last year's potting soil

pearsaml(6)May 1, 2011

I used a few self-watering containers last year--was going to mix some perlite into the same potting soil and fertilize for this year and the soil in the bottom of the container smells-- presumeably anaerobic bacteria? If I let this soil dry out and add perlite, should it still be useable this year?

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Pear, plants are too expensive to risk losing them for the price of some potting soil. Toss the soil into your composter. Scrub your pots thoroughly, using some bleach to disinfect them. Then use fresh potting soil.

    Bookmark   May 1, 2011 at 10:40PM
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Part of the reason that I really would like to rescue the soil is that it's very difficult for me to get potting soil as I live in brooklyn and don't have a car, and have about 50 gallons worth of potting mix from last year when a car was an option. basically-- if I can't use this soil, my garden will have a lot less plants as there are no garden stores near me and I won't be able to manage many bags of soil on the train.

is soil that's gone anaerobic dangerous just to the transplant that goes in it--or is it dangerous to eat a plant that's grown in it as well? If I do plant something in it and it produces, is it safe to eat?

I had already transplanted some lettuce and arugula into the soil a little over a week ago without drying it out first, not really thinking about why some of the soil had an odor. They seem to be doing find but I am sure that it's not a guarantee that they are out of the woods. And also, I guess now I'm wondering if it's unsafe to eat from those plants?

Basically--I'm willing to risk the life of some plants and waste a few bucks just so I can at least potentially have them if it works. But, of course, not really willing to risk my own health by eating some poison.

    Bookmark   May 1, 2011 at 11:11PM
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Hi pearsaml,
If you can dump it out on the patio,let it dry out for a couple of days, You will be fine, It isn't going to hurt you at all,Plus.... if the plants grew in it, it has to be fine... seems to me,....... but we are all so different with so many different kinds of obstacles to deal with, good for you for trying in your very tough situation.

Let us know how it are always nice too ;-)

    Bookmark   May 2, 2011 at 12:09AM
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I agree with zuni. Compost or get rid of the old soil. However potting soil can be reused if you have a good enzyme system going. "Hygrozym" is an enzyme that cleans everything and makes it usefull for the plants. Look it up and the benefits it holds.

    Bookmark   May 2, 2011 at 12:15AM
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Hi there.

Could someone please explain why the potting soil would have to be disposed of?

When a person grows plants in natural soil, they don't throw it away every year and replace it.

    Bookmark   May 2, 2011 at 2:41AM
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Most garden writers strongly urge you to dump your potting soil each year because of the potential of plant diseases that might be in that potting soil, and for many people more, new, potting soil is fairly readily available, it is easy to get.

    Bookmark   May 2, 2011 at 6:57AM
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I will probably manage a couple of bags of potting soil via the usual methods of public transportation so I can have at least a couple of container plants in fresh soil.

Another thing I'm starting to wonder about now that I'm realizing how many people don't advise that you reuse potting soil-- I have some perennial plants in containers-- 3 mint plants, an oregano, and 2 hops plants. One of the plants is in a self-watering container. If the soil in the bottom of this has any anaerobic bacterial activity, is it dangerous to use the mint in food? I don't cook the food I use mint in (ice cream, drinks, etc).

Is there a difference between re-using potting soil that has perennial plants vs. soil that had tomatoes or basil or peppers in them?

I guess my number 1 concern at this point is whether or not it's a danger to my health to eat plants that DO grow in the old soil, if any do-- I already have seedlings ready for transplant that I started inside, and won't be able to transport enough new soil for all of them. (The problem isn't finding the potting soil, but transporting it). I've spread out a couple container's worth of the soil in the yard in hopes that it will dry out.

Basically, I'm going to give it a try in a couple of containers since I already have more plants than I can get new soil for... UNLESS,I come across some information that says it's dangerous to eat from plants that grow in this soil.

Thanks for everybody's input! If I do re-use the soil against the advise that some of you are giving that it won't be as good of soil as fresh soil, it's not that I'm just disregarding you... I am sure you are right that it's not as good-- I just really love gardening and want to be able to at least TRY more plants rather than less.

    Bookmark   May 2, 2011 at 8:01AM
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It is not dangerous to eat anything grown in that soil. And...I reuse my potting soil every year. Some of it is 19 years old. I add compost to it every year. I use a lot of potting soil and I certainly can't afford to throw it away every year.

I used to cook mine in the oven, which never bothered me a bit, but people here claim it smells. I never noticed, but I grew up with a father who sterilized manure in my mother's oven.

    Bookmark   May 2, 2011 at 8:16AM
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The primary risk of reusing potting soil is the degradation that occurs during the course of the previous growing season and also through the winter if the pots and soil remain out and exposed to weather. Unless a very durable mix (much like Al's 'gritty mix', described in detail on the Container Gardening forum), the particles breakdown and compact and you lose porosity, reducing oxygen and compromising drainage. The anaerobic conditions and "smell" you notice now is a result of that compaction.....oxygen is lacking.

