The best thing to do with that potting soil is to compost it, or you could use it as a source of organic matter for your other outside soil. Few people recommend reusing potting soil from year to year because of the potential of plant diseases being harbored in that material.

I will reuse it when I can, as I have a huge container garden with also huge containers, so it's an issue: just turning them over is an issue, actually. If the plants haven't HAD diseases, no problem.

However, if container dirt lifts up solid with roots, I just throw the whole thing in the compost bin. For this reason, my compost tends to have lots of perlite and such in it......which is no bad thing for the garden, however, of course.

It is true that if you just leave the container out all winter the roots will rot and be gone, leaving just the less-rottable planting medium; I know because I've done that and was surprised all the roots were gone.

I do like to refresh the dirt in the containers yearly, however. This year I'm putting chicken manure deep down and then homemade cool compost and let the pots sit in the snow all winter, and then I'll top with a deep layer of sterile planting medium on top and hope the compost weeds won't get through. I may be sorry about that chicken manure.....but I wanted to try it.

How can soil be used up? That's what people think that want to keep using soil from year to year.
But soil does wear out; its nutritive value is taken by whatever plant was in it. Fresh potting soil only has so much to give, then its up to the gardener to add nutritive value to it with fertilizer.
At end of season, its done its job, now give it over to the compost and start fresh so that your new plants get what they deserve.

I never reuse potting soil but add it to my garden. Right now I have quite a few 'lumps' of potting soil with old plants and roots to dig in this fall or next spring - if it snows soon I can ignore them for about 5 months!

IMO it's too much work to amend depleted potting soil and easier to buy new bales. Also there could be plant diseases that were not evident this year but may affect new plants next year - not worth the risk.

I think it's probably wiser to look at the life of container media in terms of it's structure, rather than its nutrient content. For plants that grow as near as possible to their genetic potential, it's up to the grower to provide nutrition and the soil to provide a favorable balance of air and moisture.

Finished compost, coir, peat, pine bark ..... break down very slowly and cannot be relied upon to provide enough nutrition w/o supplementation. Nutrients are locked tightly in the hydrocarbon chains that make up these materials, and are only available after they are cleaved by soil life and broken down into elemental form the plants can absorb. Containers are rather hostile environments for soil life, so soil life populations are erratic, making the delivery of nutrients via soil decomposition erratic and unreliable.

Old soils that started out as peat/compost/coir/other small particles, will ALWAYS be very water retentive because of their small size. You cannot amend these broken down soils to improve aeration or drainage by adding materials like perlite and pine bark, because the fine material simply surrounds the bark or perlite. To illustrate: a cup of perlite or pine bark in a quart of pudding represents 1/5 of the mix, yet it has no effect on drainage or aeration. It's only after the perlite or pine bark becomes more than 60-70% of the o/a volume that it impacts aeration or drainage. From this, we can see that STARTING with larger particles and adding enough fine ingredients (peat/coir/compost) to balance the air:moisture ratio is a better choice.

If I was going to reuse old soil, I would include it as part of that 'balancing act'. I would start with fresh or partially composted pine bark, and add enough peat/compost/old soil, and enough perlite to give me the air/moisture balance I wanted.

I never count on my soils for any of the nutritional requirements of my plantings. Instead, and as noted, I take full responsibility for their nutritional needs, and build soils that shoulder the responsibility of making roots happy. Roots are the heart of the plant - the leaves just THINK they are. Make the roots happy, and the plant will be happy ...... and it's very likely you'll be happy with the arrangement.

Al

I always use fresh potting mix. I recycle the used into the compost or a garden bed.

Karen

I do dozens of large containers, and purchasing fresh potting soil every year would cost well north of $200 for the cheap stuff, double that for something good. So I reuse it year after year, dumping it out on a concrete slab, breaking up the root balls, adding fresh bags of soil, sifted compost, and a few shovels of very rich soil from the bottom of my pond for trace minerals.

Works fine, although this past summer, I had some problems with leaf hopper eggs over-wintering and getting started up in the greenhouse. This year, I'll be using clear plastic trash bags to solarize it.

