Feeding Seedlings: Should I Fertilize Seedlings

By: Liz Baessler

Fertilizing is a necessary aspect of gardening. Often, plants can’t get all the nutrients they need from garden soil alone, so they need a boost from extra soil amendments. But that doesn’t mean that lots of fertilizer is always a good thing. There are all kinds of fertilizers, and there are some plants and growth stages that actually suffer from the application of fertilizer. So what about seedlings? Keep reading to learn the rules of fertilizing young plants.

Should I Fertilize Seedlings?

Do seedlings need fertilizer? The short answer is yes. While seeds have enough power inside themselves to germinate, the nutrients essential to healthy growth aren’t usually present in soil. In fact, the problems that small seedlings suffer from can often be traced back to a lack of nutrients.

As with most anything, though, too much fertilizer can hurt just as much as not enough. Make sure when feeding seedlings not to give too much, and don’t let granular fertilizer come directly into contact with the plant, or your seedlings will get burnt.

How to Fertilize Seedlings

Nitrogen and phosphorus are two very important nutrients when fertilizing seedlings. This can be found in most common fertilizers that are designed to promote plant growth.

Don’t fertilize your seeds before they’ve sprouted (Some commercial farmers use a starter fertilizer for this, but you don’t need to).

Once your seedlings have emerged, water them with a common water-soluble fertilizer at ¼ regular strength. Repeat this once every week or so, gradually increasing the concentration of the fertilizer as the seedlings grow more true leaves.

Water all other times with plain water. If the seedlings start to become spindly or leggy and you’re sure they’re getting enough light, too much fertilizer may be to blame. Either reduce the concentration of your solution or skip a week or two of applications.

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I take a bit of a different approach than J. Musser does (and here I am making an assumption based on the answer), but I agree with that approach when adding commercial fertilizer. Bagged fertilizer (e.g., 10-10-10) can be quite hard on young seedlings. J. Musser didn't specifically state the use of commercial fertilizer in that answer, but I believe that is the case.

On our farm we only use "organic" fertilizers. I use the quotes because I think the term is way overused and loaded and has been co-opted for marketing reasons. What I specifically mean is that on our farm I use compost, compost tea and worm castings and worm tea when I have it. I will use composted/aged manures as well (horse, donkey, goat, chicken, rabbit) and leaf mold.

In our case I am less concerned about damaging the young seedlings with these because I tend to start seeds in soil blocks or sometimes trays or small pots with a fairly neutral soil mix and then move that seedling block to a bed where I will put a handful of the compost/castings/manure/leaf mold down in the hold and put the block atop that. Later it will get side-dressed with the compost/castings/manure/leaf mold once it has been couple of weeks. I've not had any issues doing things this way and I think it is because the amendments I'm using aren't nearly as concentrated as the bagged stuff.

Again, if you're going to use commercially available mixes, I'd follow what J. Musser said. It is sound advice.

For container gardening I would leverage the power of the compost/worm tea and (limited) side-dressing with quality compost. As J. Musser points out, it is a bit more of a challenge to use the organic stuff with containers but I think that there is considerable value in compost/worm tea and in side-dressing with compost. Whereas in the garden bed, there's always room to add a little more compost/manure, in containers it is more of a challenge. Definitely consider compost/worm tea for containers. The N-P-K values will be lower than the commercial stuff but there is considerable benefit otherwise.

10 Steps to Starting Seedlings Indoors

Step 1: Set up a lighted seed starting area:

In order to grow healthy seedlings, you will need some supplemental lighting. Seedlings need at least 12-16 hours of light each day. I set my timer on my lights for 16 hours on, then 8 hours off. Keep the lights about 2-inches above the seedlings. Adjust as the plants grow. See How to Assemble a Grow Light Shelving System.

Step 2: Gather growing containers to start your seedling:

These can be seed-starting flats, peat pots, toilet paper rolls, newspaper pot, or any recycled container with a few drainage holes poked into the bottom. You can omit growing containers all together by using a soil block maker to compress the soil into a cube. Whatever container you choose, wash them with warm soapy water and rinse well. Place them in leak proof trays or containers to prevent water from dripping. Read more about the benefits of using Soil Blocks for Growing Seedlings.

Step 3: Prepare your seed starting soil:

Use new seed starting mix that’s made for growing seedlings. Using soil from your garden or re-use potting soil from your houseplants can introduce disease to your young and vulnerable seedlings. Starting with fresh, sterile, seed starting mix will help ensure healthy seedlings.

Pre-moisten the seed starting mix before filling your containers. Use a clean bucket or bowl and mix a little warm water into the seed starting soil. You will want the soil mix slightly damp, but not soaking wet. Fill your containers with pre-moistened seed starting mix to within 1/2-inch of the top of the container. Press gently to remove any air pockets.

