Tomatoes are heavy feeders and can benefit from conservative fertilization. Too much fertilizer can cause your tomato plants to grow very large but can result in poorer yields and fruit quality. Tomatoes can be fertilized using synthetic chemical fertilizers and organic fertilizers. In some ways, chemical fertilizers are easier to apply because they have consistent percentages of nutrients.
Fertilization to encourage green vine growth and leafing early in the growing season will help you encourage strong, vigorous tomato plants. Fertilizing for healthy initial growth actually starts before planting. According to Ohio State University, add 2 1/2 to 3 lbs. of fertilizer per 100 square feet of garden area two to three weeks before you plant your tomatoes. Recommended fertilizers include 5-10-10, 5-20-20 or 8-16-16. Be careful not to apply too much nitrogen. Nitrogen encourages green growth but can have negative impacts on flowering and fruit set.
Flowering and Fruit Setting
Once the plant has flowered, watch the tomato growth carefully. Once the fruit has reached the size of a golf ball, apply 3 lbs. of 10-10-10 fertilizer to each 100 foot row. Additional nitrogen at this time will not affect flowering. Once the fruit has reached this size, the additional nitrogen will also not adversely affect fruiting.
After your initial post flowering fertilizer application, apply another 3 lbs. of 10-10-10 per 100 foot row three weeks after the initial application. A second application six weeks after the first application will also benefit fruit growth and vine productivity.
Tomatoes also respond well to organic fertilizers, as long as you keep in mind that higher nitrogen organic fertilizers can have the same negative effects on flowering and fruit setting as chemical fertilizers. When selecting organic fertilizers, select ones with lower nitrogen for pre-flowering and fruit setting fertilization. Once the fruit has set, you can use a more balanced fertilizer to maintain plant health and fruit production. Organic fertilizers are often slower acting than chemical fertilizers, so be patient while waiting for their effects to come to fruition.