Guide to Fertilizing Tomatoes: How and When to Fertilize for Growth (2024)

Tomato plants require a lot of nutrients to grow and produce healthy fruit. To ensure your tomato plants get the nutrients they need, it is important to provide them with fertile soil, and that often means adding fertilizer. A soil test can help you determine which nutrients are lacking in your soil and what type of fertilizer your soil needs for its primary and secondary nutrients to be adequate.

Here's what you need to know about how and when to fertilize tomato plants.

Should You Fertilize Tomatoes?

Tomatoes would benefit from an application of fertilizer at several different points in their growth.Applying the right product at the right time supports healthy growth, flowering and fruit development, fruit quality, and disease resistance. You can also consider amending your soil before you plant your seedlings.

If you are growing tomatoes for the first time in-ground, your soil probably has sufficient nutrients. A soil test for a new planting area or for previously used soil can tell you if your soil is deficient in any way and how to correct it.

Tomatoes grown in pots need to be fed regularly throughout the growing season, as often as twice monthly depending on the type of potting mix used. A balanced fertilizer, such as 5-5-5 or 5-10-5 works best applied during watering.

Types of Fertilizer to Use for Tomatoes

Organic and manufactured commercial fertilizers for tomatoes don't just correct deficiencies; when used correctly, they can improve the yield and quality of your harvest. Products are formulated to boost a specific stage of plant growth: vegetative, flower and fruit formation, and ripening which includes fruit quality, sugar levels, and storage characteristics. They come as liquids, granules, powders, dust, and pre-mixed formulas.

Organic Fertilizer

• Fish emulsion is a liquid fertilizer high in nitrogen and trace elements like magnesium and calcium. It's a thick liquid diluted in water and applied at soil level and as a foliar spray. It can be used repeatedly throughout the season, but used incorrectly it can burn young leaves and stems.
• Animal manure should be aged before application because nitrogen levels in fresh manure can also burn plants. This fertilizer is worked into soil before transplanting seedlings but is also added as a side dressing for established plants. Aged manure is a bulk product often sold by the bag in square foot quantities.
• Bone meal and blood meal are powders that add phosphorous and nitrogen to soil, respectively. Both take several weeks to break down before plants can use them, but remain effective for several months.

Manufactured Fertilizer

Commercial manufactured fertilizer is formulated to immediately provide specific nutrients to feed your plants. Labels indicate a NPK ratio giving a percentage of nitrogen, phosphorous and potassium. Tomatoes cannot grow without adequate amounts of these primary nutrients.

Many companies offer fertilizers specifically for tomatoes with labels that also list macronutrients like magnesium and calcium. When purchasing these products it's important to read and understand the labels.

For example: A balanced NPK, such as a 20-10-10 is made up of 20 percent nitrogen, 10 percent phosphorous and 10 percent potassium. The rest is filler and may include small amounts of other minerals and vitamins.

Products with a higher P number may be labeled as bloom boosters. Knowing at what growth stage your plants use the greatest amounts of these nutrients helps you choose the best product.

When and How Often to Fertilize Tomatoes

Tomatoes would benefit from being fertilized just before transplanting, at planting, before flowering, and when the fruits are small.

• Prior to transplanting is a good time to add aged manure or compost to your tomato plant's intended spot. The nitrogen boost gives plants a good start with healthy vines and leaves. Compost also works well applied as a side dressing mid-season to keep nitrogen levels stable.
• At planting time fertilizers with a higher percentage of nitrogen, such as a 20-10-10 can be applied to support healthy foliage and growth. Nitrogen rich fish emulsion can be added every two weeks and blood meal every six weeks during the growing season.
  
• Around two weeks before flowering is a good time to use a fertilizer with a higher P value, such as a 5-15-5. (Depending on the variety, tomato plants begin to bloom between 30 and 45 days after transplant. Tomatoes with a longer time to harvest often flower later.) Alternatively you can use a balanced fertilizer lower in nitrogen, such as a 10-10-10. Bone meal, worked in at the base of the plant, is an organic alternative that delivers phosphorous for up to four weeks.
• When the first fruits are small — about golf ball size — an application of balanced fertilizer maintains sufficient nutrition for continued healthy growth and fruit development.