Potting soils are not really soil but are made up of organic matter, peat moss, coir, finely ground bark, with some perlite or vermiculite added as well as, maybe, some fertilizer. When I used commercial potting mixes I found the plant roots so filled the container that there was little of the potting mix left to reuse.

David saying, "I do dozens of large containers, and purchasing fresh potting soil every year would cost well north of $200 for the cheap stuff, double that for something good. So I reuse it year after year, dumping it out on a concrete slab, breaking up the root balls, adding fresh bags of soil, sifted compost, and a few shovels of very rich soil from the bottom of my pond for trace minerals. "

Yes, that's just the sort of thing I do. I will throw away a ball of soil stiff with roots if it's in a small enough pot to lift and handle. The big pots don't get solid with roots, in my experience, at least not deep down. Anyway, it works well for me and I don't have disease or pest problems. I do use a lot of fertilizer -- that delayed release stuff and then liquid fertilizer through the summer. (At some level, I don't really believe the slow-release fertilizer actually works.)

While acknowledging Al, like David, I plant a lot of large containers and cannot afford to replace my potting soil every year. I keep a large outdoor bin for soil. In the fall I dump my pots into it, and after every 8 inches or so, I add compost or seaweed until the pallet-built, cardboard-lined, soil bin is full. I fertilize many of my containers with fish emulsion fertilizers, which keep them going. I do not fertilize my dock plantings. I dump them out and replant them mid-season when they look straggly.

I used to sterilize potting soil in my oven annually for seed-starting, but the last few years, I purchased Pro-mix for seed starting. I was starting to have damping off problems, and wondered about the old soil.

Annpat --- yes! Baking dirt in the oven, as soon as I read your post I remembered that so-familiar smell, from 30 years ago.

It was pretty awful. [:-)

Pro-Mix is better. I don't bake soil anymore either.

That's an interesting idea you have, to segregate the pot medium from the other compost. At this point I put it all in together, and so my garden beds have a lot of Pro-Mix and perlite and such in them.....thanks for your idea, that might be something I want to change to. One way to reuse the potting mix without the heavy weed-seed burden of my usual cool compost.

The one thing I've noticed over the years is that perlite stays intact pretty much forever, which gives that essential drainage / aeration.

Good thread full of good posts, THANKS everyone.

From reading this thread it seems that re-using potting mix that still has good structure is a viable idea. Just a few questions:

1) Al, David, et al - how many years do you re-use your potting mix for?

2) Is it necessary to flush the soil with water occasionally to reduce the build up of salts?

3) Al, how big are the chunks in your composted pine bark and does the size of bark chunks vary in proportion to the size of plant (i.e. smaller chunks for cuttings vs. a larger chunks for a large container with a tree)?

Thanks in advance for any input.

Terrene - I use 2 different basic mixes. One is comprised of partially composted pine bark, sphagnum peat, and perlite in roughly a 5:1:1 ratio. I include dolomitic (garden) lime in the mix as well. I use this soil for containerized veggies and the mixed annual plantings I display in the gardens & on decks. I sometimes use it for plantings I'll have in the same container/soil for two years. I limit the life of the soil to two years, but that isn't to say that the bark-based mixes aren't many times more durable than peat/compost/coir-based soils. They are. I just prefer to use soils that have not broken down to the point where there can be a question about their ability to ensure the kind of aeration required to ensure the plants have the opportunity to grow at as close to their genetic potential as possible.

The other soil I use is made with equal parts by volume of
Pine or fir bark - uncomposted and screened to 1/8-1/4"
Screened Turface (baked clay granules - almost ceramic-like)
crushed granite (Gran-I-Grit in 'grower' size or #2 cherrystone
I also include a small amount of gypsum in this soil as a Ca source.

The nutritional supplementation program for these soils, using soluble fertilizers in (usually) a 3:1:2 ratio (24-8-16, 12-4-8, 9-3-6 are all common 3:1:2 ratios) makes fertilizing and watering very easy, with a wide margin for grower error. The downside is you need to locate the ingredients and make the soil, and you have to water a little more often because of the added drainage/aeration these soils afford. They are, though, stellar for root health, which, because roots are the heart of the plant, is very good for o/a vitality.

