I’ve only been soil block for three or so years, so I’m not an expert on this topic. There are people like Lisa Mason-Ziegler and Eliot Coleman who have been around a lot longer than I have, doing things I haven’t even been able to wrap my brain around yet.
But as their studious pupil, the following is what I’ve learned from them and many others and put into practice in my own little grow room here in Marshfield, Missouri.
Soil Block Recipe
- Half tub peat moss
- Half tub potting soil
- 1/2 cup lime
- 3 cups greensand
- 3 cups rock phosphorus
Some people recommend using coco coir. If you value being environmentally friendly over function, be my guest. It’s been my experience that it’s much more difficult to work with, causing germination failure & poor watering, among other issues.
Mix your coco coir or peat moss into any brand name potting soil (I have used Jiffy and MiracleGro and I’ve also just sifted compost from my backyard). Just sift out the large chunks with your fingers or use a hardware cloth screen to get the pieces out if there are an obnoxious amount.
Mix in the fertilizers and lime. I’m not married to these ingredients, but they’ve proven the most easily accessible, although a bit pricey. You’re just looking for a slow release NPK in whatever form you’re most comfortable with.
Once everything is fully integrated (try not to breathe it in), pull out the amount of dry mix you’re looking to use for the moment and wet it with hot water. I use boiling water to sterilize the mixture, since I use potting soil and people have had a lot of trouble with store bought potting soil over the last few years. I’m not sure if this activates the slow release fertilizers pre-maturely, but it’s worked well over the last few years.
Make sure the mixture becomes truly wet. You want it to be so wet that when you lift it up and gently close your hands around it, it drips. But not so wet that it pours through your fingers. It needs to hold it shape when you press out the blocks, water should squeeze out the edges on the tray and then suck back up into the blocks when released.
It’s an artform, so keep at it. Your first few sets will be wonky and untidy. Press the dirt very firmly into the blocker holes by squishing the blocker back and forth on the bottom of your tray. Or use your fingers to press the mixture into the holes and flatten everything off before pressing it onto the tray. Hold the blocker from its base, not from the top handle that presses up and down. You want a firm grip to shove that dirt in there.
If your blocks are too dry, they’ll crumble when you start watering them down the road. They need to be completely saturated in water and not be able to hold a single drop more. The bottom of your tray should have little pools of water as you finish going through your mixture. If it doesn’t, it’s probably too dry.
Watch some youtube videos and enjoy the process. There’s a learning curve, but it’s not a steep one and you’ll be pressing out hundreds of blocks in no time at all.
Once they’re pressed, I recommend adding the seeds right away, as the seeds need a good contact with the soil and moisture. The blocks start drying up fairly quickly and by the next day, you can tell they’ve lightened in color. I start watering around the 2nd or 3rd day for newly pressed trays.
Don’t overthink it. You’ll know by the end of your first tray whether it’s a big puddle of water or a crumbly mess. Soon you’ll be doing it with your eyes shut!
