So do you think I could re-use this soil for dahlias?

You will get a wide range of opinions on this.

Do *I* think you can re-use it for dahlias? Yes.

Will it be the 'best' thing to do? Depends on how you define best. It will certainly be less expensive than replacing it all so that is one factor to consider. As you noted the organic portion of the mix breaks down and compacts a bit.

I, personally, don't consider this an issue as it still offers enough aeration to grow healthy plants. Some folks though wanting to get every last ounce of performance from their plants would not consider it acceptable. Only you can make this call.

I would re-use it as would some others, but plenty of others would not ;-) So, that is the definitive word on that ;-)

How to store it? Doesn't much matter. It isn't sterile at this point so keeping it sterile is a moot point. If you pile it on a tarp in the woods I wouldn't be surprised to find the winter weather blow it away or the water wash it away so I would say put it in something that will effectively contain it. Other than that one place/container is about as good as the next. Keeping it on the cool/cold side and also dry will bring a halt or at least a slow down to further decomposition of the organic material.

In my estimation, the only case to be made for reusing container soils is one of economics, and you'll never find me argue against making that decision. If you can't afford, you can't afford it. That said and setting economics aside, you might decide to reuse soil for reasons other than economical. Perhaps the effort involved with acquiring (or making your own) soil is something you might not wish to go through or be bothered with.

In any case, it would be difficult to show that soils in a more advanced state of structural collapse can somehow be preferred to a soil that can be counted on to maintain its structure for the entire growth cycle. So, if the economic aspect is set aside, at some point you must decide that "my used soil is good enough" and that you're willing to accept whatever the results of that decision are.

All soils are not created equal. The soils I grow in are usually pine bark based & collapse structurally at a much slower rate that peat based soils, yet I usually choose to turn them into the garden or give them over to a compost pile where they serve a better purpose than as a container soil after a year of service. Some plantings (like woody materials and some perennials) do pretty well the second year in the same bark-based soil, and with careful watering, I'm usually able to get them through a third year w/o root issues.

Watering habits are an extremely important part of container gardening. Well structured soils that drain well are much more forgiving and certainly favor success on the part of the more inexperienced gardeners. As soils age, water retention increases and growing becomes increasingly difficult. If your (anyone's) excellence in watering skills allows you to grow in an aging medium, or if your decision that "good enough" is good enough for you, then it's (your decision) is good enough for me, too.

The phrases "it works for me" or "I've done it this way for years w/o problems" is often offered up as good reason to continue the status quo, but there's not much substance there.

I'm being called away now, but I'll leave with something I offered in reply on a recent thread:
"... First, plants really aren't particular about what soil is made of. As long as you're willing to stand over your plant & water every 10 minutes, you can grow most plants perfectly well in a bucket of marbles. Mix a little of the proper fertilizers in the water & you're good to go. The plant has all it needs - water, nutrients, air in the root zone, and something to hold it in place. So, if we can grow in marbles, how can a soil fail?

Our growing skills fail us more often than our soils fail. We often lack the experience or knowledge to recognize the shortcomings of our soils and to adjust for them. The lower our experience/knowledge levels are, the more nearly perfect should be the soils we grow in, but this is a catch 22 situation because hidden in the inexperience is the inability to even recognize differences between good and bad soil(s).

Container soils fail when their structure fails. When we select soils with components that break down quickly or that are so small they find their way into and clog macro-pores, we begin our growing attempts under a handicap. I see anecdotes about reusing soils, even recommendations to do it all over these forums. I don't argue with the practice, but I (very) rarely do it, even when growing flowery annuals, meant only for a single season.

Soils don't break down at an even rate. If you assign a soil a life of two years and imagine that the soil goes from perfect to unusable in that time, it's likely it would be fine for the first year, lose about 25% of its suitability in the first half of the second year, and lose the other 75% in the last half of the second year. This is an approximation & is only meant to illustrate the exponential rate at which soils collapse. Soils that are suitable for only a growing season show a similar rate of decline, but at an accelerated rate. When a used soil is mixed with fresh soil after a growing season, the old soil particles are in or about to begin a period of accelerated decay. I choose to turn them into the garden or they find their way to a compost pile.

