Potting soil vs potting mix

Recently I have been reading about the differences between potting soil and (soiless) potting mix. Historically I have always planted fruit trees in the ground but am now living in an urban setting where potted is my only option. I want to plant some citrus. For those who grow in pots, do you use P. Soil or P mix? Also, what size pot minimum and how long can trees be kept in pots?

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Potting soil and mix are probably the same thing: peat moss, perlite, vermiculite and maybe a small amount of slow release fertilizer. Light ingredients that drain well. They are generally ‘sterile’ so as to not introduce any pests or disease.

Garden soil contains heavier ingredients that retain moisture better: the above + compost, mulch, and sand. Garden soil can have some myco and insects in it so it’s not ideal for indoor use.

So just use whatever potting soil you can get your hands on, you don’t need a specific mix. You will also want to use some kind of slow release fertilizer, I’m a big fan of Osmocote. Doesn’t have to be citrus specific or even fruit tree specific.

For pot size I think that 5 gal is minimum and 10 gal is the sweet spot of root space and portability. High durability black nursery pots are my go to, let me know if you want a link to the ones I use.

EDIT: Regarding how long trees can be kept in pots: indefinitely. Look at bonsai trees - you just need to routinely prune and root prune them. Worst case scenario you just airlayer it and then start over.

EDIT2: Indoor watering is pretty tricky for fruit trees - highly dependant on a lot of different variables: plant size vs pot size, type of mix, how much you water each time, etc. Some plants I can ignore for a week with no problems, other plants wilt if I don’t water them every day. Honestly, I still don’t really get it.

it all depends… what matters is PH, nutrients, drainage, and anchoring. You give them that and they are happy. Learn how to mix soil, it is not that complicated. I get screened top soil, a yard or two. I start by mixing it half and half with well cured horse manure, which adds nutrition and improves texture (the screened topsoil can be a bit on the clay side).

Then depending on how I use it I further cut it. If for a tree or bush on the ground, I mix it in 50/50 with the native sand (my soil under the top layer is nothing but rocks and sand without any organic material). For potting I may cut in some peat moss. Heck if I have vermiculite I would throw some in; it helps with water retention. For a seed/cuttings bed I would do mostly sand and peat moss, you don’t want seedlings worrying about sucking up nutrients that early.

Most trees can be kept on pots forever provided you take care of their needs. Depending on the variety root pruning can be a must. Also picking a very dwarf variety else you’ll be fighting the tree more than you need to.

For citrus get a meyer lemon, I have two here in Alaska (indoor plants of course). Root development is extremely slow so it will forgive you if you forget to prune them.


Potting soil, potting mix, and seed start mix are all different things and each should be used for the purpose it is made. Don’t use potting mix to start tomato seed for example. It will lead to lots of spectacular failures.

Potting Soil - This contains actual soil with some amount of organic matter. It often includes bark and/or wood chips. It is generally used for ornamentals and other plants that are grown in containers.

Potting Mix - Does not contain soil. It is a mix of peat moss, perlite, and sometimes other organic materials such as pine bark. It may have a small amount of fertilizer added to maintain plant growth. Don’t use potting mix to start seed because it may have fungi in the soil that kill seedlings.

Seed start mix - This is a mix of peat, perlite, and vermiculite that is made specifically to start seed. It is steam sterilized to eliminate fungi that can harm seedlings. It may be amended with a small amount of fertilizer and is often blended with mycorrhizae to encourage long term plant health. This is the only material acceptable to start tomato and pepper seed. I use Promix MPX but there are others such as Miracle Grow, Fafards, and Peters.


I start my tomatoes and peppers (and most other seeds) in a potting soil mixture that is not sterilized and includes a bit of topsoil and compost, and never have had any problems. It probably depends on your specific local soil microbes, but I don’t think it’s as clear cut as you’re saying for all locations.

I’ve had problems with damping off papayas, which seemed to be a fungal issue in the mix, but never tomatoes or peppers.

But as far as container trees go, I’d say err on the side of excess drainage, so add plenty of perlite to the mix even though you’ll need to water and fertilize more often. And measure the pH. With good drainage and good pH, the precise mixture doesn’t matter much.

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Dimitri - with reference to containers, I reference an earlier post on containers 14 to 18" square, how would this size correlate to gallon size?

