Feed Your Tomatoes! A Complete Guide to Fertilizing Tomatoes

The Importance of Fertilizing Tomatoes
Gardeners talk about tomatoes as being “heavy feeders.” However, this doesn’t mean that you need to dump a lot of fertilizer on your tomato garden. It just means that tomato plants use a lot of nutrients. If you already have nutrient-rich soil, your tomatoes will grow just fine. But, if you want a maximum harvest or are starting from less than optimal soil, then extra fertilizer will be critical. And because tomatoes are heavy feeders, adding extra nutrients will result in healthier plants and a bigger harvest.

Should I fertilize my tomatoes?
Before adding fertilizer to your tomato plants, start with a soil test. You can get a soil test through your local University Extension Service or a commercial soil testing service.

The results you get will tell you what nutrients are in your garden soil and what deficiencies it may have. Everyone’s soil is a little different because of what they’ve already been growing and what has been added. So the results of your soil testing may be different from your neighbors.

Along with nutrients, a soil test can also tell you the pH of the soil. A pH range of 6.2 to 6.8 is optimal for tomatoes. Anything out of that range will affect tomato growth.

Once you know the nutrients in your soil, you can go forward with fertilizer applications.

The benefits of fertilizing tomatoes
When used appropriately, fertilizer can increase tomato yield and help produce better fruit. Using the right fertilizer at the right time will result in big healthy plants producing flowers and then developing delicious tomatoes.

When to fertilize tomatoes?
Timing is crucial when it comes to fertilizing tomato plants. Applying the right type of fertilizer at the right time ensures that the plants receive the nutrients they need at each growth stage.

So when is the right time to fertilize tomato plants? An easy way to know when to apply what nutrients is to break the tomato plant growth into 4 stages. It helps to think of your plants as seedlings, transplants, flowering, and fruiting.

Fertilizing tomatoes as seedlings –
You can start fertilizing tomatoes when they are seedlings. If you start your heirloom tomatoes from seed in a seed-starting mix, they will soon need additional nutrients. When tomato seedlings have at least two sets of true leaves, you can start applying a diluted fertilizer when watering. Be careful with applying fertilizer at this stage. These tender plants can easily be burned by too much fertilizer. Using a liquid fertilizer that is diluted to half strength will be sufficient. Water from the bottom to avoid getting fertilizer on the stem. A balanced fertilizer is best at this stage, as all parts of the plant are growing. Phosphorus will help with tissue development, and nitrogen will help with leaf growth.

Fertilizing tomatoes when transplanting –
When you transplant your tomato seedlings outdoors, ensure the garden soil is nutrient rich. If needed, before translating, work in some well-aged compost.

During this stage, tomato plants focus on leaf and stem growth. Nitrogen is particularly important during this phase, as it promotes lush foliage. Use a fertilizer with a higher nitrogen content, such as a 20-10-10 or 16-6-4 formulation. Apply the fertilizer around the base of the plants, keeping it about 6 inches away from the stem to prevent burning.

Fertilizing tomatoes when flowering –
Once your tomato plants are growing in their garden space, they will start producing flowers. When your tomato plants start putting out flowers, it is time to add phosphorus.

Switch to a fertilizer with a higher phosphorus content, such as a 10-20-10 or 5-10-10 formulation. Apply a dry fertilizer in a band around the plants, ensuring it is well-distributed and incorporated into the soil.

green tomato on vine
Fertilizing tomatoes when the fruit is starting to set –
During the fruit development stage, tomato plants require a balanced supply of nutrients to support healthy fruit growth and prevent nutrient deficiencies. Continue using a balanced fertilizer or switch to a slightly higher potassium content formulation, such as a 10-10-20 or 5-10-20 formulation.

At this stage, higher potassium content can result in more sugar in the fruit, which affects the flavor. Too much nitrogen will result in tomato puffiness, and the flavor won’t be as good.

You can continue to apply the fertilizer as directed, taking care not to over-fertilize, as this can lead to excessive foliage growth and reduced fruit production. When the season is coming to an end, you can stop applying fertilizer.

red tomatoes on vine
How to apply fertilizer to tomato plants
Now that you know when to apply fertilizer. How do you do it without harming your plants and making sure that it’s available for the roots? We’ll take a look at how to apply liquid and dry fertilizers.

Liquid Fertilizers
Some types of fertilizers, such as fish emulsions, can be mixed with water, sprayed onto the leaves, and added during regular watering. A liquid or water-soluble fertilizer is a great option for container tomatoes.

Liquid fertilizers should be diluted and are usually applied more often. When using a liquid fertilizer, don’t water too close to the stem to avoid burning the stem and roots. Unless it is a fertilizer specifically formulated for spraying on leaves, you will want to avoid getting fertilizer on leaves and stems.

Dry Fertilizers
Dry fertilizers are sprinkled on the soil and then worked in gently and watered. Dry fertilizers usually release slowly over time and aren’t reapplied.

Before applying dry fertilizer, water your tomato plants. This will keep the roots from soaking up the fertilizer too greedily. When applying dry fertilizer, spread it around the plant without touching the stem. Fertilizer touching the stem and leaves can burn them.