Written by Logan Hailey Horticulture review by Sarah Jay Last updated: March 17, 2025 | 4 min read

Starting seeds is the highlight of spring gardening. Once you choose the right seed starting mix, the best trays, and your favorite varieties, the process is pretty simple. But after seedlings sprout, you may be wondering if they need more nutrients to keep them going.

Tomatoes are known as heavy-feeders in the garden, and these fast-growing plants suck up a lot of nitrogen and phosphorus in their early days as they develop lush foliage and strong roots. However, fertilizing them too soon or too late can be problematic for later growth. 

Let’s dig into whether or not you should fertilize tomato seedlings and the best products to use.

Vegetable Fertilizer

All-Purpose Vegetable Fertilizer

Tomato-tone Plant Food

Espoma Organic Plant Food (4 lbs.)

Worm
Castings

Urban Worm Company Worm Castings

The Short Answer

Most healthy tomato seedlings do not need fertilizer until they are going to be transplanted into the garden. Many seed-starting mixes already contain sufficient nutrients for seedling growth, so you may not need to add anything additional if the blend already includes sources of N-P-K, micronutrients, and organic material like compost. However, if seedlings are yellow or pale, nutrient additions may be helpful.

Never fertilize tomato seedlings right after they sprout, as they still have the necessary nutrients from the seed endosperm and they are vulnerable to nutrient burn. Instead, wait until the young plants have two to three sets of true leaves. These are the leaves that develop after the initial moon-shaped baby leaves called cotyledons.

At this point, you can side-dress with a slow-release balanced organic fertilizer or diluted liquid organic fertilizer. Avoid synthetic nitrates or quick-release products that may burn the fragile young plants. Balanced products have almost equal rates of N-P-K such as 4-4-4.

The Long Answer

Fertilize transplants with granular amendments in planting holes once they have strong foliage and root growth.

It is most common to fertilize tomatoes at the time of transplanting when seedlings already have robust foliage and root growth. You can sprinkle granular amendments into the planting hole and water to promote uptake. 

If the seedlings show signs of yellowing or pale foliage before transplanting time, it may help to add diluted liquid fish emulsion or another gentle, organic nutrient solution. Here are more details about how and when to fertilize tomato seedlings.

Sources of Nutrients in Seed-Starting Mix

Blends with coco coir, peat, vermiculite, and perlite often lack nutrients to support plants beyond cotyledons.

Most soil blends contain mixes of plant nutrients, including nitrogen, phosphorus, potassium, and micronutrients. If your mix contains any of the following ingredients, you probably won’t need to fertilize your tomato seedlings until they are larger and ready to go in the ground:

• Compost
• Rotted manure
• Worm castings
• Fish meal
• Seaweed or kelp
• Feather meal
• Calcium shell 
• Bone meal
• Blood meal
• Guano
• Mycorrhizal fungi
• General organic fertilizer

You may notice phrases like “enhanced with…,” nutrient-rich,” or “amended with organic fertilizer” that indicate minerals have already been added to the blend. If the mix contains only ingredients like coco coir, peat moss, vermiculite, and perlite, then it likely lacks enough nutrition to support developing plants past the cotyledon stage. 

Signs That Seedlings Need Fertilizer

Nutrient-rich mixes with compost or worm castings delay the need for feeding until transplant time.

Yellowing, pale foliage is the first sign that young tomatoes are deficient in some nutrients. The leaves may also turn purple or brown and fall from the plant. Slow, stunted growth may also be apparent if there is a lack of nitrogen in the soil. 

However, you must be careful not to mistake signs of malnutrition for other common issues. Overcrowded seedlings and disease issues can cause similar symptoms. Check your seed starting mix ingredients first, and then eliminate other issues before proceeding with fertilizer. 

How to Avoid Nutrient Burn

Pale, yellow, or purple leaves with slow growth can signal nutrient deficiencies, overcrowding, or disease.

Nutrient burn is the most common issue with fertilizing tomato seedlings. This can lead to browning, yellowing, curling leaves, wilting, stunted growth, and even death. Tender young plant tissues are vulnerable to nutrient burn from harsh fertilizers, so it’s crucial to be very careful when applying plant food to seedling trays. 

Best Fertilizers for Tomato Seedlings

Use organic granular fertilizer, worm castings, or composted manure sparingly, always following package instructions.

All-purpose organic granular fertilizer, Garden-tone organic plant food, worm castings, composted manure, and diluted organic liquid tomato plant food are all great options. Be sure to follow package instructions closely. When in doubt, remember that less is more.

Use smaller amounts of fertilizer than you think you need to ensure that you don’t damage your seedlings. You can always add more later! Water thoroughly after application to ensure uptake. Add a large flush of water or transplant to new soil if you think you’ve overfertilized.

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