soil ... aka topsoil ... is mother earth ... she is not in a pot ...

in a pot.. you use a media ... or medium ... never call it soil ... and you will be well on your way to understanding the difference ... [and dont get me starting on the word: potting soil ... HEY!!!! ... it not soil ....]...

anyway ... the most important thing in a pot.. is the media.. not the plant ... it is a water management system ... defined by the plant you put in it ...

many annuals and perennials ... and houseplants.. are water hogs .... so you use a material that is almost sponge like in its ability to hold water ...

BUT ... when you are putting trees in pots ... then you need a media that take the water provided.. and allows a vast majority of it.. to flow right out the bottom ... because that is how trees prefer water ...in mother earth.. as well as in a pot ...

if you provide the water holding media for a tree.. odds are.. you will rot the roots off it ...

reading up on al's method.. with the explanation above ... will get you all the info you need ...

but the only real downside to al's formulas.. is that you might end up with a cubic yard of it ... lol .. and most of us dont need that much ... so plan ahead ...

if you want to cheat.. you might be able to buy a common cactus mix.. and cut it with a peat product ... so you get teh fast sharp drainage.. with the ability to hold just a bit of water ...

conifers are trees ... but you might find some interesting posts in the conifer forum ... especially the two current ones about seedlings ... at least one.. the one about pine bark siftings ...

i just saved you 30 years of screwing around with pots and media... i hope you appreciate it.. lol ...

ken

ps: you confused me with the title versus the facts ... you may have to make adjustments for how old/large the seedlings are ...

pps: to answer your question directly ... soil ... will not allow water to run freely thru the pot.. eventually caking and hardening.. until water pools at the top.. and then runs down the side ...

ppps: third word ... DIRT ... dirt is what is all over you.. after you havbe played int eh soil.. or the media ... it is commonly removing with a washing machine ... words mean things .. go figure..

If you are growing them in pots for a relatively short time (a year or two), composted bark fines are usually the easiest solution. If you go to a few nurseries and look at what they use, you'll find this is the most common potting media for trees and shrubs. Nature's Helper is a brand that is fairly widely available as a bagged product in bigbox stores.

I live in a place where commercial American products like potting medium are not known or available. Everybody here grows things in pots filled with topsoil. It works fine. I've got about 30 pots indoors this winter, including two lemon trees, a rosemary, 5 geraniums, and lots of familiar succulent houseplants.

Most of the containerized plant issues folks come to these fora seeking resolution for are directly related to poor soils; in particular, soils that compact easily and/or hold excessive amounts of water for an extended period. I, and an extremely high % of experienced growers avoid using topsoil in their container soils, this, for the simple reason that it promotes compaction and excessive water retention, so I'll go out on a limb and say topsoil should be avoided if the grower is intent on maximizing the opportunity for his/her plants to realize as much of the genetic potential imparted by Mother Nature as possible. It's far better to use soils that are formulated to minimize the amount of excess water (a perched water table or column) the soil can hold. This is achieved by starting with a very large fraction of coarse material (it doesn't matter if it's organic or inorganic) and amending it with a smaller fraction of finer material to adjust water retention so it's appropriate for the plant/planting.

BTW - I've been talking about soils for use in various forms of container culture for more than 30 years. A fair measure of these conversations were with folks who have degrees in soil science or a closely related subject, and they all interchange the words soil, media/medium, and substrate. I generally try to use media and medium, but most of the things I write have a generous sprinkling of the word "soil", sometimes purposefully, sometimes not. I've found that as long as your meaning is clear, it doesn't matter what word you use for what goes in a pot. I generally make the differentiation by calling a soil of the earth a mineral soil, and container soil a medium, substrate, or container soil - so it's clear what's being discussed.

FWIW - using a container medium that requires very frequent watering offers greater opportunity for plants to realize more of their genetic potential, by virtue of the fact that they significantly reduce the limitations associated with the lack of oxygen that occurs when container media support a perched water table/column. In a perfect world, the entire soil mass would be evenly moist or damp, with all water existing inside of porous particles, on the surface of all soil particles, and at the interface where soil particles contact each other, leaving all the air spaces between particles full of air, from the top of the soil column to the bottom of the pot. That's not achievable by commonly used growing techniques if the medium is composed of a significant fraction of fine material, like peat, coir, compost, fine sand, topsoil, .......

Al

@tapla, I'm curious about your experience / opinion on using expanded clay aggregate (LECA) as a growing medium, either as main part of the mix or just as an additive. It'd seem mostly to fit what you describe as above (absorbs some water and releases while being chunky enough to let oxygen in).

Also happens to be the easiest / cheapest additive I have available, so I'm not disinterested.

To what end would you be using it - your goal?

Al

As part of a growing medium firfor potted plants.

Lol - I got that part. I'm wondering what advantage you're thinking it offers? What does it do to make the soil you intend to build better than, say, grower grit, or #2 cherrystone, or calcined clay or calcined DE - any of which would be considerably less expensive..

Al

I don't have any of those, and the clay aggregate I'm getting is quite cheap (less than a dollar and a half for 20L bags). Even if others were cheap, this is far more convenient.

