Tomatoes need quite a big food supply over the season -- they're what we call "heavy feeders." This is no surprise when you look at all the work they're doing: extending the stem, putting out more branches, leaves and blossoms; developing, nurturing and ripening all those fruits! To do all this work they need a steady diet of water and nutrients.
All About Side-Dressing
In most gardens, it's a good idea to side-dress tomatoes. That simply means placing fertilizer around the plants to give them extra nourishment through the season. One or two side-dressings is fine for most gardens.
Many kinds of fertilizers can give tomatoes the extra nutrients they need. Some gardeners prefer to use a complete fertilizer (such as 5-10-10 or 10-10-10). Organic fertilizers such as bone meal, dried manure or cottonseed meal are also good. Just remember that most organic fertilizers don't contain balanced amounts of the three major nutrients - nitrogen, phosphorus and potassium. For example, manure tends to be low in phosphorus so you could add bone meal at the same time to provide a more complete diet.
Stay away from high-nitrogen fertilizers such as urea, ammonium sulfate or fresh manure because it's easy to use too much. When you overfertilize, you get tall, dark green plants with few tomatoes.
When To Side-Dress
Start side-dressing when the first tomatoes have just formed - when they're about the size of golf balls. Make repeat side-dressings every three weeks after that. About a pound (two cups) of 5-10-10 should be enough for all the plants in a 30-foot row (about 20 plants). This works out to about 1-1/2 tablespoons per plant spread in a one-inch-deep circular furrow five to six inches away from the stem, usually right under the drip line of the plant. Be careful not to get any of this fertilizer on the leaves or stem because it will burn them. Cover the fertilizer with one to two inches of soil. The next rain or watering will start carrying the nutrients down into the root zone of the plants.