It IS necessary to have in place some sort of mechanism to prevent solubles from accumulating in your soils. Heavy soils comprised of peat/compost/coir as their primary fractions often don't allow you to water thoroughly enough to flush salts from the soil w/o the soil remaining wet for extended periods, which can promote root rot, and if not root rot, the cyclic death and regeneration of some fraction of the fine roots due to temporary anaerobic (airless) conditions in the soil. The practice of watering in sips to avoid root rot ensures the build-up of solubles from fertilizers and irrigation water. I have a thread devoted to dealing with water-retentive soils somewhere. If you're interested, I'll look it up and provide a link.

The ideal size of bark for the 5:1:1 mix I described is about dust to 3/8" in size, though some larger pieces wouldn't be the end of the world if the primary fraction was in the size range I noted. I already mentioned that the ideal size for the other (gritty) mix is 1/8-1/4, but again, if a small fraction was a little larger than that, it wouldn't matter much. The gritty mix is designed around particle size, with the ideal size of the inorganic fraction being around 1/10-1/8" and the bark a little larger to allow for some decrease in particle size as the soil ages. The gritty mix should retain it's structure for much longer that it would be prudent to push the interval between repots. This ensures that the plant being severely root-bound would be the (growth/vitality) limiting factor before soil (structural) collapse.

I use the same size particles in each soil, each time I make it, but I do vary the ingredients in each soil to adjust the water retention to whatever I want/need for a particular plant. IOW, it's sort of a 1-size-fits-all, but the composition might vary slightly.

Al

Al, thanks for all the info! I appreciate your response, it has been very enlightening to learn about container growing over the last couple years, and mostly this is because of your posts. Mostly I grow in the ground, but what few containers I tried outside always failed. The fact that I often forget to water them didn't help, LOL. The houseplants and seedlings seem to do okay though, although I doubt they are growing to their potential.

I happen to have some orchid bark laying around, it sounds like the chunks might be too big though. Where do you usually obtain your bark chips?

I have several suppliers within a few miles that supply bark either in bulk or in 2 cu ft bags for the 5:1:1 mix. That can also be screened & used in the gritty mix. What I prefer for the gritty mix, though, is pre-screened fir bark in 1/8-1/4" size. That, I usually buy when I get to Chicago, but if you check, you might find it at orchid supply houses. It comes in 3 cu ft bags. When I but 20 bags, it's $16/bag. Less than 20, it's $18.

Fir bark at the top - pine bark from 3 different sources at 3, 6, and 9. Dry 5:1:1 mix in the middle:
{{gwi:2389}}

the gritty mix - used for all my houseplants and all long-term plantings:
{{gwi:1295}}

Al

Terrene, re how long I re-use potting soil. 10 years and still going. As per salt build up, I'll see it on the sides of the pots, not so much in the soil. Wash it off the pots.

I live in a very dry climate, humidity routinely in the 10 - 20% range, so when the container plants get large - 3 feet high tomatoes, eggplant, cucumbers, okra, etc, they need heavy watering frequently, as in the pots running water out the drainage hole. This seems to be enough to keep the salt buildup to a minimum.

Re fertilization, I have a couple of 5 gallon cheap buckets where I'll stick a shovel full of compost and let it sit 24 hours, with an occasional stir. Pour that into the plants, and wow, do they love it.

Whenever you use a soil that supports a perched water table, which is a layer of soil at the bottom of the container, you sacrifice some degree of growth and vitality. The reason is, a fraction of roots suddenly subjected to anaerobic conditions, as they are when you water soils with a large fraction of very small particles as in soils >10 yrs old, begin to die quickly - within hours, depending on temperature. In fairness, sometimes there is a compromise required if the grower can't or won't commit to watering plantings as frequently as a more open soil requires. In this equation, lies the difference between judging the quality of a soil from different perspectives. The plant will assuredly wish for the more open soil with better drainage/aeration, while the grower may wish to sacrifice some degree of growth and vitality for the convenience of not having to water so frequently .... but you can't have it both ways.