Step 4: Sow your seeds:

Check the seed packet instructions to see how deep to sow your seeds. Poke holes into the soil in the center of your containers and sprinkle 2 or 3 seeds. Cover the seeds with soil, press down gently so the seed makes contact with the soil, and mist the soil surface with water. Label the containers with the seed variety and sowing date. Cover the containers with a humidity dome to keep in moisture.

Alternatively, you could pre-sprout your seeds and actually SEE the seeds sprout before planting into your containers. See the Benefits of Pre-sprouting Seeds.

Most seeds need temperatures of 65°F to 75°F (18°C to 24°C) to germinate. Place the trays in a warm location near a heat source, on top of a refrigerator, or use a seedling heat mat.

Check your seed trays daily for germination, mist with water if the soil surface has dried out, and wait for seeds to emerge from the soil. Once the seeds sprout, remove the humidity dome and place the trays under lights. Keep the lights within 2-inches of the tops of seedlings.

Step 5: Keep soil moist but not soggy:

Use a mister or turkey baster to water the young plants when needed. The goal is to keep the soil moist but not soggy. Too much water will encourage mold. As the seedlings grow and the roots begin to grow into the soil, water the plants from underneath by adding water to the leak proof tray or setting the containers in a tray of water so the roots can draw in moisture. Don’t allow the soil to become waterlogged or the seedlings will drown. Once the seedlings become established, let the soil dry slightly between watering.

Step 6: Begin fertilizing the seedlings once true leaves sprout:

Most seed starting mixes do not contain any nutrients. When seeds first sprout, they are able to acquire nutrients from the seed’s endosperm. Once the second set of leaves form, also referred to the plants “true leaves” it is time to begin fertilizing your seedlings. Begin a fertilizing regimen using half-strength, organic liquid fertilizer such as liquid fish fertilizer or worm casting tea. Each brand is different follow the instructions on the label for best results.

Step 7: Thin the plants so the strongest survive:

Ideally, each container should have only one seedling in order for it to grow strong and healthy. Thinning involves selecting the strongest plant and removing the extras. The easiest way to do this and with the least amount of root disturbance is to snip the unwanted seedlings at the soil line. You can also try to transplant the extras into separate pots, but you risk damaging the roots and stunting growth. This is another reason why I like to pre-sprout seeds. Then I only plant the seeds that sprout one per soil block or container…No thinning required.

Step 8: Pot up the seedlings to larger containers:

Some seedlings will outgrow their pots before it is time to transplant them outdoors. These plants will require larger containers, so they can continue to grow at a healthy pace. Once the roots fill the container, or you find that you need to water the plants constantly, it is time to repot the transplants into larger containers. I like to use 16 oz. plastic drinking cups with some holes poked in the bottom. These are washed and re-used for many years.

Water the seedlings well before transplanting. This will help contain the soil around the roots and reduce transplant shock. Use a good quality organic potting mix and pre-moisten before filling your containers just as you did with the seed starting mix above.

Fill your containers part way with the moistened potting mix leaving enough room for the seedling’s root ball to sit about 1/2-inch below the rim of the new container. (Exception: If you are transplanting tomatoes, try to bury as much of the stem as you can. Unlike other plants, tomatoes will grow extra roots along the portion of the stem below the soil.).

Remove the seedling gently from its original container by squeezing the sides of the container and inverting it while holding your hand over the soil so the base of the plant is between the index and middle fingers. Tap the bottom of the container several times and the root ball should slide out of the container. Try not to mangle the roots or pull from the stem.

Gently center the seedling the new container, fill in the sides with potting mix, and tamp it in lightly until you have filled the gaps. Be sure to leave about 1/2-inch below the rim of the new container to accommodate watering. Water the repotted transplant well, and then allow the soil surface to dry out before watering again. Label your container and return the plant to the lighting shelf.

Step 9: Adapt your seedlings to outdoors:

Several weeks before transplanting your seedlings to the garden, begin to harden off your seedlings to outdoor conditions. Hardening off is the process of adapting plants to the outside, so they can get used to sunlight, wind, rain, cool nights, and less frequent watering and fertilizing. The hardening off period allows your seedling to transition from the comfortable growing conditions under lights to the normal conditions they will experience in the garden. Learn more about How to Harden Off Your Seedlings Before Planting.

Step 10: Transplant your seedlings to the garden:

After your seedlings are hardened off, they are ready to be transplanted into their permanent location in the garden. Prepare your garden beds ahead of time. If the weather has been dry, water the bed thoroughly the day before you plant. Choose a cloudy day with no wind and transplant in the late afternoon or evening to give your plants time to adjust without the additional challenge of the sun. Water the seedlings well after planting.

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