Tomatoes should be fertilized before planting and side-dressed with a nitrogen fertilizer three times during the season.
The first sidedressing should go down one to two weeks before the first tomato ripens. The second should be applied two weeks after the first tomato ripens and the third one month after the second. Common sources of nitrogen-only fertilizers include nitrate of soda, urea and ammonium sulfate. Blood meal is an organic fertilizer that contains primarily, but not exclusively, nitrogen.