Optimal Seed Starting Mix Recipe

Since we’re working to get better production and economy on our organik* farm this year, I researched a few different sources for what would make the best seed starting and transplant mix. I’m doing most of our farming this year based on Brett Markham’s book Mini Farming: Self-Sufficiency on 1/4 Acre but a few of his ingredient suggestions were a bit pricey. Still, his recipe is very similar to one I used last year that was based on advice from some garden forum online (and did work well). I went looking to see what else was out there (OK, I lost last year’s recipe too) and I found this paper from Cornell which looked systematically at various combinations of nutrients and came up with a clear winner which did far better than all of the other options.

Their paper switches back and forth between various percents of volumes and ratios and lbs-per-cubic-yards (quick, how many quarts are in a cubic yard?) so I sat down to do the math (hopefully I did it right, corrections welcome) to come up with a recipe that would scale more closely to our first planting and be near amounts that can be easily purchased at the local garden center. Without further ado, the recipe:

5 gallons vermicompost (worm castings)

3 gallons vermiculite

3 gallons perlite

14 gallons peat moss

2 c blood meal (dried blood)

1 c green sand

2 c bone meal (not pelletized)

2/3 c garden lime (pulverized not pellitized)

All this adds up to 1/8th of a cubic yard of starting mix. To help with shopping, 5 gallons of vermicompost is 1 bag from the garden center. 3 gallons of vermiculite and/or perlite is about a bag and a half of the size they sell at Walmart. 14 gallons of peat moss is somewhere around half a 2.2 cubic foot bale at Walmart. The blood meal and bone meal can be had at Walmart but the greensand has to be purchased from a garden center. The garden lime also came from Walmart (our local fancy garden center charges 250% of the prices at the big box stores for similar items, but if yours does better, might as well get it all in one trip). The blood meal and the bone meal come in bags big enough for ~4 recipes. The lime and greensand probably have enough to make 50 recipes. Those two are ancient stones, so they should last for years – just keep them dry.

To buy all of that around here cost about $85. The second to fourth batches would cost about $40, and then the fifth batch would cost about $60. It works out to about $400 a cubic yard doing it this way, which isn’t a good idea if you’re doing this in volume. Around here a cubic yard of seed starter mix goes for about $150 though with ingredients Cornell found to not be helpful (so what is the true value?). $200 of my cost is the worm castings, though, which I should be able to generate on the farm practically for free, so once that is going the real cost should be closer to $200/cu yard which is probably OK for an optimal seed starting mix. Even at the full price, it’s cheaper than buying Miracle Grow starting mix at the big box store if you’re doing any kind of volume.

We were surprised how much less of the amendments this recipe calls for than the one we used from online last year. But if the Cornell folks say this works, I’m happy to stretch the budget further (save the rest for the 2nd planting of the season!). Just because last year’s mix had enough of the nutrients doesn’t mean it didn’t have way more than was needed. Time will tell.

I plan to make planting blocks with this mix, using this soil blocker. The basic idea is you wet it down to an oatmeal-like consistency, make the blocks, and then let them dry. Then you plant right in the soil block and plant the block right in the ground, with nothing to constrict the roots.

Two caveats then: first, this mix is intended to go from seed to transplant because I’m only using the medium soil blocker. Next year we might use some farmstand revenue to buy the mini-soil-blocker, which lets you start more blocks for just the sprouting stage, then move them to a medium block if they germinate and start to sprout. This is theoretically more efficient, and this mix then might be better for the transplants than the initial seed starting in that case. There shouldn’t be a real need for the soil amendments in the first week or so.

Second, the Cornell study was focused on tomato plants, and certainly we’ll be growing many tomatoes but we’re also growing other plants. It’s possible that a better mix might be available for other plant types. But, like I said, we used a similar mix last year for everything and had no trouble.

I’ll update this post later on with some pictures and a report on how the mix did for us.

  • screw you USDA for stealing a word from the people