I did a lecture out at one of the local high schools today on growing Citrus in our area. For the most part, they were very attentive. There were a couple that really didn't seem that interested, but that's okay, it is high school after all.
The ones that seemed interested asked some very good questions. One in particular had to do with using a soiless medium in containers. Both the teacher and I thought this was an excellent question. Why do we use a soiless medium instead of good old garden soil?
Well, there a numerous reasons. To begin with, I don't care how good your soil is it will not provide adequate drainage, and over time it will become compacted. This will cause it to not have enough oxygen and interfere with the absorption of nutrients. Garden soil also tends to have clay in it which when dry resists water. When this happens the soil will tend to pull away from the sides of the pot, watering then becomes a real challenge as the water will just run down the sides and out the drainage holes.
There is also the hazard of introducing different non beneficial organisms. Potting mix that you buy in the bag has been sterilized to keep from introducing fungi, weeds, insects, nemetodes, etc.
So what exactly is a soiless potting mix?
The potting mixes you buy in a garden center are comprised of three basic ingredients: Peat moss, Pine bark, and either perlite or vermiculite.
Peat moss comes from the peat bogs of the northern United States and Canada. There are some peat bogs in the southern US, but the quality usually isn't as good.
Pine Bark comes from paper mills from all over and acts to provide moisture and fertilizer retention. It also gives a little more air space to the roots.
Perlite & Vermiculite are both volcanic in origin and are put into potting mix to provide additional air space. It is also used to lighten the mix up so it is not as dense and heavy. I have seen people use very small pieces of styrofoam to lighten it up and give more air space also.
One last piece of advice. Have you ever run out of potting soil in the middle of planting and had to improvise or go out and get some? Only to find out you have to get a different kind because they were out of what you were using?! I know I have. Make sure you mix the two different types or kinds of soil very well. You will have a better mix this way. If you just put one kind of potting mix on top of a completely different one and don’t mix them together there is the possibility that if the two mixes are different enough your plants will have a hard time growing down through one mix and into the other. The top mix might be heavier and compact the bottom soil for example. Thus, not allowing root penetration or air and water. It is kind of the same thing with planting in the ground. When you dig the hole, mix together at least part of the soil you took out with whatever you were wanting to add. If you don't, the roots may blow through the nice soft soil, then, when it gets to the garden soil they may start to curl around, like in a container. Basically creating a rootbound plant. Water can also sit in the bottom of the hole, like a tub.
I hope this has answered any questions about WHY you should use a potting mix instead of your garden soil. Your plants will thank you for reading this!