I generally tend to look at things from the plant's perspective without much consideration given to what's cheap/free or convenient. If those are the primary considerations for you, the best way I can help is to say that combining soil ingredients that are widely divergent in size doesn't usually work well. As a part of a container medium, it won't do much to increase aeration unless there is not enough fine material to fill the spaces between the balls; but if that's the case, you'll end up with a layer of balls at the top of the soil that dries very quickly. Actually, mixing the balls with almost any commercially prepared mix or peat/coir/compost will reduce o/a air porosity and air porosity at container capacity. Container capacity is a measure of how much water the medium holds after the soil has been fully saturated, at the point in time when the medium has stopped draining as a response to gravity alone.

The best way to use it, other than in some form of aquaculture would be as a type of ballast in the lower part of the pot only. By using it as ballast, you can effectively reduce the volume of water that can be held in any perched water column the soil holds. There is a significant difference between using it as ballast (which can be very effective if done correctly) and using it as a "drainage layer" which would be counterproductive because it doesn't improve drainage. Used as a drainage layer it would move any perched water the medium supports from immediately above the pot bottom to immediately above the drainage layer - not something you want.

Personally, I'd only use it in some form of aquaculture. When creating a conventional medium for growing in containers, there are too many choices that will do whatever the Leca does, but better.

Al

What do you mean by “topsoil”? Something you dug off the top of your yard, or the bags of “topsoil “ sold at the store. Because they are very different things.

The bags of topsoil are manure and sawdust or fine wood chips usually. If you mix in enough diatomaceous earth to ensure drainage, I expect it would work, although I question whether a pine tree would like that much N.

To be sure, you would probably have to add some sulfur (preferably well in advance of planting) to get the ph right. Possibly some micro nutrients too. A soil test would be well advised.

Unless you are doing several trees or you are curious about your soil chemistry, it’s probably not worth it.

I categorically would not add real topsoil to a container in any significant amount.

Most "store-bought topsoil" I've seen has been black sand with a small amount of OM mixed into it, and has served poorly in any application where it's been pressed into service as a growth medium or a part thereof.

In order to get topsoil to work in containers, (for the sake of accuracy) you'd be better to say that you'd need to add a small fraction of the topsoil to a large fraction of the calcined DE you mentioned to come up with an overly water-retentive medium that's very unlikely to work for conifers or any other plants using the methods commonly associated with conventional container culture.

Please allow me to explain: If you have a quart jar half full of topsoil (a pint) that supports 4" of perched water, and you mix in a pint of calcined DE screened to 2.5mm (roughly 1/10") to 5mm, what do you have in terms of an improvement in aeration or drainage? Since the volume of topsoil is more than enough to fill every air space between the DE particles, you still have a medium that supports just south of 4" of perched water and virtually no increase in aeration or drainage. Change the mix so you have a 60/40 mix of DE and topsoil, and what do we have? About the same thing, but with a slight decrease in the ht of the PWT - aeration/drainage remains about constant.

It's only when you've increased the volume of DE to the threshold point where there is no longer enough topsoil to fill the air spaces between the DE particles that aeration/drainage starts to increase with any significance. If you have 80% DE and 20% topsoil, it's hard to suggest you're amending the topsoil with DE, the equivalent of saying you add coffee to your cream. Even if you had 100% screened DE 2.5mm-5mm, you'd still have a medium too water-retentive to keep an overwhelming % of plants happy.

If you need to lower pH for container media, one would use an acid-forming fertilizer, or acidify the irrigation water. Adding S to acidify and 'some micronutrients' sounds to me like a recipe for problems. A fertilizer like Foliage-Pro 9-3-6 is an acid-forming fertilizer that has ALL the nutrients plants normally get by the root pathway, in a ratio that closely matches the ratio at which the average plant actually absorbs nutrients, and in an appropriate ratio to all other nutrients in the solution. IOW, the average plant uses about 6X as much N as P, and about 3/5 as much K as N, and that's reflected in the NPK %s of FP 9-3-6.

Al

Well, I guess I agree with you (and didn’t make my point very well) that a plant like a conifer that requires a somewhat special mix isn‘t a very good candidate for improvisation using bagged “topsoil“.

I can’t address your observation that bagged ”topsoil“ consists mostly of black sand. We live a few thousand miles apart and so far as I know, there’s no fixed, let alone legal, definition of what can be called topsoil, although I suppose a pure bag of manure wouldn’t count.

The “topsoil” we buy out west behaves about the way you would expect any manure-rich growing medium to behave. Lush, if somewhat over- nitrogenated growth. That’s what happened in my strawberry raised beds, which is not all that different from an actual container.

I can’t speak to a mixture of bagged “topsoil” and diatomaceous earth in a pot, but I have a mixture of about 2:1 finely screened compost to peat and DE added at about the same concentration as perlite that would be found in a commercial potting mix, which should behave similarly. I water nearly every day. The mixture drains very well. It’s not soggy. If I really went overboard with the watering it’s clear to me that the flooding would be the immediate issue.

I have a 7 foot tall ’Brown Turkey‘ fig tree growing in my living room very well in this way. Aside from the soil structure which is manifestly OK, I had the benefit of a soil test for a bunch of similar pots, so I was able to put the soil chemistry right too.

As I pointed out in my original post, as separate expenses these costs would very quickly swallow any cost savings for improvising a mix using bagged “topsoil”

I'm too lazy to get complicated with container media so I just use cactus mix. It is readily available, pumice is the largest component, and it drains superbly.