Let’s talk about how to fertilize tomato plants. I’ll start by saying I do a pretty poor job of fertilizing anything, not just tomatoes. I feel pretty good about myself if I simply throw a handful of compost into the planting hole, but I end up doing little beyond that. I have come to realize I am making the life of my plants pretty hard by not giving them plant food and just water. This is the year I begin fertilizing my plants properly, starting with my tomatoes, which are known to be hungry plants.

Proper fertilizing of tomatoes is really a two-step process.

Step 1: Fertilize Tomato Plants When Planting

Throwing some compost in the planting hole isn’t enough for tomatoes. They should be fertilized from the get-go. To get tomato plants off on the right foot, feed them with a fertilizer that has a balanced formulation. That would be something like a 4-4-4 or 5-5-5. This means that Nitrogen, Phosphorus and Potassium are all present and in the same amounts. The Nitrogen will help the young plant develop lots of new leaves.

This initial fertilizing should keep the plant going for a while.

Step 2: Fertilize After Fruit Set

Once the tomato’s flowers bloom, they’ll begin to form tiny tomatoes. This is known as fruit set, and it’s at this point you should begin fertilizing again. This time, however, fertilize with a formulation that has less Nitrogen and more Phosphorus and Potassium. Nitrogen enhances vegetative growth, and at this point you want to encourage fruit growth instead. A formulation such as 5-10-10 or 3-4-6, where there is more Phosphorus (the middle number) and Potassium (the third number) than Nitrogen (the first number).

Fertilizers formulated specifically for tomatoes often include Calcium and Magnesium. Calcium helps grow healthy fruit tissues, and Magnesium helps with fruit development.

How often should you fertilize tomato plants once fruit set occurs? That depends. If you are growing in the ground, then every two weeks should be satisfactory. If you are growing in containers, fertilize a little more often.

A Note About Granule Fertilizer

If you are using a granular form of fertilizer, scratch this into the planting hole and then add some soil on top of the fertilizer before placing the plant in the hole. This helps prevent the fertilizer from “burning” the roots with direct contact. Or, you can also scratch the fertilizer into the surface of the soil once the tomato is planted. Scatter it a few inches from the plant’s main stem to prevent the fertilizer from washing its way to the stem where it can burn the plant.