My Plan for Seed Starting Mix


IF YOU’RE PLANNING to start seeds indoors this winter or early spring, you’ll need seed starting mix or seed starting soil to get started.

While the experts agree in principle on the list of ingredients that should be in seed starting mix, there’s no clear consensus on the optimum recipe. But just about everybody agrees that the two key points are that it should hold moisture and drain well.

Almost everyone’s recipe includes at least some of the following ingredients:

— peat moss or coir
— vermiculite
— perlite
— clean sand
— organic soil amendments or non-organic fertilizers

Worm castings or screened compost are often mentioned as organic amendments. Lime is frequently added to alter the PH and greensand to add potassium. For purposes of this blog, I won’t discuss non-organic alternatives.

Winter harvest expert Eliot Coleman, who starts a lot of vegetables in hoop houses, uses this recipe:

— Three 8-quart buckets of peat
— Three 8-quart buckets of compost
— Two cups of organic fertilizer
— One 8-quart bucket of perlite

Another area of consensus is that even the best garden soil is NOT a good media for starting seeds. The most common problem indoor seed starters face is dampening off and garden soil encourage it. It also drains badly and is difficult for young roots to penetrate because it tends to compact.

Peat and Coir

Peat moss and the more environmentally-friendly coir* are popular because they retain moisture well. Peat is often harvested un-sustainably, ruining peat bogs in the process. Coir, on the other hand, is made from coconuts, a sustainable resource.

Vermiculite also aids in moisture retention and is light weight, making it easy for young roots to move it aside. Perlite holds both water and air. Sand, which must be clean and sterile, promotes good drainage so that seeds and roots aren’t sitting in water for long periods of time. Finally, soil amendments provide nutrients for the seedling once the seed’s food supply storehouse is exhausted.

The big question

Many commercially-available starter mixes do not include nutrients. I guess that the plan is to feed the seedling liquid fertilizer until the plant is planted outside in your garden or repotted. If you plan to keep your seedlings inside for a while without repotting, this could be a problem, especially if the plants are heavy feeders.

Some experts suggest that liquid fertilizer is not enough and you must add soil amendments or chemical fertilizers for good results. But the potential problem with this is the nutrients may promote the development of problem- causing bacteria. The big question is should you use seed starter mix that includes organic soil amendments, such as screened compost or worm castings?

My plan

Last year, I started my seedlings in small pet pots filled with Ferry-Morse’s Jiffy Seed Starting Mix. Then, once the seedlings were well established, I transplanted them into larger peat pots with Winterwood Farm planting mix and added Neptune’s Harvest liquid fertilizer at one quarter strength every two weeks. This worked pretty well considering some of the tomatoes were under the lights for as much as six weeks.

One of the brandywine tomatoes from this method was exceptionally productive. I put it under the cold frame early and the tomatoes started ripening around the Fourth of July, and it kept giving until early October, which is really exceptional in New Hampshire.

But I still would like to avoid repotting my seedlings if I can help it. I’d like to keep them under the lights for less time and spare the expense and effort of repotting.

This year, I plan to use paper pots. I’ll fill the bottom quarter of the pot with planting mix fortified with a little of my worm castings. The next two thirds of the pot will be filled with Jiffy Seed Starting Mix. I’m hoping that this will allow me to avoid repotting and, at the same time, protect me from problems with damping off.

I’ll use successive plantings of each variety so that it will increase the chances that I’ll have plants ready for the outdoors that are at the optimum vigor. This will also let me make a course correction if my strategy isn’t working out. I’ll let you know how it turns out.

If you agree or disagree with my strategy, let me know by commenting below.

Related articles you might enjoy:

1. Build an Indoor Seedling Starting Unit
2. Transplant Seedlings at Proper Soil Temperature
3. Getting Rid of Gnats on Seedlings

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19 Responses to “My Plan for Seed Starting Mix”

  1. John Walker
    March 20, 2011 at 8:16 pm #

    Nice set of instructions
    Have you tried soil blocks. This is my third year and an really enamoured with them.

    • March 21, 2011 at 6:43 am #

      It seems that most of the experienced gardeners tend to prefer soil blocks. Most people that have tried it really like it.

  2. Organicdisciple
    July 21, 2012 at 4:45 pm #

    I disagree with this strategy, mainly for the reason that transplanting really benefits tomatoes.

