Basically the title. I am new to gardening indoors and wondering why is it best to use potting soil for potted plants? I mean for growing seedlings and also plants that spent whole lifetime in pots.
There are some obvious reasons, such as that the potting soil doesn't get "compressed" over time and that it is probably sterilized.
But one could use cocos fibres mixed with normal soil and well... hoped that the the normal soil isn't infected with some fungi or pests.
Potting soil also gets compressed over time. The most important thing about potting soil is that it shouln't contain fungus spores, insect eggs and weeds. I say "it shouldn't" because I have bought potting soil with earthworms and some germinated seeds, probably weeds, but not sure since I have removed them immediatly.
Also, potting soil has a label that lists its properties: pH, NPK content, microelements, so you know for sure what can be grown in it. One of the reasons I use potting soil is the easiness of procurement. I don't have to take soil from the edge of the forest, add decomposed leaves and sand, because I buy it from the store.
I know people who use garden soil in their pots and their plants also look healthy (Aloe, Pelargonium and Saintpaulia), so in the end it's up to you to decide if it's enough to hope that garden soil is free of pests, or you want to take the prevention route.
"Normal soil" is so variable that it's a fairly meaningless term unless you qualify it.
I used to live in a part of the UK where the topsoil was about 30% sand and 70% fine silt, with the water table two feet below the surface all the year round. If you cut off literally any part of a plant and just stuck it in the ground, it would grow. The local joke was that even bamboo canes used to stake plants sometimes sprouted leaves! In that situation, buying potting compost was a complete waste of money.
On the other hand, where I live now most of the soil is heavy red clay, and if left alone it quickly becomes the same color and texture as house-bricks - completely useless for potted plants!
Ideally, you also need different types of compost for different uses. For example, compost for growing seedlings needs to contain nutrients that are easily for the young plants to take up through their roots, but in fairly small amounts so the plants are not "burned" by the overdose - it doesn't matter that the nutrients are all used up by the time the plants are moved on to other pots or into the garden. On the other hand, for growing mature plants that will only be repotted every few years, you need a bigger supply of slow-release fertilizer. Of course there are also more specialized compost requirements for lime-hating or acid-loving plants, etc.
What you want is a product with a controlled and known specification, that suits the plants you want to grow. If you are new to gardening, the simplest way to do that is buy a reputable brand from a garden store. As you get more experience, you might want to try alternative methods.
The real question is why the use of garden soil is not recommended in pots. Garden soil has a host of life forms within it, including bacteria, pathogens, fungi and various microfauna. All of this is more or less kept in balance in open ground, but if you dig some up and put it in a pot, its possible it will contain pathogens - those pathogens, whilst harmless in open ground, will not be so contained in a pot. There may also be issues with poor drainage with many garden soils. However, many people do just use garden soil, and the plants growing in it are okay, but there's a risk it won't be. That is one reason why sterilised potting soils are used.
Most potting soils are created from composted materials - it's processed aerobically, so the contents are naturally sterilised by the heat generated in this process. Even so, there is plenty of humus rich material within it, which is great for plants. Other things can be added to potting mixes, such as sterilised loam, perlite, peat, fertilizer and so on, depending on the intended use of the particular potting soil. Seed and cutting potting soil has much less or no fertilizer within it, so that's one formulation; there are other specialist formulations for bonsai, orchids, ericaceous mix for acid loving plants, houseplants, and so on. There are also the general use potting soils (called potting compost here) - in the UK, these are known as multi purpose, but we also have John Innes, which comes in three grades or 'strengths'. Formulation of JI varies between 1, 2 and 3 in terms of ratio of loam to peat to other ingredients, including more or less fertilizer. The king of potting soils, John Innes, but not available in the USA so far as I'm aware.
If you are starting growing in pots or containers from scratch, then its better to invest in the best potting soil you can find that's appropriate for what you'll be growing, rather than attempting to sterilise ordinary soil and then working out what you need to put back in to make it a useful growing medium.
This question and answers here might be helpful Why use potting mix instead of natural soil?
Regular run of the mill Potting soil might not be good in long term for the plants!!
I was hoping that someone will chip in with the link to container soils. Since nobody has done this, here is the link. Have a good read.
In a nut shell what it says is that it is impossible to recreate in a pot what mother nature does in soil. Even with potting soils.
