Whenever the urge strikes me to try out a new container garden idea, I happily select my pottery and plants, but get to the potting soil section and nearly have a heart attack when I see the price of my favorite bagged potting soil. It doesn’t seem to make any sense that dirt actually costs big money! But in fact, a lot of science and labor (yes, labor) goes into that OMRI listed organic potting soil, and it’s important to understand why you shouldn’t try to substitute it for that cheapo bag of top soil.
What's the difference between potting soil and garden soil?
The first thing to know about the differences in bagged soils is to understand that almost all plants are adapted to grow and thrive in the ground. Plants growing in containers are growing in a non-ideal setting and because of that, soil mixes that go into containers like pots and planters have to do a lot of extra work for those plants. The #1 chore for potting soils is to allow air to get to the roots of your container plants, while still providing them with moisture. In fact, “potting soils” or “potting mixes” actually contain little to no “soil” and are usually made of a blend of peat moss, vermiculite, and screened compost.
Garden soil often consists of dense shredded bark and other fillers, and needs to be worked into the ground, which naturally allows for good drainage and airflow (unless you have clay soil or swampy soil — then all bets are off!). If you put garden soil into a container, it retains way too much water and drowns the roots of plants. So don’t mistake Potting Soil for Garden Soil and don’t try to substitute one for the other!
Why are there different types of potting soils?
Most potting soils are labeled “All-Purpose” and that means most plants will do quite well planted in it, but there are a few specialty potting soil mixes that are good to know about, and most of them are self-explanatory. You might run across orchid mixes, African Violet mixes, and desert-plant mixes in your local garden center. Because these potting soils are formulated for plants with very special growing needs, you should only use those specialty mixes as directed.
Organic products can be treated a little differently though. You can use organic potting mix like an all-purpose mix, and buy it for all your pots and planters, which is great. I like to use organic products for everything, but since I have many containers, I can make my dollars go a bit farther by reserving the organic potting soil for growing food, and the non-organic products for flowers. Black Gold All-Purpose Potting Soil is a fantastic potting soil, that can be used to start seeds and fill containers.
If you are dedicated to using organic products from soil to tip, go for it!. But if you are looking to stretch your dollar, consider growing flower plants from organic seed, buying organically grown plants, and fertilizing your flowers with organic products instead. Save the really good organic potting soil for growing food!
What about soil for raised garden beds?
If your raised beds are going to lay on top of your existing soil, you can can get away with partially filling them with less expensive garden soil. There are many, many, different ways to improve top soil or garden soil to prepare it for planting, and many experienced gardeners have their own preferred mixes, depending on the climate or the vegetables they plan to grow.
A good general mix, is to combine garden or top soil with a compost product (like composted manure or lobster compost). If you find that this combination makes your soil too heavy or dense, or if water sits on its surface for more than a few seconds, you can add in coarse sand (not play sand!) or vermiculite to lighten it up. Of course, you could simply skip all this mixing, and fill your raised beds with potting soil too!
If you plan on using an elevated raised garden bed, or need to put your raised bed on concrete or stone, you should definitely avoid garden or top soil, and stick with that all-purpose potting or organic potting soil.
How much soil should go into containers?
A good rule of thumb (the green, not the black kind!), is to leave one to two inches of space from the top of your container when it’s filled with your plantings. If you fill your containers all the way to the top, when you water them, the soil will expand and run out over the top, wasting that precious black stuff.
Some gardeners advise that you should fill the bottom of your containers with stone or broken pottery to improve drainage or save on potting soil. This practice is generally frowned upon by professional horticulturalists for a number of reasons. For one, stone can add a lot of weight to heavy pots. More importantly, the stone can fill up with water and your roots might actually rot, leading to sickly plants!
If you are concerned about drainage, be sure there are enough drainage holes in your container and place your containers on pot risers or plant stands. If you have a really large or tall container, you can fill some of the volume with packing peanuts, old milk jugs, or aluminum cans. Be sure to leave approximately enough room to fill your container with potting soil that is as deep as your tallest container plant. More soil leads to less watering, more space for your plants roots to develop, and healthier plants in general.
While you may feel like dirt is one place where you don’t want your precious gardening budget to go, it actually might be the most important thing for the health of your garden! So for better plant health and more beautiful and vigorous plants, leave the garden soil for your raised garden beds and go for the good stuff and fill your containers with good quality potting soil.
Original source: http://blog.greatgardensupply.com/potting-soil-101-select-soil-container-plants-raised-gardens-beds/#.X0UwFNNKhBx