Unless the reasons are economical, I find it difficult to imagine why anyone would add garden soils to container soils. It destroys aeration and usually causes soils to retain too much water for too long. Sand (unless approaching the size of BB's), has the same effect. I don't use compost in soils because of the negative effect on aeration/drainage. The small amount of micro-nutrients provided by compost can be more efficiently added, organically or inorganically, via other vehicles.

To boil this all down, a container soil fails when the inverse relationship between aeration/drainage goes awry. When aeration is reduced, soggy soil is the result, and trouble is in the making."

Al

As to where to store your soil, why not just leave the soil in their containers over the winter? I do this for many of my containers and so far they haven't shown any wear or tear even suffering through sub zero winter temperatures here in Chicago. In the spring I empty many of them into my soil mixing station, which is a large Rubbermaid storage bin, and rejuvenate the soil with compost and peat and manure, etc. I'm still working on a good recipe but I have absolutely no problems with plants recycling soil like this. This year I did have problems with my morning glory pots but I think that was because I added too much sand and not enough compost making the pots drain too well. Some of the morning glory vines faded way too early this year but I should have fertilized them more and used more compost or peat.

It's not economics that keeps me from replacing new soil every year, it's practicality. My entire garden consists of containers that hold more than 300 cubic feet of soil. Replacing that much soil every year would be a gigantic PITA . Plus many of my big containers have perennials so they can't be emptied at the end of the year. I have too many other things to do each spring than to lug soil around. My garden expansion next spring will requite 75 more cubic feet of new container space which alone will keep me completely occupied. I compost a lot of soil, mainly boxes that have had problems this summer with the hope that I can use it next spring.

I mentioned the effort factor in my first paragraph. Though I understand your reluctance to change that much soil, when the reason is distilled, it still points to your being unable (time) or unwilling (effort) to use new soil and that the aging soil is good enough. I'm not pointing this out to be disparaging, only to illustrate the decision to reuse soil is something of a compromise.

Re. your morning glories. Sand in soil does help soil drain quickly, but it also clogs valuable macro-pores. Compost also clogs the same pores and both destroy aeration. Even though the sand causes water to flow through the soil faster, it reduces macro-pores to micro-pores, which increases capillarity and causes soil to retain too much water for most container culture. Sand in containers should be uniform in size and large - BB size or maybe slightly smaller. Swimming pool filter sand works fairly well in containers if you feel a need to use sand. Chicken or turkey grit (crushed granite) is better.

Al

The above authors seem to be in agreement, but let's consider another alternative. Take container #1. remove about 80% of the soil, and add a 2" layer of leaves. Put some soil from container #2 into container #1, and add more leaves, Put some soil from container #3 into container #1 and add some leaves. Now add some fresh cow manure to Container #1, and some soil from #4. Container #1 is now full, and will be ready next Spring. Container #2 becomes container #1, and the whole process is repeated.
What material you have left over goes into a "holding" container, and as you add spent soil to it you also add leaves between layers. I would leave the covers off for now, to let the containers "work". If, in the Spring, it get's to rainy, cover them. The soil from the "holding" container will be used to top-off the display containers in the Spring. Lot's of work, yes, but the results should pay off well next Spring. Coments appreciated.

Hi corgicorner - well just to continue the discussion I'll say that I would not plan on leaves digesting completely at my place in an open container unless they were shredded first. Of course there is a person on the Perennials forum who composts in trash cans on an unseen side of her house. She's posted pictures of it. It appears to work very well for her - I've seen photos of her garden.

I clean a lot of whole leaves out of my gardens in the spring - it seems like as many as I took out in the fall. We live in the woods and I have plenty of leaves - they never all decompose during the winter. I think what Al is saying is that too much compost might be a bad thing in containers. It holds water like a sponge. I've experienced that myself only I didn't completely understand what the problem was. And then there's his whole discussion on another post of the plateau level of water due to gravity and some other term I can't remember but understand the concept. I think if I save the soil at my house I'm going to dump it in couple large garbage cans. In the spring I'll put in leaf mold that I compost elsewhere and some large grain solids, like builders sand, perlite, etc. And if all else fails, I can always use extra soil in the compost bins or filling holes.

