When you grow a lot of plants, soil becomes a big expense. It also becomes a big concern, as you want soil that will help your plants thrive. Here's what we use in the greenhouse:
Seed Starting Mix
The following recipe is based on workshops we've taken at Rancho Santa Ana Botanic Garden, where they grow a lot of California native plants from seed. We found it worked better than the best commercial “seed starting mix” we'd been buying up until then.
* 2 parts peat moss
* 2 parts perlite (keep damp, and don't inhale perlite dust)
* 1 part horticultural sand (sterile and large grained - it will say “horticultural grade” on the bag)
Mix it up well, and put into your seed tray of choice, patting it down well. Soak the tray in water until saturated. Sow the seed, covering it as per directions, and then sprinkle with a little more sand (we find it helps to keep the top of the tray from forming a crust). If in doubt as to whether tiny seed should be covered or not, we just sprinkle on the seed and then "scrunch it up" (i.e., move your fingers in circles around the soil, so that some seed is covered a little but it's all fairly random). Because of the need to keep the seed/soil damp, and the heat in Los Angeles, just leaving seed sitting on top of the soil doesn't make sense to us, so this is how we hedge our bets!
As the seedlings grow, we give them a very light weekly dose of organic liquid fertilizer until they are ready for moving up to small pots.
We worked our way through all the usual stuff at the local nurseries, eventually realizing that none of it is very good. Sterile potting soil sounds great in theory (no harmful fungi, right?), but it also means there's none of the good stuff either. And most soils have little (or no) fertilizer to help the plants grow strong.
Mountain Valley Growers also had trouble getting good soil for their nursery operation a few years back, so they decided to mix their own. After a lot of experimentation, they learned something that goes against what some may think is conventional wisdom: They add compost, with active biological organisms, to their potting soil. They found this helps plants take up nutrients. We read about their results, and ran our own test. We were astounded to see how much better seedlings potted up with a mix that included a bit of compost performed compared with the same plants in a potting soil mixture (the best we could find at the store). After a few weeks, it was like night and day (most visitors couldn't believe the two sets of plants were from the same crop!).
Over the years we've arrived at the following mix for potting soil, with equal eyes towards cost and performance:
* SunGro's Sunshine #4 soilless mix (fast-draining, comes with fertilizer mixed in, reasonably priced by the bale)
* Time-release fertilizer (we use Apex 18-6-12 purchased from McConkeys)
* Organic transplanting fertilizer with mycorrhizal beneficial bacteria (we use EB Stone Sure Start)
* A dollop of organic compost from our own bins
* Liquid organic fertilizer and minerals if/as needed.
If you're growing plants on a large scale, you really should check out buying them in bulk from a wholesaler. A pot that costs 40 cents at the store, could be 6 cents wholesale (so long as you don't mind buying 200 of them!). We find that no matter how many pots we buy, we eventually go through them and have to buy more.
We start by potting up seedlings to 2" pots (called "liners" or "rose pots"); you can fit 49 (7 x 7) in a square tray. Depending on how vigorous the plants are, we move these liners up to either 4" or one gallons to grow out to final size. Vigorous seedings we may move directly to 3" pots, if the roots are already filling a 2".
And don't forget to label everything!