When To Start Fertilizing Tomato Seedlings?

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Tomatoes are one of the craziest plants to grow, and it’s not even their fault! These fruits are often mistaken for vegetables and are close relatives of potatoes, resulting in grafted plants that grow both.

But even worse, walk into any garden center, and chances are you’ll find an entire section devoted to tomato care.

Fertilizing Tomato SeedlingsPin

Of course, tomato plants aren’t nearly as difficult to grow as all the “experts” make it seem. In fact, the trusted resource, the Old Farmer’s Almanac, has been known to suggest using beer instead of fertilizer for these plants!

And thus, we come to the dilemma at hand – weeding through the endless arguments about feeding these popular crop plants so we can determine.

When To Start Fertilizing Tomato Seedlings?

The answer to this is (as with all things tomato) unnecessarily complicated.



Generally speaking, you will want to feed them after the first 1 to 3 sets of true leaves appear.

What About Fertilizing Tomato Seeds?

And now we’re immediately going to play devil’s advocate and discuss exceptions to the rule.

For starters (literally and figuratively), you don’t need to use a seed starter for tomatoes.

As long as the potting soil is fresh (i.e., not recycled), it should have all the nutrients your seeds need to germinate.

Likewise, the seed casing contains more than the seedling, and it also contains some additional nutrients to help feed the plant until its starter leaves reach sunlight.

But some people use a soil-free mix, which may or may not contain nutrients.

In cases like this, you may need to add some type of starter food unless you’ve added nutrient-rich ingredients such as worm castings to your mix.

However, any fertilizer you use should be minimal since (again) the seed mostly provides for itself until the first true leaves have formed.

Different Fertilizers For Different Stages

The next common stumbling block is that your tomatoes do best when you provide for their nutritional needs at each stage of life.

Like children, a tomato plant’s nutritional needs will change as it ages. This means you shouldn’t use the same fertilizer for tomato seedlings as you do for a fruiting plant.

In addition to micronutrients, fertilizers contain nitrogen, phosphorus, and potassium (the N-P-K on the package).

Nitrogen promotes strong and healthy foliage and stems, phosphorus is key to healthy blooms and fruits, and potassium strengthens stems and boosts the plant’s immune system.

For many, a balanced fertilizer (10-10-10) works best, although it must be diluted to ½ or even ¼ strength.

Keep in mind that the quality of soil you’re using affects how much additional nutrition your plant will need, so ¼ strength is best for organically rich soil, while ½ is better for poorer soils or soil-free mixes.

Never use full strength, as this can produce chemical burns to the fragile seedling.

Another popular NPK ratio is 24-6-16, a ratio provided by one of Miracle-Gro’s popular water-soluble plant foods.

When using this for your seedling, begin at ¼ strength after the first two sets of true leaves appear, then boost to ½ strength when the plant is several inches tall with at least twice as many sets of true leaves.

Once your tomato reaches its juvenile stage, you will need to switch to a higher phosphorus blend.

Liquid Soluble vs. Granular

Another important thing we need to touch upon is the date between liquid soluble fertilizers and slow-release granular fertilizers.

The latter is popular because growers are told it’s a set-and-forget, suggesting there’s less work involved.

Unfortunately, granular fertilizers contain nutrients that degrade at different rates, meaning your plant can get a burst of one nutrient and a deficiency in another.

As a result, your seedlings won’t get the nutrition they need right when they need it most.

Conversely, liquid-soluble fertilizers must be applied more often but can be given when watering the seedling.

The nutrients absorb more quickly into the soil and break down more efficiently, meaning your tomatoes get what they need as they need them.

It’s also much easier to dilute liquid fertilizers, and they leave behind fewer mineral salts which can become toxic to your plants in large quantities.

Bringing It All Together

So now that we’ve covered many nuances, let’s look at the entire process, including the exceptions.

The medium you use when sowing tomato seeds will usually have nutrients already present unless you go with straight peat moss or coconut coir.

Adding worm castings or a similar substance to bare-bones soil-free mediums will give the seeds any extra boost they might need.

The seed provides plenty of nutrients and contains two starter leaves known as cotyledons.

The cotyledons are more rounded than true leaves and jump-start the process of photosynthesis while the first true leaves form.

When your tomato seedling has two pairs of true leaves, it will have exhausted the nutrients in the seed.

At this point, you will want to give it the first feeding, using either a balanced 10-10-10 or high nitrogen 26-6-16 NPK liquid soluble fertilizer diluted to ¼ strength.

Follow the instructions on the packaging for frequency.

When the seedling has grown several inches and has more than four sets of leaves, increase the dosage to ½ strength until the plant has achieved its next growth stage.

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