My stuff was all originally a peat and perlite Pro-Mix blend for commercial growers. The downside to a soil-less mix like that is the peat collapses as it rots and becomes more dense. That is not helpful for dahlias anyway, they need sharper drainage. In a way, the 2 year old Pro-mix IS the organic portion, now I need to lighten it up next year with other additives and fertilizers.

One thing I will share that I've had to do when I reuse soil is to drench the soil after planting with an short half life insecticide. While I hate to kill earthworms, it important to know that various creepy crawleys could have laid eggs in the soil and you don't want them waking up to munch your plants. The other reason for me is that the skunks, chipmunks, squirrels and raccoons find containers easy picking for digging up any grubs or worms. I've had many containers completely destroyed and all over the deck from the marauders.

I'm lucky enough to have plenty of space in my gardens but out of my whole yard we get the most sun on the deck. I know, its backwards. So the sunlovers go in containers.
Lots of ways to make it work.

That is not helpful for dahlias anyway, they need sharper drainage.

I think you would be surprised at the results if you tested this belief.

I only dabble in dahlias, but my wife's grandfather is a dahlia nut who enters his in competitions and all that and he grows his in amended clay soil (in the ground) that doesn't offer anything even approaching the drainage in even the most worn out peat based potting mix. He is an old dog so he only uses peat to amend the clay soil and then feeds them synthetics. I call him an old dog, but I can't argue with his dahlias ;-)

Isn't that the screamin' truth? But clay can have a lot of nutrition in it, and he probably has more sun than me to grow them in. I bet I watered my containers no more than 10 times all summer - some less.

You know what I think is that some people just have a feel for what a plant will like.

I always dump all my containers (I have two sets--one for spring and one for summer) if they are in the "dumpable" category. The Japanese maples in big cedar boxes don't get dumped. The great big faux granite planters out front don't get dumped. The maples do get an annual top-dressing of fresh potting soil and new redwood bark every spring. The faux granite planters get about the top 8-12" scooped out and replaced with fresh soil. Everything else gets dumped, and called "adding organic material to the soil".

I use commercial potting mix, anything I can get reasonably cheaply, but I'm certain that most of the "good" of these is gone by the end of one season--what they still have going for them is good drainage. But my argument is against dumping the soil, and not just because it's expensive and troublesome. It's hard on perennials, especially big ones, to replant and it provides nutrition only briefly. Much better to fertilize them regularly during the growing season, and pot up only when necessary because of size. I'm somewhat suspicious of corgicorner's routine, though possibly that's because I'm a city gardener without access to his manure etc. I've found annuals to grow in my windowboxes quite cheerfully in old potting mix as long as they are fertilized.

Are people putting their used container soil into their gardens or throwing it out in the trash? If you wouldn't put it into your perennial beds, would you use it in an area for plants like nasturtiums that like poor soil?

I never put potting soil into the trash. I can't imagine any reason to ever do that. I mean I paid good money for the stuff.

I would put it anywhere I wanted some softish soil added whether it be a vegetable garden, around perennials, annuals, the lawn, wherever.

Most commercial mixes are just peat and some inorganic ingredients and none of them will do anything bad to any ground soil.

Pretty much any potting mix no matter how 'worn out' it is has better tilth and drainage than the clayish soil around my yard.

wow, I've just learned so much from all of you! but I have a question about timing: when is the best time to replace or refresh the soil in containers of perennials? I have a roof garden with about 50 containers between 10" and 20" in diameter. My plants (mostly dwarf evergreens) have been doing remarkably well considering that I've never replaced soil (other than a few repottings to larger pots). I'm not sure how I'd approach this project logistically, but before I start planning I'd like to understand whether it needs to be done before the first frost or after the last frost (or in between?)

First, there is a difference between "repotting" and "potting-up". Potting-up is simply removing the plant from its container & moving it to another container with a minimal amount of root disturbance and the addition of an appropriate amount of new soil. Repotting includes removing all or most of the old soil and pruning of larger roots. Repeating the former presents some long range problems including having dissimilar soils in the same container.

