Buy the good soil, cheap out on everything else
We've spent four lessons on frost, sun, and what to plant. Now we're getting into the dirt itself, because none of the cold frame building we're about to do matters if what's underneath it can't grow anything.
Here's my opinion, and I'll just say it plain: buy the good soil, cheap out on everything else. People walk into a garden center, drop real money on a fancy cedar planter or some powder-coated raised bed kit, and then fill it with whatever bagged dirt is cheapest that week. That's backwards. The planter is just a box. The soil is the whole operation. Your yield lives and dies in there, not in the wood grain.
So flip your budget. Scrap lumber, an old window, cinder blocks you find on a classifieds listing, whatever you've got, that's fine for structure. Save the real money for what goes inside the box.
What "good soil" actually means for a raised bed
You're not just buying topsoil. For a raised bed you want a mix, roughly:
- Compost — about a third. This is your nutrition and your biology. Don't skip it or go light on it to save a bag.
- A quality garden soil or topsoil blend — another third. Something with actual organic matter in it, not just screened fill dirt.
- Something for drainage and structure — the last third. Coarse sand, perlite, or a soil conditioner. Our valley soil is heavy clay in a lot of spots, and if you fill a bed with straight topsoil from a cheap source you'll get compaction and standing water by July.
Read the bag. If the ingredient list is just "soil" and nothing else, put it back. You want to see compost, forest products, something living in there.
Run the volume math before you buy anything
I mentioned a while back that I built a bed a foot too deep once because I read that deeper is better for root crops and didn't stop to do the arithmetic. Eleven wheelbarrow loads to fill it, and I still had to make a second run to buy more bags. Don't do that.
Here's the actual math. Multiply length times width times depth, all in feet, and that gives you cubic feet. Divide by 27 to get cubic yards if you're buying bulk, or check the bag, most bagged soil tells you square footage coverage at a given depth right on the front.
A 4-by-8 bed at 12 inches deep is 32 cubic feet. That's a lot of bagged soil, usually 40 or more bags depending on size, so at that volume it's almost always cheaper to call around and get a bulk delivery of a garden blend instead. Run the numbers before you commit to a depth. Six to eight inches is plenty for lettuce, spinach, and most root crops. You don't need twelve unless you're doing something like a long carrot variety, and even then you can go deep in one section instead of the whole bed.
The part nobody warns you about: your water
This is where I have to bring up something that cost me most of a summer to figure out. My second season, I had drip irrigation running to a bed and the far end kept dying no matter what I did. Same soil, same sun, same everything, and one end just wouldn't thrive. I checked the emitters, checked the tubing, checked my watering schedule about four times. Took me until fall to realize the emitters on the far end were half clogged with mineral scale from our hard water. I ended up soaking the whole line in a vinegar solution to clear it out, and it fixed the problem overnight.
Half the valley has this same water, and most people don't know it's happening until a section of the garden just quietly underperforms all season. If you're setting up drip irrigation on a new bed, plan on cleaning your emitters at least once a season, more if you notice uneven growth. It's cheap insurance and it takes an evening.
The dry air out here matters too. Good soil with real compost in it holds moisture a lot better than cheap fill dirt, which is one more reason not to cut corners there. A bed that dries out in a day in July is going to fight you all season no matter how good your cold frame is come October.
A few practical notes before you build
- Test your existing soil if you're not doing a fully raised bed. A basic pH and nutrient test from the extension office is worth the small fee.
- Don't buy soil in direct sun and let bags sit sealed on hot pavement for a week before you use them. It cooks whatever biology is in there.
- If you're building the bed yourself, treat the lumber question as separate from the soil question entirely. Untreated scrap is fine. Don't spend on cedar to hold garbage dirt.
Before next time
Measure the space where you're planning your first cold frame or raised bed, and run the cubic footage math we just went through. Bring the numbers to next class. We'll start building.