Why use potting mix instead of natural soil?

Question

Being a layman who is thinking of starting to garden in the yard I have a question in mind. Why does one need to buy potting mix and why not use natural soil for gardening? I live in Melbourne Australia and I cannot understand why mostly smaller plants or seeds are in potting mix but on the other hand I saw trees and many other small to medium plants planted in soil.

Your guidance will help me growing more greenery on earth :)

Thanks for your guidance, help and time.

Answer 1

An artificial soil mix offers advantages to growers and some advantages to the people who buy the plants. For the grower:

• consistent artificial soil quality and mix across multiple batches
• free of pathogens and the crowd of bacteria/virus/fungi that live in soil
• different mixes are designed for seedlings, acid loving plants or plants that benefit from endophytes
• the major cause of death for indoor plants is over watering. By adding vermiculite and perlite drainage is increased and the chance of soggy roots is decreased

For the new plant owner:

• soil less or artificial mixes are lighter than top soil. Easier to carry

In the long run I have seen these issues with artificial soil:

• a much larger percentage of the soil is organic matter. Great for retaining water and air but it tends to dry out and diminish in volume over time.
• as it is lacking in the flora and fauna of a soil mix if something does get established due to over watering or a pre existing virus/fungi/bacteria then there is nothing aside from chemical controls to stop it's spread
• large plants like dracaena massangea or ficus benjamina do not get enough support and will lean in an artificial mix
• when a soil less mix dries out it physically contracts in size from around the rim of the pot. When you try and water this the water runs off the side and it is hard to rewet the root ball without a surfactant
• once the initial charge of fertilizer is used up heavy feeders like flowering plants need supplementary feeding to keep flowering. This is sort of a moot point in most cases as the majority of indoor plants never receive enough light to require additional nitrogen. Micro nutrients are another matter.

After maintaining tropical plants for over ten years the longest lived healthiest dracaena massangea I saw were potted in some kind of clay soil. They weighed a ton and even after fifteen or twenty years in an office were still growing and flowering without any leaf spotting. Not a large enough sample for research grade study but interesting...

Some further references.

Answer 2

Just for interest's sake, I'll add something, though Kevinsky has given a comprehensive answer already. The reason you don't just dig up a bit of soil from your garden, put it in a pot and then plant something in it is simple - any pathogens present are now contained in that pot and multiplying, rather than being diluted in open ground, which means there's a risk your potted plant will be killed. Certainly, I've known people do just this, and the plant's been fine, but there is still a risk.

The other thing to note is that some potting composts are loam based, which makes them look more like 'ordinary' soil and which are very heavy when wet. These are the most suitable for large trees and shrubs in pots, so John Innes No. 3 is the one of choice for large, permanent potted subjects - unless the weight is an issue (in a roof garden, for instance).

As a further point, many people can and do make their own potting composts - they use a hot, aerobic composting system for garden waste, and when that is ready, they add other materials (sand, vermiculite, grit, peat, etc) in various ratios to get the potting medium they want for various plants. A hot, aerobic composting system kills off the majority of pathogens and most weed seeds too, which is why that system needs to be used if you want to make your own potting medium.