I grew tomatoes and peppers with no problems at all for 10 years. Then came the year that
“damping off” killed half my seedlings. Many people grow for years with no issues. You might be lucky and never have a problem. But when you run a business selling tomato and pepper seedlings, you can’t afford to make a mistake like that. Seed start mix is finely tuned to produce healthy seedlings with predictable growth rate.


I just grab compost from my pile and microwave all pathogens

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My neighbor, an avid and very experienced gardener, lost all his starts because he used an “organic” potting mix he bought at a nursery and came begging me for pepper and tomato plants the other day. That was a special case but there is a lot of difference between commercial potting soils out there, but if you purchase a 3 cubic ft. compressed bale that calls itself pro-mix from Canada it probably is top shelf. They also contain wetting agents and more often than not, quick release fertilizer, although Scotts and perhaps a few other companies that sell it loose do include a slow release. I’d rather not have any so I can customize fertilization to my own needs. Seeds don’t need fertilizer until they start to show mature leaves. Until then they are more or less canabalizing themselves. Some are inoculated with mycorrhizal fungi but I’m not sure how much difference that makes if you are using a fertilizer with available P. Seems like a good idea if you are growing things in pots permanently.

One of the biggest benefits of mycorrhizae is that they tend to prevent growth of competing soil pathogens that can kill plants. They also make nutrients more readily available in the soil. I compared Promix BX vs Promix MPX several years ago to see if I could tell a difference. MPX produced noticeably healthier and faster growing seedlings. Until then, I considered BX one of the best available.

I disagree re needing fertilizer in a seed start mix, but with a caveat that this applies to species with small seed such as tomato and pepper. The endosperm in a small seed is not large enough to push the plant to grow normally beyond the 1st leaf. This was brought home in spades about 20 years ago when a friend tried to grow tomato seedlings in coir (coconut fiber). The seedlings stopped growing, turned pale yellow, and started dying after the first leaf had grown. There was no available nitrogen in the coir. He was advised to add a dilute liquid fertilizer which brought the remaining seedlings out of their stunted growth pattern. Lesson learned is that the seed start mix needs a minimum amount of fertilizer to get seed off to a good start. It does not matter if the fertilizer is in the mix when purchased or added by the grower when the seed are started. If it helps with growing seedlings, Promix BX and MPX both contain a small amount of added fertilizer. It is just enough to push a tomato seedling in a 48 cell tray to grow to the 4 leaf stage. It is not enough to produce a healthy 8 leaf seedling ready to go in the garden. I add 1/4 teaspoon of miracle grow 15-30-15 in a quart of water to fertilize 48 seedlings. This is enough to grow a healthy ready to transplant seedling but not enough to encourage leggy growth.

I use store bought “top soil” mixed with real ‘dirt’ from the ground.

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Why? I can think of no other reason other than inadequate soluble P. Can you? If that wasn’t the difference I doubt any commercial grower would buy what doesn’t contain mycorrhizal fungi. Word gets around.

But you are the commercial grower here, I never have done a comparison, but I use Osmocote mixed in before planting.

For starting plants in pots?

I don’t know for sure re the MPX vs BX difference. I could speculate 3 or 4 things like better nutrient availability in the MPX or maybe it has more added fertilizer. I could see a difference in the MPX being milled finer than the BX. The growth difference was pronounced enough that I have used MPX or BX-M since.

I deliberately grow my seedlings dry meaning that I let the soil dry out to the point the seedlings wilt, then water heavily. This produces a tropism (where a plant grows toward or away from something like growing toward light) where the roots expand much faster than the top. The larger root system is retained with a significant increase in production.

I also deliberately chill my plants once or twice as they grow. This was proven back in the 1930’s to significantly improve production. Chilling has to be VERY specific down to 40 degrees F. The next day I close the greenhouse and let the temperature go up to 90 degrees or above. Chilling causes the stem to stop transporting rubisco (look it up) but 90F the next day reverses this effect. If the plants are chilled without the high temps the next day, they will stop growing for a week or two. Chill them then give them high temps the next day and they won’t take a break from growing. Also worth knowing, tomato seedlings can easily handle 120F so long as they have plenty of water.

Another thing to be aware of with tomato seedlings is that they have a sharp transition from juvenile to adult growth phase. Generally, the adult phase is shown by flower formation. The plants need to grow as large as possible before the transition in order to get the best production. People who buy a 2 foot high tomato plant in a pot loaded with fruit for $$ are shooting themselves in the foot. That plant has already gone through the transition to adult phase and will never produce as much fruit as a plant that grows and expands properly before fruiting. There are a couple of exceptions to this mostly involving precocious fruiting genes such as ft (fruiting temperature).

