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The soil for container gardening must be different from that found in "traditional" gardening. In a container, the plants depend entirely on YOU to provide the nutrients and growing conditions necessary for them to thrive. They can't sink their roots deeper or spread them wider to reach the necessary food or water and so you must meet those requirements. You can start off by purchasing a commercial mix or you can make your own soil for container gardening.
Let's consider what makes up soil in the first place.
About 50% of a fertile soil, whether it's earth soil or the soil for a container garden, isn't soil as we know it at all. It's actually pores, or the spaces between soil particles. In other words, soil is "holey". Keeping this in mind is critical to the success of your garden, regardless of where you're planting it. Air and water must be able to enter the soil and flow freely through it and a good potting soil allows exactly that.
| Remember that a plant is a living organism and its roots are the arteries through which nutrients travel. They are also living tissue and life requires two things to survive: air and water. If the soil for your container garden is so dense that water and air can't penetrate it, your plant isn't going to thrive. The plant will put out fewer roots and will thus take up less water and nutrients, resulting in a smaller plant and less product.
We tend to think of soil as if it's simply particles of matter, but in reality, it's made up of particles of matter plus all the spaces of air between those particles.
A typical potting soil for container gardening is a combination of two basic components: something that will absorb water and something that the water will flow around. Many different ingredients will meet this criteria but the most common blend is a mix of sphagnum peat moss and perlite.
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Sphagnum peat moss is the partially decomposed remains of ancient sphagnum moss. Much of it is harvested from centuries-old bogs in Canada and it holds both air and water. It's an organic product but it decays very slowly which makes it an excellent ingredient in potting soil for container gardening. Because of its slow rate of decay, it contributes very little to the nutrient level of the soil mix. Keep in mind that it is very acidic, with a pH of 3.5 to 4.5.
Another option is to use coco peat, or coir. This is simply recycled coconut husks and is a more ecologically sound alternative to sphagnum peat, which is a very slowly renewing resource. It holds more water than sphagnum peat and has a pH range of 5.7 to 6.8. Coir comes in compressed bricks that must be soaked in warm water before use.
Perlite is made from volcanic rock that has been crushed, screened, and then heated to 1,800 degrees F (1,093 degrees C), causing the particles to expand. The result is a teeny tiny volcanic rock version of popcorn. Perlite holds water on its surface in all the little nooks and crannies and it helps to keep the soil light and fluffy. It adds no nutrients whatsoever and has a neutral pH.
Limestone (either calcitic or dolomitic) is usually added to the soil for a container garden to counteract the acidity of sphagnum peat moss. It also contains some magnesium, another mineral much appreciated by plants.
These are just a few of the ingredients used to make potting soil for a container garden. Many other options or combinations are available but keep in mind that you want a potting soil that is nice and "holey". You want to be very sure that your plants get enough, but not too much, water and air to encourage good root development. Look for these ingredients on the bag of potting soil you are considering and you won't go wrong on choosing the best potting soil for your container garden.
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