When someone says they are using a soil that is 10 years old, it's a certainty they are sacrificing some degree of potential growth and vitality on the altar of convenience or frugality. Soggy soils kill roots and it takes energy that could be channeled to fruits/flowers/increase in biomass to replace those roots. Some growers might be perfectly happy with the results they get, but that doesn't change the fact that the plant would have more to offer if it didn't have to deal with the cyclic death and regeneration of roots lost to soggy soils every time we water to container capacity.

Personally, I prefer the reliability and immediate availability of soluble fertilizers that provide nutrients in a ratio very close to what plants use and in a ratio favorable to each other. I can use something like Foliage-Pro 9-3-6 and be certain I am keeping TDS/EC at the lowest rates possible w/o nutritional deficiencies. IF you depend only on compost tea as a nutritional source, there is NO way to determine whether plants are getting sufficient NPK (compost is very low in macronutrients) which they most likely wouldn't be, or if there were other deficiency issues from secondary macros or micronutrients that might be in short supply or missing. In the end, it's almost a certainty that you'll need to supplement nutrition anyway, at least if you aren't willing to leave a lot of potential growth lying on the table. Since you don't know what you started with (in the tea) you have to guess at what fertilizer might be appropriate ..... with the thought in mind that an excess of any single nutrient can be as bad as a deficiency.

Just my reasoning. As always, YMMV.

Al

Pardon my error, please. I said a perched water table was a layer of 'soil' at the bottom of the container. It's actually WATER that occupies heavy soils (soils comprised of fine material) and forms a soggy layer of saturated soil at the bottom of containers and won't drain until it is eventually used by the plant or evaporates.

Al

This is very educational, and thank you for the pics Al. I googled around a bit, and couldn't really find anyone selling a reasonable quantity of bark fines - Amazon is selling a pallet of about 60 bags for several hundred dollars, but I can't buy a single bag. How odd. I wonder why these are not a more common component of potting mixes and why they're not readily available? I will have to check out the local nurseries to see what they sell.

David do you use anything beside compost tea to fertilize? That doesn't seem like enough nutrients for a container, at least for veggies or flowers. Many houseplants seem to do okay, though, with a lot less than optimal conditions and I'm sure some of mine have some of that muck on the bottom.

You can find a LOT more information about container soils and container culture in general over on the Container Gardening Forum, and on this thread in particular.

You'll also find threads that talk about finding various components, like different types of bark. If you pursue your interest, you'll discover that bark is found where you find it. I have several fairly reliable sources of appropriate material near me, but I also often stumble on suppliers (sometimes even big box stores) that by chance have an exceptionally good bark product. If I find something exceptional, I usually get a pallet or two, because it's a sure bet that when the season comes around, there will be lots of beggars showing up looking for bark, or for help making their container soils for their spring/summer plantings. I always end up buying Turface (a calcined clay product) and crushed granite by the pallet (40 bags), too - even though I only go through about 10 bags of each for my own purposes.

Al

Al, wow, where do you store pallets of rocks and bark? :) Do you have a nice big shed or garage for potting your containers? Thank you for the link. I read your post closely. Very interesting, and thank you for all your efforts to help educate your fellow GWers.

I'm going to be on the lookout for a bag (or 2) of bark fines.

Too bad you don't live near - I'd be glad to share my stash & help you make your soil(s). ;o)

In addition to a pretty big garage, I own a business (glass company) with a large warehouse where we store cases of glass & mirror, as well as metal extrusions for storefronts, and other supplies; so when I need to/feel like buying something in bulk, it works out well. I can't say the guys are always 100% cheerful about having to screen all the Turface I use, but they manage to pull through. ;o)

I do a lot of potting on the driveway or in the garage. I use this arrangement for most of my potting/deadheading/pruning/tidying-up plantings/and working on my bonsai trees because it has storage drawers for lots of tools and is hydraulically adjustable for height:
{{gwi:43058}}

It's an early spring picture and the front garden is just waking up. Later in the year it will look more like a garden.

