Guide to Fertilizing Tomatoes: How and When to Do It

Tomato Fertilizer

Zbynek Pospisil / Getty Images

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.


Soil pH for tomatoes is 6.2 to 6.8. No matter what fertilizer you add, if your soil pH is too acidic or too alkaline, tomatoes will grow poorly. Soil tests include a result for pH level. When it's too high or low, correct this before applying fertilizer.

Types of Fertilizer to Use for Tomatoes

Organic and manufactured commercial fertilizers don't just correct deficiencies; when used correctly, improve 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.

Organic Fertilizers vs. Soil Conditioners

Organic fertilizers differ from soil conditioners because they feed your plants. Conditioners improve soil health and structure and generally take longer to produce results. Compost is an excellent organic soil conditioner applied annually to gardens.

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.


Manufactured fertilizer can correct deficiencies quickly but plants also use them up quickly. Overuse with repeated applications can cause harmful salts to build up in soil. Tomato plants benefit from secondary macronutrients (e.g., magnesium, calcium, etc.) along with minerals and vitamins. Even when your soil is healthy starting out, a nutrient boost correctly applied several times during the growing season can improve your crop.

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.
  • What is the best fertilizer for tomatoes?

    The best fertilizer for tomatoes comes down to preferences. Fish emulsion is a mild, nitrogen rich organic that can be used all season long. If only one type of formulated fertilizer is used, it's best to choose a balanced product such as NPK 10-10-10.

  • When should I fertilize my tomato plants?

    You should fertilize your tomato plants just before or during planting, around two weeks before bloom begins, and again when the first tomatoes are small.

  • What does Epsom salt do for tomatoes?

    Epsom salt adds micronutrients magnesium and sulfer to soil. It promotes chlorophyll for healthy green foliage and helps the tomato plant take up nitrogen and phosphorous.