Recycled SOIL MIX - is it possible?

Savannah, GA

Can SOIL MIX be recycled that been used for two years in my outdoor containers? I would like to reuse these many gallons of soil MIX, but not sure what to do it for the health and recycling of my mix. In addition to cow manure compost, it contains some topsoil, but it's mostly mix. It is November in Georgia and I have a compost bin with coffee grounds (Starbucks), and lawn and garden waste that is composting until Spring 2011. And, I also have a worm compost bin and my worms only eat ground fresh veggies, crushed egg shells, and fruit scraps. SURELY, between all these resources I should be able to recycle-refresh-renew this soil mix. Someone told me to throw it all out, and by fresh every year. That advice ain't sitting to well. That mix cost $$$.

Bay City, MI(Zone 6a)

In my estimation, and approaching this primarily from the perspective of what it best for the plants, I think 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 (any one'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. The horticultural world changes rapidly, and a large % of practices in vogue 20 years ago are laying by the way today.

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.


Morrisville, PA(Zone 6b)

I would go with tapala's recommendation. I recently learned of tapala's container mixture formula and I see the difference in the few plants I recently repotted.

I have nine earthboxes that I am emptying the contents into the garden and compost pile. Next year I will use tapala's recipe and no more store bought potting mix for any of my containers. I want my plants to have healthy roots!


Savannah, GA

Tapla...thanks for your comprehensive response, and it was quick too. You've given me much to consider and I appreciate your advice. In addition, Merrymath also responded to my ( $$$ ) thoughts in relating that she has nine earthboxes ready to be emptied into her garden and compost pile. Way to go Mary! Then she mentions her success using YOUR recipe for soil mix. Question: Where can I view your recipe? Hopefully it is posted here on the DG site. After sending my question earlier, I then recalled the importance of having growth mediums which are free of harmful microbes, fungus, etc. Since there is no way to sterilize old soil mix, and get good bugs to chase all the baddies away, that consideration becomes academic. Joining this site is one of my best garden investments this year.

Morrisville, PA(Zone 6b)

Tapala's recipes are on this thread

I also add some osmocote to the mixture.

In a nutshell it is 5 parts shredded pine bark fines, 1 part perlite, 1 part peat moss.


Bay City, MI(Zone 6a)

There are a couple of ways to look at container culture. One is, you can try to use/apply what you learned in your gardens & beds to container culture. The other is you can treat container culture as something entirely apart from gardening in the earth - which it is. On a scale of 1-10, with 1 being growing in the ground and 10 being full hydroponics, growing in containers is probably a 7 or 8, so why adhere to the practices you apply to gardening? Why not borrow a page or two from hydroponics and see how that works. ;o)

If we strip away the politics and ideologies associated with growing in the garden, you'll see that healthy plants are about soil structure and nutrition (we'll disregard other potentially limiting factors for the moment).

good structure: poor nutrition = poor plants
poor structure: good nutrition = poor plants
good structure: good nutrition = good plants

Technically, if you have good soil structure, you can raise beautifully healthy plants with nothing but soluble synthetic fertilizers, even in the ground, and it won't hurt the populations of soil life. The rub arrives on the shoulders of the fact you need to continually add OM to the soil in gardens to retain good structure. In containers, you can BUILD the structure into the soil by starting with ingredients that ENSURE good aeration and drainage, and amending the soil to adjust water retention as you see fit from there. If you start with a used and structurally collapsed soil, you can't amend it.

What I just said sounds like an irrational statement, but let me explain. Adding perlite or other large particles to fine particles like peat/compost/coir/used soil doesn't improve the drainage characteristics of a soil or the height of the perched water table (PWT). To visualize this, think of how well a pot full of BBs would drain, then think of how poorly a pot full of pudding would drain. Even mixing the pudding and BBs together 1:1 in a separate pot yields a mix that retains the drainage characteristics and PWT height of the pudding. It's only after the BBs become the largest fraction of the mix (60-75%) that drainage & PWT height begins to improve.

You cannot add coarse material, and certainly not fine material, to used soil or the other ingredients I listed and expect improve drainage or to reduce the ht of the PWT. Use the same example as above & replace the pudding with play sand or peat moss - same results. The benefit in adding perlite to heavy soils doesn't come from the fact that they drain better. The fine peat or pudding particles simply 'fill in' around the perlite, so drainage & the ht of the PWT remains the same. All perlite does in heavy soils is occupy space that would otherwise be full of water. Perlite simply reduces the amount of water a soil is capable of holding because it is not internally porous. IOW - all it does is take up space.

If you want to profit from a soil that offers superior drainage and aeration, you need to build it into the soil from the start, by ensuring that the soil is primarily comprised of particles much larger than those in peat/compost/coir, which is why the recipes I offer as suggested starting points all direct readers to START with the foremost fraction of the soil being large particles, to ensure excellent aeration. From there, if you choose, you can add an appropriate volume of finer particles to increase water retention. You do not have that option with a soil that is already extremely water-retentive right out of the bag.

I said all that so I can say that water doesn't behave in containers like it does in the ground, so those paying close attention to the structure of their container soils are going to be a leg up on those trying to build a soil that feeds the plants and the 'micro-herd'. None of that is necessary, and in fact, I find it counterproductive to introduce soil amendments that produce explosions of soil biota in containers; after all, these organisms feed on soil particles and collapse the soil, destroying the aeration that is so valuable in container soils.

I'd never say you can't make heavy soils work if you work at it, but I will say that they offer reduced potential for plants to grow to their own genetic potential within the limits of other cultural influences. They also ensure a much narrower margin for grower error in the areas of fertilizing and watering.

Here is a link that I think you might find helpful:


Thumbnail by tapla
Mid-Cape, MA(Zone 7a)

SURELY, between all these resources I should be able to recycle-refresh-renew this soil mix.

MrDonald, I share your wish to utilize used soil mix--I hate the throw stuff out! But Tapla's advice is too good to ignore. Creating new mix for your container plants is really the best thing.
I take my used mix--from containers, Earth Boxes, etc., --and after emptying them out and breaking up the lumps, I scatter the mix over my garden beds in the fall. I cover this will a thick layer of shredded leaves and leave the beds until spring. By then the earthworms do their part and the old soil seems to be renewed.

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