Different crops require different amounts of nitrogen for optimum growth. A high availability of most other minerals might have a negligible effect on a crop, but too much nitrogen can be a huge problem.
Too little, on the other hand, can halt growth altogether. Add the ease with which it can be lost from the soil, and we need to closely monitor this element. Some guidelines:
With tomatoes, the plant needs enough nitrogen to make a good-sized plant. Unless leaching takes place, there’s usually sufficient nitrogen from the basic fertiliser application at this stage.
When flowering is in full swing, lower nitrogen levels will encourage fruit set.
When fruit is filling, you need to increase the nitrogen to provide size and get the plant to fill the trusses.
Get this right by observing the colour of the leaves. Also, don’t overdo the nitrogen at a later stage. This can result in a change in fruit shape and firmness.