We found out that tomatoes need fertilizer in our second growing season (2006). That may seem obvious, but we didn't have a mineral deficiency problem in our first year. We used Miracle Grow soil and cruised. But when we found purple and black spots on our leaves last year, it meant we needed to put phosphorus on the menu.
Saiffee Hardware on 7th St. and 1st Ave. in Manhattan had the goods. Hoffman's Tomato Food 5-10-10 includes the proper balance of phosphorous, nitrogen and potassium and restored our babies to health.
This year we were prepared with Hoffman's when the spots showed up. However, next season, we'll start feeding them before the problems start and continue with regular fertilizing. I bought Miracle Grow tomato food, a balanced mineral diet, and will try using it after the seedlings are transfered to outdoor pots and are strong enough for fertilizer. Of course, if the spots show up, I'll be back with the Hoffman's.