisions about nitrogen are best made in the field.