Hello gardeners! I was doing a bit of "field research" for my Movable Garden business and discovered the surprisinging truth behind bagged soil. If you're a gardener of any sort, from large-scale to containers (pots), please read on.
Soil is composed of inorganic materials (minerals, broken-down rock, sand, silt, and/or clay) and organic materials (peat moss and "compost," which can be made from manure, kitchen scraps/plant matter, and/or wood chips). Optimal garden soil mostly contains inorganic materials and only 2-10% organic matter. Adding organic matter to garden soil can be a good way to replenish certain minerals and improve garden health, however, there is a very common misconception that "more is better" when it comes to adding manure, compost, or peat to garden soil. By adding too much organic matter, you can actually 'kill your garden with kindness.'
If you are a container or lasagna gardener, most likely you will be purchasing soil. Recently I stopped at a few of my local Home & Garden stores and checked out their pre-bagged soil. When I read the fine print of the "soil" ingredients, I was shocked. All of the bags explicitly labeled "Garden Soil" and "Top Soil" contained absolutely no inorganic matter. The "soil" was made from 100% organic matter- manure, compost, peat moss and/or wood chips. These were not bags of soil, they were mislabeled bags of compost. I went to a several different stores and could not find any bags of real soil that contained any amount of inorganic matter.
Before you buy a bag of soil, read the label carefully. Remember, it should contain about 95% inorganic matter and 5% organic matter. If you cannot find any proper soil that is pre-bagged, your best bet is to purchase some un-bagged soil from your local Garden Center. Many "Pa & Ma style" Garden Centers have big piles of top soil, loam (garden loam usually contains ideal parts of inorganic and organic matter), and manure for sale. You can shovel the soil into the back of your car or truck, or to make it easier for yourself, shovel the soil directly into the containers that you'll use for growing. Ask the Garden Center workers what the soil contains and if it is from a reputable source. A good garden has good soil!
If you can't find labels or answers about soil ingredients, here's the rule of thumb: "real" garden soil is black or dark brown. It is rich-looking, retains moisture well, and is quite heavy. Soil is not too dense, but not so light and fluffy that it will blow away. It is made of bigger clumps and particles as well as smaller particles. Fake "soil" that is mostly or all organic matter is often a lighter brown color and dries out quickly. It is usually much lighter than soil and may blow away when you drop a handful of it. It is mostly fluffy and not so dense. Often, the majority of its composition is small stick-like pieces or fluffy bark. Be careful: water is added to fake "soil" to give it the perception that it is dark, rich, and heavy.
Have fun gardening and remember to investigate!