Tomatoes are the crowning jewel of the summer garden. Appropriate use of fertilizers will help enhance your tomato crop, but excessive use of high-nitrogen fertilizers can lead to overgrown vines with no fruiting, or burning the tomato plant leaves. Use a well-balanced fertilizer, and time nitrogen-based fertilizer applications to the needs of your tomato plants as they grow from seedlings to mature fruit-producing vines.
Seedlings and Transplants
Starting tomato seedlings at home gives you a huge spectrum of varieties to choose from. Tomato seedlings should be started in a pasteurized seed starting medium, according to the University of Missouri Extension Service. The Clemson University Extension adds that to avoid damping-off and other seedling issues, you should not use seed-starting medium that has a high nitrogen content. Clemson Extension advises that you should not add nitrogen fertilizer to your tomato seedlings until they have established their first set of true leaves.
Of the three primary elements of garden fertilizer--nitrogen, phosphorous, and potassium--nitrogen is most strongly associated with leaf and vine growth. The West Virginia Cooperative Extension Service advises that tomato plants need most of the fertilizer during their second and third months of growth--when they are developing the large vines and leaf systems that will support the fruit in the harvest months to come. Most tomatoes are transplanted to the garden at between 6 and 10 weeks, so this need for nitrogen fertilizer coincides with the time of transplanting out, making it easy to incorporate fertilizer to support most of the tomato plant's nitrogen needs into the soil during preparation for planting. The West Virginia Extension suggests top-dressing each tomato plant with about a teaspoon of nitrate of soda when the plants are young, while the Ohio State Extension Service suggests turning in a complete, low-nitrogen fertilizer, such as a 5-10-10 or 5-20-20, at the time of planting at a rate of about 3 pounds per 100 square feet of garden area.
Mature Tomato Plants
According to the Ohio State University Extension, excessive nitrogen fertilizer may lead to prolific vine and leaf growth but reduced fruiting in tomato plants--just the opposite of what most gardeners are shooting for. Both the University of Missouri Extension and the Clemson University Extension advise avoiding urea-based or ammonium nitrate fertilizers for tomato plants, as these can promote bacterial and fungal diseases in the tomato vines. Tomatoes also require numerous micronutrients, most notably calcium, and well-drained, moist soil with lots of organic matter in it, so consider adding significant quantities of compost or organic matter like seed meal to provide most of the tomato plants' nitrogen and other fertilizer needs, as well as to improve soil tilth and fertility.