{{gwi:43083}}

{{gwi:3281}}

{{gwi:3277}}

Al

Nice Photos.

Really nice photos and beautiful gardens! Your gardens are very artistic and the plants look happy and well cared for.

If I lived close I would love some supplies! Hopefully somebody will sell them around here. I am not so much interested in containers outside, but would like to upgrade many of the houseplants. The Thanksgiving cactus bloomed nicely this year, can only imagine how nice it and some of the other plants would look when they get repotted.

Many hundreds of people have switched their houseplants to some variation of the mix you see upthread with the dime in the middle. At first, people scoffed when I told them that was what I grew all my houseplants, cacti, succulents, and even all the very herbaceous plants in, but now there is a very large group that has discovered that with a little effort to find the proper ingredients and mix the soil, they can grow houseplants with far less trouble and a MUCH wider margin for grower error - especially in the area of watering and fertilizing.

I don't want to completely hijack EDs thread unless he's cool with it, so if you have more interest, just slide over to the Container Gardening Forum and join any one of the threads about 'the gritty mix'.

{{gwi:5839}}

{{gwi:3244}}

{{gwi:3243}}

{{gwi:3252}}

{{gwi:2010}}

{{gwi:43055}}

All the above are in the gritty mix. Notice that everything is perfectly healthy and there is no evidence of burned leaf tips or margins to be found anywhere. I have dozens of similar pictures of equally happy plants in the gritty mix.

Al

You are the Man, Al

Happy New Year and Thanks for not giving up or giving in...

I'm pretty thin skinned, I'm not a fighter, I go right to surrender... ;-)
Happy New Year Brother

{{gwi:15175}}

{{gwi:11399}}

{{gwi:15174}}

{{gwi:80192}}

{{gwi:132558}}

Beautiful gardens, folks.

Hope to clarify, I have been remixing and reusing potting soil for 10 years, of course the oldest stuff is now nothing but perlite, everything else has long-decomposed. So in practice, this 'recycling' creates a very well drained, porous soil.

As for watering, root rot is the bane of container gardening, so its a bit touch and go early on.

There is another phenomenon that I'm not really sure about. If you take a cutting from a tomato or other plant, stick it in water, it will root. The roots are healthy, there submerged, and the plants will grow happily. To a point. These 'water' roots are not the same thing as normal roots, plants also need some 'air' roots, and transplanting a water-rooted plant into normal soil may not work.

So, those pots that have a gallon or so reservoir in the bottom, the roots grow right in there, sucking up water, and the air roots are up there in the soil. They work great. In the fall, when I dump them out, there is a huge mass of roots down in the 'well'.

As for fertilizer, I find I'm far more likely to over-fertilize than under-fertilze, and compost, which I make by the cubic yard, works pretty well.

It's all good fun, and as this thread demonstrates, there are several viable ways of going about the same thing.

David - this is written in a conversational tone, even though I disagree with you on a couple of points.

First, the reason I don't reuse my potting soil, and I start with a soil that has as its primary fraction, large particles of pine bark, which takes 4-5 times longer than peat to break down, is that I have found it VERY advantageous to keep my focus on aeration. There are actually ways to quantify a good soil and compare it to a lesser soil by examining it's total porosity and air porosity at container capacity. Container capacity is the soils state of water retention after it has been completely saturated and has just stopped draining.

You cannot achieve a level of either total porosity OR air porosity at container capacity using broken down particles of peat/compost/coir as the primary fraction of your soil, even with a considerable volume of perlite added. If you are left with nothing but perlite, which doesn't seem like something that would occur as a common scenario in normal cases of soil aging, then we really aren't talking about reusing an old soil; rather, we'd be talking about reusing the old perlite fraction, which is something quite different.

So, if you are reusing the old soil as a significant fraction of the current soil, its very likely you're putting yourself at a distinct disadvantage because of the inherent reduction in aeration and probable excess water retention.

