I really like the 2” blocks, and I think the 1.5” one has potential, but I don’t think the mini is worth the bother, especially since my heating box space is not too precious, since I built it large enough for two flats at a time (three if I’m not worried about light).
Pros: Growing with soil blocks in trays leads to way less to sterilize than cell packs, plus easier to transplant and easier on roots than just growing in flats. Also, despite reusing the cell packs, the flimsy plastic cracks after a few uses, so soil blocking = less plastic waste.
Cons: uses more soil than cell packs, because they require packed soil.
For seed starting soil, I’m currently using Pro-Mix, which comes in a compressed 3.8 cu ft block. I’ve used a couple of other potting soils, and I’ve seen recipes for making your own. I use this mostly because it’s affordable, and available easily near me, but I think most any general purpose seed starting/potting soil would work. I’d prefer to make my own using coconut fiber instead of the peat, but practicalities of time lead me to compromises.
Soil blocker (a cookie cutter for soil)
Potting soil (bought or home-made)
Flats (or trays) – sterilized with chlorine bleach solution if re-using and especially if found
A flat bottom container for the wet soil
Using the soil blocker
1. Moisten some soil the night before, with the goal of very wet but not soupy soil.
2. Take the soil blocker and push down firmly into the soil. I generally really pack it in.
3. Twist the blocker right and left a bit to loosen the soil in the container from that in the blocker
4. Tip the blocker slightly to release any liquid seal/suction and then pick it up
5. Set it down on the tray and then
a. Push down on both the blocker and the handle on the top of the blocker
b. Gently allow the bottom half of the blocker to come upward as you keep pushing down on the handle
c. Tip the blocker slightly to release suction
d. Lift the blocker (leaving soil blocks behind)
This seems complicated, but it really is rather simple and fast once you have the hang of it. I think it may be a bit faster, and is certainly more pleasant, than sterilizing cell packs for reuse.
Once I’ve completed a whole tray of soil blocks, I start seeding, either in the dimples, or on the surface depending on the seed. I use a bit more of the moistened soil to fill the dimples and cover the seed (unless it’s something that needs light to germinate).
To Water: I used to spray the blocks carefully with a mister, but now I just use a watering can that gives a fairly gentle shower. It does destroy the pretty, orderly look of the blocks, but they hold together enough that it’s still easy to separate the blocks for planting.
I do still grow onions in flats, and I grow my artichokes in cell packs so I can vernalize them, but for most of my other indoor seed starting soil blocks are my method of choice.