Tomatoes are one of the most common plants found in summer vegetable gardens. Many varieties of this subtropical vegetable exist, from the small, pop-in-your-mouth cherry tomato to the enormous beefsteak varieties that weigh in at over 1 lb. each. Whichever type you grow, tomatoes benefit from some basic plant nutrients such as phosphorus, nitrogen and potassium.
Nitrogen is an essential nutrient that all plants require. Many fertilizers include a good amount of nitrogen. If you notice the label, it will have three numbers listed, such as 10-10-10; this is the N-P-K ratio of the nutrients it contains, with the N indicating nitrogen, the P standing for phosphorus and the K meaning potassium. Nitrogen also occurs in compost and fresh green plant parts, especially grass clippings. Young tomato plants benefit from a moderate amount of supplemental nitrogen, which causes lots of healthy foliage to grow. However, if they get too much nitrogen after they begin to bloom and set fruit, tomatoes will produce foliage at the expense of flowers and fruit; switch to a low nitrogen, or "blossom booster" plant food to encourage fruiting.
Throughout their lives, tomato plants need a certain amount of supplemental phosphorus. You can ensure that your plants get enough if you feed them with a plant food having a "P" rating of 5 or 10, for example one with 5-10-10. Ohio State University recommends digging about 3 lbs. of a 5-10-10 fertilizer into a 100-foot row of tomato plants two weeks before you plant them, and again when your plants have begun to set fruit.
The "K" in an N-P-K listing stands for potassium, which is an essential plant nutrient tomatoes need in order to thrive, especially in peaty, chalky or sandy soil where potassium is often lacking. A potassium deficiency causes the edges of leaves to become discolored and appear as if they were scorched. To avoid this, fertilize your tomato with a plant food that has at least a "10" rating for potassium, which is the final number listed in the N-P-K listing.
In addition to nitrogen, phosphorus and potassium, tomatoes also need some trace elements or secondary minerals such as calcium, magnesium and sulfur in order to grow large and strong and produce the largest amount of fruit. A lack of calcium can contribute to the condition known as blossom end rot on tomatoes, so watch for it and fertilize your plants with a plant food that includes minerals such as calcium. A magnesium deficiency sometimes occurs in tomatoes if you grow them in light, sandy or overly acidic soil---you'll know if this is affecting your plants if their leaves turn yellow but the veins remain green.