I'm not pointing out these facts to disparage how you grow, only so we can understand that SAYING a soil drains well (or is well-aerated) doesn't mean it actually does (or is). We KNOW that fine particles hold more water than coarse particles (of the same material) and vine organic particles tend to compact, further reducing aeration and increasing water retention.

IF you were to use inexpensive pine bark as the primary fraction of your soil, and add enough of your used soil to arrive at a favorable state of water retention, you would end up with a soil that DOES have favorable total porosity and air porosity at container capacity. If the soil is entirely broken down as you suggest it is, you STILL need to buy something to replace the lost volume, unless you're using compost ..... and then you end up right back where you started - so why not buy inexpensive pine bark instead of another finer ingredient?

Something I wrote on another thread about rooting in water:

Though roots form readily and often seemingly more quickly on many plants propagated in water, the roots produced are quite different from those produced in a soil-like or highly aerated medium (perlite - screened Turface - calcined DE - seed starting mix, e.g.). Physiologically, you will find these roots to be much more brittle than normal roots due to a much higher percentage of aerenchyma (a tissue with a greater percentage of intercellular air spaces than normal parenchyma).

Aerenchyma tissue is filled with airy compartments. It usually forms in already rooted plants as a result of highly selective cell death and dissolution in the root cortex in response to hypoxic conditions in the rhizosphere (root zone). There are 2 types of aerenchymous tissue. One type is formed by cell differentiation and subsequent collapse, and the other type is formed by cell separation without collapse ( as in water-rooted plants). In both cases, the long continuous air spaces allow diffusion of oxygen (and probably ethylene) from shoots to roots that would normally be unavailable to plants with roots growing in hypoxic media. In fresh cuttings placed in water, aerenchymous tissue forms due to the same hypoxic conditions w/o cell death & dissolution.

Note too, that under hypoxic (airless - low O2 levels) conditions, ethylene is necessary for aerenchyma to form. This parallels the fact that low oxygen concentrations, as found in water rooting, generally stimulate trees (I'm a tree guy) and other plants to produce ethylene. For a long while it was believed that high levels of ethylene stimulate adventitious root formation, but lots of recent research proves the reverse to be true. Under hypoxic conditions, like submergence in water, ethylene actually slows down adventitious root formation and elongation.

If you wish to eventually plant your rooted cuttings in soil, it is probably best not to root them in water because of the frequent difficulty in transplanting them to soil. The brittle "water-formed" roots often break during transplant & those that don't break are very poor at water absorption and often die. The effect is equivalent to beginning the cutting process over again with a cutting in which vitality has likely been reduced.

If you do a side by side comparison of cuttings rooted in water & cuttings rooted in soil, the cuttings in soil will always (for an extremely high percentage of plants) have a leg up in development on those moved from water to a soil medium for the reasons outlined above.

Al

Al, we may well be discussing two separate things here.

I grow a couple thousand plants in a green house, for the seedling pots and such, I'll buy potting soil because the risk of disease transmission or a pour mix is too high. These plants are sold or planted out in the garden, but a few, 60 or so, I plant in containers because they warm so much more quickly than the ground, and I can grow stuff like eggplant and okra which other wise I can't, and some cucumbers for early season use.

These pots are approx 5-10 gallons in capacity, about a dozen have a built in reservoir at the bottom with slots for water roots. If I refill them every year with fresh soil, I'd spend more on that potting soil than everything else put together, and it makes no economic sense to put $15 worth of soil in a container where I'll pick 6 eggplant.

The problem I have is the soil drying out, largely through plant transpiration. And to say that the roots are not taking advantage of the available space, well I know what a that looks like, and when I whack apart a root ball from a container, thats not what I see.

I do see your point, and I happen to have a large pile of pine/aspen bark in my driveway, the residue of 10 cords of firewood being dumped there, and I'll try a dozen containers with it this summer.

Linked is a really nice self-watering pot, they used to be about half that much. When I empty these out at the end of the season, the bottom reservoir is an impressive mass of roots. I'm trying to do something similar on the cheap - remember here, we're at 7000 feet in the high desert, with remarkably low humidity and it does get windy in the spring.

Here is a link that might be useful: link