There are a lot of ways you can be frugal in your garden and save some money. The potting mix you use in your container gardens should be something you feel comfortable splurging on. In my opinion, the best potting soil for edible container garden is the Happy Frog line of potting soil distributed by FoxFarm.
Here is what the FoxFarm website says, in part, about the Happy Frog potting soil:
"Happy Frog® Potting Soil is alive with beneficial microbes and fungi that help break down organic matter and feed the plant roots. Between the earthworm castings, the bat guano, and the composted forest humus, your container plants have never felt so good. And don't worry--FoxFarm uses only the highest quality, premium ingredients--no cheap fillers, no topsoil, no sludge. Never. We promise."
Sounds good right? This year I put it to the test in my container garden on the back porch. The first picture in the photo gallery is of lettuce greens on 5/08/10. The second picture is of the same lettuce greens on on 5/26/10. Now, compare the picture taken on 5/26 to the lettuce greens in the windowbox. They're the same seeds, from the same seed packet, started at the same time and growing under the same conditions on my porch. The only difference is that the greens growing in the window box were sown with a "cheap" potting soil and the larger container is filled with Happy Frog potting soil. I managed to get several salads out of the lettuce growing in the Happy Frog potting soil, while I haven't even touched the greens growing in the window box. The lettuce in the window box looks exactly the same today as it looks in that picture. If I didn't sow the seeds myself I'd swear the lettuce was fake.
How to choose a good potting soil mix
Carefully read the ingredients list on any bag of potting soil that you purchase. If you don't have time to take notes and research the ingredients; I've found that picking up the bag of soil and squeezing it helps me determine if the potting mix is good. If you squeeze the bag of soil does it feel like the soil inside is crumbling or forming a clump? If it feels like the soil is clumping together then ratio of ingredients are not suitable for container gardening. A good potting mix should be loose to allow water to drain. Finally, the price. If the bag of soil costs less than you'd pay for a cup of coffee chances are that it isn't going to be very good for your plants. Whenever I see those $2.99 bags of potting soil at drug stores I have to shake my head. Just by handling the bag I can tell gardeners will be faced with disappointment if they try to grow plants in it. Those cheap potting soils harden to the point that they're only good for building mud houses. Don't skimp on potting soil for your container garden, especially if you're growing vegetables and herbs that you plan on eating.
Where to buy Happy Frog potting soil in Chicago.
Last year I visited just about every independent garden center in the Chicago in search of Happy Frog potting soil. From the reactions I got when I asked for it by name I discerned that it must be popular with marijuana growers. I learned about this potting mix during a visit to the organic rooftop farm at Uncommon Ground on Devon, where their raised beds are filled with Happy Frog potting soil. It seems like the only retailer for Happy Frog potting soil in Chicago is Gethsemane, where a 2 cu ft bag set me back $20.00.
UPDATE: For those not looking to buy container garden soil check out the post Where to Buy Garden Soil in Chicago.