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Most of the time, yes. Unless I’m doing tiny seeds or something.

I have not found too much of a difference In soils other than cost assuming it has potting in the name in terms of performance. In fact in terms of performance I have found my cheap at home potting mix to be just as good if not better. This year I got miracle grow soil from Costco and got some that were my own mix. The mix of peat moss, osmocote fertilizer and perlite has my trees performing much better than the trees with miracle grow. My citrus tree is in a happy frog potting mix. There has been some growth on my citrus but not as much growth as with my cherry, plum and apple trees. My pears just came so no growth on them and I forget the potting mix I used for my peaches but also little growth on my peaches as well so far.

I would pop open the bag before committing to buying in quantity. On a lot of big box bags, it will say the mix is “regional”, so it’s not a consistent product everyone across the country will see.

In mid-atlantic/ south, my experience with big box mixes recently is that they are all mostly bagged pine bark that’s been finely shredded. Pine bark must be really cheap…


EL1, the differences you are describing are typical of not properly fertilizing the mix. The critical characteristics of a growing mix are:

  1. moisture retention (enough but not too much moisture to encourage growth)
  2. Aeration (yes, roots need oxygen to grow)
  3. nutrition (fertilizer of some sort)
  4. micronutrients (separate from fertilizer because lack of proper micronutrients can KILL some plants)
  5. pathogens (there are many mini chompers happy to kill your seedlings)
  6. pests (fungus gnats among others can cause a ton of problems)

There are a few others, but the above is the critical list.

To give some specifics, I grew a few dozen pecan trees in containers 4 years ago. Unfortunately, I fertilized with plain NPK in Promix. The mix was missing Nickel and Zinc which are critical for pecan. Over half the seedlings died. Lesson learned, for many trees, micronutrients are a critical limiting factor. Want to see an apple curl leaves and croak? Try growing them without a tiny amount of boron.

I grew tomato and pepper seedlings many times over the years (prior to starting a commercial business) using a mix of promix and sandy loam topsoil. I heat sterilized the mix in a large stainless pan over a wood fire. The results were impressive, hands down much better than promix alone. Heat sterilization can work wonders where soil pathogens are a problem.

I contracted to grow 10,000 organic seedlings about 10 years ago. This was a huge problem because nobody sells a good organic seed start mix. I wound up making my own mix using peat moss, perlite, worm castings, and using bird guano as fertilizer. The worm castings were not sterile so I fought an ongoing battle to keep the pathogens from killing the newly emerged seedlings. I eventually figured out a way to heat sterilize the mix and obtained top notch results from that point on. If I could make the organic mix in bulk, I would prefer to use it for all my seedlings.

When it comes to growing plants, harsh reality will cause you to solve problems or else go out of business.

I would not be getting bags if I needed quantity, which is what I use.

For $30 I just picked a cubic yard of screened soil, that’s the volume of 27 5-gallon buckets. It gets mixed 50/50 with well-cured horse manure (free for the picking). That’s a total of 2 cubic yards, 52 5-gallon buckets, of potting soil for $30 bucks plus some effort.

You use half soil and half of anything that calls itself potting mix and get satisfactory results? I would get drowned plants. I have seen plants started in clumps of soil without pots, which I thought was the trend of the future, but it hasn’t caught on with big suppliers. Pots require special aeration of their soil to achieve the right balance between air and water because they create a perched water table that defies capillary and even gravitational flow.

So I lose a plant from time to time to root rot. I accept that if it means I don’t have to carry water to plants in dry spells. (But, no, I don’t have a 50/50 rule, I often make do, continuing to pot if I run out of one of the ingredients I’d prefer in my pots.)

(Also, recycling dirt from pots of plants that died, recently, or in some cases 15 years ago, also gives the soil and fungi aspect that isn’t going to be in a bag of stuff from a box store or most garden centers.) (I mix some of that together/into a bag from the store if I’m doing small quantity little projects.)

The kind of actual topsoil is a consideration. Sandy soil from mountain/hill tops is going to drain rather nicely…soil that has much clay and higher limestone content can’t be used 50/50 for some potting purposes…but mixed to existing soil and adding compost certainly makes nice raised beds for gardening in.

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