While it is considered ideal to replace potting soil each season, there are often reasons why that is impractical and you can work around it. Dumping the soil out of the containers, allowing it to dry and then adding a good portion of a chunky, durable component - like composted bark fines - will rejuvenate the soil to a reasonable degree that should allow you to replant without concern.

There is NO concern that plants grown in last year's soil will not be edible or somehow contaminated :-) The concern rests with how well this season's plants may grow - unless the soil is properly refreshed, any new plants may struggle under poor drainage conditions and insufficient oxygen exchange. And it goes without saying that the soil from any containers that might have produced disease issues should be dumped rather than reused.

nutsaboutflowers, one cannot make fair comparisons between inground gardening and growing in containers :-) The two situations are very different and different requirements including the type and quality of growing media (soil), fertilization and watering come into play. Since container gardening is pretty much dealing with a closed system, fast drainage and good aeration/high porosity are overriding concerns.

pearsaml, if you can locate and haul home a few bags of bark fines to mix with last season's soil, you should be good to go....won't hurt to add some fresh potting soil as well. And you may find you need to anyway - that degradation that has occurred effectively 'shrinks' the soil volume over time. If you can't locate bark fines at your local garden center or home improvement store, try a pet store :-) A very similar and perfectly adequate product is often sold as pet, especially reptile, bedding.

    Bookmark   May 2, 2011 at 10:08AM
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Some people will tell you that growing perennials and annuals is different and they need different things, or something. However, plants grow in soils that provide them with both nutrients and moisture, whether they are annuals or perennials, and many people grow plants, perennials, in the same soil for years (wife had African Violets that were only repotted about every 5 years).
If the plants the grew in that soil last year had some disease and the plants you plan on putting in that soil this year are in the same family, or are susceptible to that disease, they could well get that disease. However, if the plants had no disease last year it is unlikely plants growing in that media will get a disease that did not exist last year.

    Bookmark   May 3, 2011 at 6:35AM
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Pear, Drying the soil and re-aerating is a reasonable option, provided there was no disease in the plants you grew previously. Container growing is totally unlike growing in natural soil. You simply do not have the biosystems and natural compost that develop in a garden. (Most greenhouses do not even use soil, but a soil-less mix that relies entirely on fertilizers to force-feed the plants.)

You might consider worm composting to provide a high-quality soil amendment that can be mixed with your potting soils after they have been sun-dried. The worm castings will replenish your potting soil with air and good micro-organisms.

    Bookmark   May 3, 2011 at 8:13AM
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I do what annpat does - recycle potting soil year after year. The organic matter breaks down, leaving a high percentage of perlite, which serves for aeration - much like adding the specific size bark-bit they talk about chez the container gardening forum.

I dump out all the containers onto a concrete slab, smash the big clod with a shovel, shake the stuff of the dead roots, and usually add some fresh potting soil - this year, I found sifted compost in cubic foot bags for $1.29 or something, so added 10 bags of fresh compost, stirred it all up, refilled the containers, and off we go for another fun summer of plant growing. Where I live, the garden soil warms up sometime in July and cools down in Sept, so for plants that like hot weather - egg plant, peppers, okra, i use containers.

I know that some,when reading that I didn't use home-made compost, must have gasped in shock. But this year I'm recovering from surgery and the compost was really wet and heavy and time and all that and next year, I'll do better. Honest.

    Bookmark   May 3, 2011 at 9:36AM
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In addition to a large garden, I use a number of earthboxes each year to get early lettuce, radishes, spinach and tomatoes. Each fall, I take the old potting mix out, mix in some new perlite and a little of my compost and then reuse. The smell, as others have noted, is due to anaerobic metabolism. Mixing your old soil with perlite, or something similar, will provide missing porosity and new mixture should serve you well. Potting mix is too expensive to use once and then discard unless you know the soil is definitely contaminated with plant disease.

    Bookmark   May 3, 2011 at 9:44AM
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Tiffany, purpleinopp GardenWeb, Z8b Opp, AL(8B AL)

I wanted to register another vote for you to re-use it. And agree with the advice that if you had no diseases last year, you should have no reason to assume your re-used soil will cause any.

But perlite is the last thing I would add to it. There's nothing beneficial in perlite for plants and, as you said, volume of planting medium is your priority. Put compost in there as it will solve both your problems - volume and depleted nutrients.

As for the odor, it sounds like a drainage issue unless it is just because the pots are without plants and left outside all winter. If the latter, dumping it all out and mixing in some compost should cure this. If this odor persists after you do this and repot your plants, it is definitely a drainage problem. Before you pot up again, make sure you address the drain holes (insert preferred personal method.)

gardengal48, I've not heard the term "bark fines" before. Do you mean finely ground hardwood mulch? If so, I've not had positive results trying to add this inside container soils unless it is for houseplants, or if it is a leftover bag from last year. It does improve drainage but new mulch mixed-in seems to make the plants grow slower. I do like a layer of mulch on top of container soil, which can be mixed in next year.