The best time to repot most evergreens is when most of the deciduous plants in the landscape have leafed out. They are generally repotted 2-3 weeks later than their deciduous counterparts.

Though you can realize quite a few years of reasonable vigor from plants that are potted-up, they will eventually begin to decline as rootage becomes more and more massive & roots begin to interfere with each other. The old soil in the original rootball also will become so hard it will restrict the plants ability to lay down tissue in the root cambial layer, effectively girdling roots. Water penetration into the older soil often becomes almost impossible to achieve w/o soaking the entire plant.

The first symptom that this is occurring is decreasing branch extension (you can gauge this by looking at the distance between leaf bundle scars) followed by decline/death of individual branches and an overall loss of vitality.

Al

With all due respect to Al and his scientific reasons for using fresh soil, my experience as a container gardener for the last 20 years has taught me to reuse soil for several years before replacing it. I grow a lot of perennials, and so far the predicted languishing of plants hasn't happened. I think anecdotal evidence is helpful, otherwise why have a garden forum at all if not to share gardening practices with the expectation that what others do will help you?

Here's what I do. I remove the top 2-3" of old soil, poke holes quite deep in the soil with a pencil, and pour in Vermont compost. Seems to work fine for me.

However, when it comes to bulbs (spring blooming -- I do keep my lilies in the same soil), I generally replace the soil every year. I treat my container bulbs as annuals. In order to plant new bulbs I have to get down to the bottom of the pot to get rid of the old bulbs, so I just get rid of everything and start over with fresh soil.

I have been growing in containers for nearly 20 years now, mostly in Earth box type planters. I have been using the same soil that I started with. All I do is each year I add some organic fert. as a top dressing to each planter, and keep an organic mulch on them. My containers are never dumped out and only dug enough to plant transplants or make a seed bed for lettuce, carrots or beets. My original mix was a peat and perlite mix and is as good now as when I started.

John in Pa.
containerseeds.com

Hi, Im a newbie on this forum. I have been using "bagged" potting soil for about 5 years; my entire veg garden is in containers. Currently there is a 32 gal plastic garbage can in the back yard full of this stuff, and I have been layering kitchen scraps, grass cuttings, and red wriggler worms in this container. Mostly I have used this wormy mixture for some potted rose bushes, and have been careful not to plant tomato plants in any "used soil". I guess the decision to reuse potting mix depends on any diseases your flower tubers are prone to, and if the soil has a smell to it. When you flip a container over, dump out soil and you smell decomposition or "swamp" you probably don't want to reuse that mix that year--compost it.

***new post June 2006***

(I thought it might be better to revive an old thread than start a new one)

I made up a couple batches of the small recipe of Al's mix last year for my containers. The exact recipe can be found at http://forums2.gardenweb.com/forums/load/contain/msg031557203792.html

Now, it appears I didn't quite use pine bark fines. I had trouble locating it and according to Al, the fines are already partly composted. What I had was, I believe, pine bark mulch. The right size of particle, or perhaps just a touch larger, but uncomposted. The CRF I used was Osmocote, and I had seaweed fertilizer as the micronutrient source.

Fast forward this year. Mom just gave me a bunch of extra tomato plants. I already have mine in the ground, so I need to put them in containers.

I would like to reuse my soil from last year. I've read this thread in its entirety and understand the compromise I am making. I would like to amend the soil to get the best possible results.

The problem is, that batch I made is spread out into a dozen different containers. I'm not sure how much soil is in each container. I'm not growing as much this year, so I'm only reusing a few container's worth of soil. The soil is still in the original containers. I am going to dump the soil together in my wheelbarrow, mix in some amendments, then put back in the containers.

So my questions are:

1. What would be the best stuff with which to amend the original mixture?

2. I'm only using a few containers of soil. I don't know how much volume it is. How can I *safely* guesstimate how much amenedments I can add, if I don't know the volume of the soil I'm amending? I am a newbie gardener and haven't the experience to be able to eyeball volume.

Any help is much appreciated.