James asked, via email:
"I am starting to prepare soil in a flat to grow seedlings. I was advised that a good mix is 1/3 peat, 1/3 perlite, and 1/3 horticultural sand. Horticultural sand seems difficult to obtain except in a small 2-quart box. (And it's rather pricey.) Is there a substitute for horticultural sand? I understand the sand should have sharp edges so that it drains well."
There are a couple of ways to answer this.
The first is that you could try a different seed starting mix that doesn't require you to use horticultural sand. You are already getting good drainage in the mix thanks to the perlite in the recipe. James could just swap the horticultural sand for finished compost or vermicompost and get some nutrients into this otherwise fairly inert mix while still having good drainage. My seed starting mix doesn't use horticultural sand, and it drains very well.
If you're set on using horticultural sand or something close, look for "coarse builder's sand." Note: NOT play sand or sandbox sand -- these will just turn your mix into cement. They're way too fine for our purposes. Coarse builder's sand is often found in the masonry department of most home centers, where you'd find bags of cement and things like that. It has a much larger grain than play sand -- perfect for drainage. We've used it in both seed starting mixes and potting soil here at our house, and it works great.
You could also try vermiculite, but between coarse builders sand and vermiculite, I prefer the builder's sand -- less dusty and it's super cheap.