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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.

  • This is one of my favorite topics. But Bamboo's said it better than I could. Taking a scoop of out the garden soil ecosystem is like taking a scoop out of Mars and judging the entire plant by that one scoop. Garden soil has not only pathogens but beneficials that keep the pathogens in check. Checks and balances. You wouldn't know what the balance in that scoop of soil. Sterilization is important. Some potting soils also ADD fungal spores and bacteria to give some life to the soil in the pot to help the plant uptake chemicals it needs. Indigenous soils work only within the large garden. – stormy Apr 20 '17 at 17:56
  • Just to point out, this question was asked on 4/20 and hit the HNQ list pretty quickly. – WBT Apr 21 '17 at 0:21
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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.

  • Alina, your comment is very good, I have a problem with 'taking the soil from the edge' of a forest thinking it is the best soil. It is full of organic matter but that doesn't mean it has the chemistry necessary for plants. Macro and micro ecosystems thrive because all the important NPK and micro nutrients are in the biomass, the live part of the ecosystem. Nutrients are released little by little when part of that ecosystem dies. Then it is decomposed. What little chemistry that is allowed back into the system is either taken up by the plants already in that system so they can continue... – stormy Apr 20 '17 at 18:04
  • ...doing photosynthesis, making their own food. Very little is left in the soil to allow newbie plants into that system. Organic matter feeds the macro and micro organisms, improves the TILTH of the soil so that these lifeforms in the soil are able to get air and move freely, roots are able to grow a bit deeper to find available water surrounded by pockets of air. Plants are tough tough tough. Just because it's steward didn't use the best soil for pots doesn't mean plants won't grow in soil from the garden. The organic matter quickly is eaten by the soil organisms and after all the... – stormy Apr 20 '17 at 18:09
  • ...decomposed organic matter (DECOMPOSED) is gone, the soil's tilth is turned into brick. The organisms die of starvation. The soil gets even tougher to live in as a plant. Potting soils are sterilized CORRECTLY the heat from decomposition is not enough to kill weed seeds and pathogens. Potting soils actually have little real 'soil'. So it is tough to get compressed by life eating the organic matter, and lots of that tilth is composed of inorganic or inedible materials like vermiculite that won't be eaten or removed. – stormy Apr 20 '17 at 18:16
  • Sooo...is this in the U.S.? Easier to buy potting soil than make it. I've found that large areas of other countries are unable to purchase soil. Unbelievable. My point was just thinking soil by the forest or in the forest is nutrient rich. It is not. Tilth might be yummy but not for growing newbie plants. That was my only point. Chemistry is different in forests and long term ecosystems. The soil does NOT have the chemistry to promote NEW plants. Which makes ecosystems sensible. No chance of overpopulation. If I ever had to buy a garden I wouldn't be here now. – stormy Apr 20 '17 at 22:16
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"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.

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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?

  • Thank Bamboo. You talk about balance in open ground and pathogens in pots. Is there any rule of thumb, how big should the pot be, to keep the bad guys in balance, such as in open ground? I have a couple of pots most are big enough to fit 15-20kg of dirt, but the big ones go way over 200kg. I will deep plant tomatoes and peppers. – sanjihan Apr 21 '17 at 6:44
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    @sanjihan how long is a piece of string? Its the being contained that's the issue, no way to know what's in any particular large or small area of garden soil in regard to pathogens, nutrients or life forms without a laboratory, and a chemist/soil scientist. You risk it, or you don't, it's as simple as that. – Bamboo Apr 21 '17 at 8:52
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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.

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    Whilst the thread contains some useful information, the statement 'its impossible to recreate in a pot....' isn't at question. The pertinent factor is whether its okay to use garden soil in containers, and if not, why not, rather than the long or short term effects of different potting mixes in containers. – Bamboo Apr 21 '17 at 9:01

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