    Re potting with the stems covered right up to the leaves produces more roots, and a stronger and therefore healthier plant. Re potting two or three times (of course in larger pots each time, to allow for root vigor) before finally planting into garden, and then sniping off some lower branches, to promote even more root growth.

    It could be said also that you may compromise the sterile seed mix by adding worm cast and/or compost at the seed raising stage.

    Regards.

    • July 21, 2012 at 6:44 pm #

      Thanks for the input. I really appreciate your feedback even when you disagree with my strategy. Everyone benefits differing viewpoints. I agree with you that transplanting tomatoes into larger pots benefits tomatoes. As you said it allows the tomatoes to grow more roots because you cover the stems during the transplanting process.

      So far I haven’t had any problems with contaminating the sterile seed starting mix by back filling my paper pots with fortified planting mix. It actually works out very well. By the time the roots make it out of the seed starting mix and into the planting mix, the tomato plants seem to be past the stage where they are in danger of damping off. The last two years I haven’t had a single tomato that has suffered from damping off.

      On the other hand, I might just be lucky. It’s certainly possible. I know that studies have shown that using compost tea on seedlings can increase damping off. Of course, you put compost tea on top of the seed starting mix and the plants themselves, so there’s no insulation from microbes and the nutrients.

  3. Tomato Lover
    March 30, 2011 at 1:47 pm #

    I decided to make my own seed starter “with nutrients” this year since I don’t want to repot and I don’t like any of the commercial mixes. I may not have done a great job on the amounts of each nutrient so we will see how it turns out. The recipe is as follows:

    3 cu ft bale of sphagnum peat

    30 lbs of worm castings

    8 cups of perlite

    1 cup greensand

    2 cups bat guano (NPK 1-10-0.2)

    1 1/2 cups dried blood

    1 cup horticultural lime

    1 cup azomite trace minerals

    8 oz wood ash

    1/2 cup TSP (tri-sodium phosphate)

    I have started 2 trays of seedlings with this mix and a friend started 3 trays with it. All seeds were presoaked in warm water with 1 TBS peroxide. Also, mine were put under a red light for an hour during the soak. All were planted in 4 inch pots on heat mats. Within 48 hours I had 5 seedlings emerge, 72 hours/9 seedlings. In a week I had 80% germination. They have been under cool white lights since sprouting, inside a small plastic covered store bought greenhouse in front of a southern facing window. I also put a small fan in the bottom that blows air upward under the shelves 24 hours a day. In one and half weeks time I have tomato seedlings 1 1/2 to 3 1/2 inches tall, all with first true leaves. Peppers are 1 1/2 inches with first leaves only. Chives are 3 inches tall. All appear to be healthy and happy. I plan to start fertilizing with dilute fish emulsion in one more week. This is a whole new method for me but seems to be working. In the past I would still be waiting for seeds to sprout by now. Wish me luck!

  4. March 30, 2011 at 6:15 pm #

    Good luck Joe. I have a question on the fish emulsion. How much fish emulsion to water will you be using and will you be watering with fish emulsion from the bottom or the top. Just curious. I’m at the point where I need to start adding fish emulsion and am trying to decide the best plan.

  5. Tomato Lover
    April 13, 2011 at 9:45 am #

    CORRECTION:

    The above recipe for my soil mix has only 2 five gallon buckets of peat. I inadvertently typed the amount for a larger batch. Sorry for the mistake :(

  6. Tomato Lover Joe
    March 30, 2011 at 7:30 pm #

    I’m going to start out with 1 teaspoon per quart of water. As they get bigger and have more roots I will increase it gradually, every couple weeks. I will be watering from the top, which is what I have been doing. I know everyone says bottom up, but the mix I made seems to be quite dense from the worm castings, so I was afraid that it wouldn’t wick it up from bottom. I have been watering them every other day from the top, and always within minutes the soil mix feels only slightly damp. I haven’t had any water go all the way through to the bottom tray. Apparently this mix holds the water very well without being soggy. If watering from the bottom works for you, you should probably continue, since your mix is different than mine. I also gave them a little spritzing with some Immunox fungicide to prevent damping off, because I’m just too afraid not to use something to prevent it. Good Luck with your plantings, Joe

  7. March 30, 2011 at 8:03 pm #

    Thanks for the info. Good luck with your plantings, too.

  8. April 13, 2011 at 11:14 am #

    Thanks for the update Joe. The readers will appreciate it.

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