    Bookmark   May 3, 2011 at 10:33AM
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Lots of people use their old potting soil/mix by incorporating what is considered 'good' soil. Compost, whether commercial or home-grown has to be considered a good amendment to that old stuff which has given its all to the plants that were grown in it.
Potting soil, when fresh, is said to contain sufficient food to do well by plants for a limited time and then, because of its low volume, and the volume of water that goes through it, is soon depleted of its nutritive values.
But if we re-mix it with what is considered top notch soil....such as compost, then are we not giving fresh life to a good medium that our plants will re-discover.

Its hard to say for certain whether a plant given the old stuff, and a plant given fresh, what the outcome of its growth and flowering says about either.
I believe though fresh is best. If we put value on our plants and wish them to do good by us, then spend the few dollars and think we are doing good by them.
But the rub is, there's good potting soil and there's some really awful stuff sold in the marketplace. Having been cheated by thinking inexpensive is just as good, one really has to take a second look at "price". When going by the adage, 'you gets what you pay for' is not that trusted anymore, there's just as many lousy expensive bags out there as there are that cost less. One really has to examine the product before putting down the dollars and good sense.

    Bookmark   May 3, 2011 at 11:23AM
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purpleinopp, bark fines are sold under various names - soil conditioner, mulch, even the pet bedding I referred to above. And you can get it in various sizes and from various materials (pine, fir, redwood, etc.) but it needs to have some size and texture......I've attached a link to another thread that illustrates bark fines or just review a few posts on the Container Gardening forum - it is discussed endlessly :-)

The benefit of this textural component should not be underestimated. It is the aeration and fast drainage this provides that is key. You will need to supply nutrients on a pretty regular basis - any container planting requires this due to leaching from frequent watering and the lack of nutrients typically present in most potting soils.

FWIW, I would avoid adding compost. This does nothing in the way of providing aeration/porosity or improving drainage - in fact, it tends to do just the opposite, by continuing to degrade and decompose, resulting in compaction and lack of porosity and oxygen exchange and the anaerobic conditions you are trying to rectify.

Hopefully Al (tapla) will weigh in on this discussion and provide his incredible insight on container soils. He always presents the facts in a much more detailed and understandable manner than I can :-)

Here is a link that might be useful: check me out for an visual on bark fines

    Bookmark   May 3, 2011 at 11:48AM
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I think the concept of all those different bark diameters is fascinating, but this isn't available anywhere near here - the only thing I can find is big pine bark chunks for landscaping.

For what ever reason, a bag of decent potting soil now runs just shy of $20 here, and just for filling the pots I use for starting stuff in the greenhouse, this is now, far and away, the greatest gardening expense. No if I add on the few dozen containers I do, then we're talking serous money. So it doesn't make a whole lotto sense to spend $10 to fill a container that will grow 3 bell pepper plants, and in the short season I deal with, maybe harvest 10 bell peppers per container. It does start to make sense if I spend 50 cents re-cycling the existing potting soil.

You do have to be careful watering the containers, but here, rainfall isn't an issue, so I have pretty good control on what gets poured in.

    Bookmark   May 3, 2011 at 1:10PM
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tapla (mid-MI z5b-6a)

Something I often leave on threads like this - a copy/paste from something I wrote on another thread:
In my estimation, the only convincing 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.

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 medium 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.


    Bookmark   May 3, 2011 at 5:55PM
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taz6122(N.W. AR.6b)

I use mine year after year if the plants did well and weren't diseased. I add new sphagnum and perlite every year. I disagree with anyone saying add compost or perlite has nothing for the plants. As stated by gardengal you want drainage and aeration which perlite helps and compost hurts. I really wish I could find bark fines but I've looked and there's nothing in this area except orchid bark in very expensive and little bags.

    Bookmark   May 3, 2011 at 6:18PM
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I have added a small amount of pond bottom soil - say 2% - to the recycled mix every few years, the theory being that my pond bottom soil verges on pure fertilizer, and it contains, theoretically, lots of micro-nutrients and scarce minerals.

But to be honest, the best results - in terms of vegetables produced - is to water every week with compost 'tea'. And not the fancy, aerated stuff, just a shovel of home-made compost in a 5 gallon bucket, stir up, let sit overnight, stir up again, and use this to water the containers.

    Bookmark   May 4, 2011 at 10:30AM
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David, I just had to comment......$20 bucks for a bag of potting soil??!! That's outrageous! And I thought the west coast was overpriced. No wonder you re-use your soil :-)

I did a radical downsize recently from a full, inground garden and now garden almost exclusively in containers - trees, dwarf conifers, perennials, seasonal color spots and a few veggies and edibles......maybe about 40 pots in all, most pretty large. At $20 bucks a bag, I'd go bankrupt filling them all! Fortunately, the best potting soil I can lay my hands on - a variety that includes ingredients very similar to Al's gritty mix - is only about $9 for a 1.5 cu.ft. bag.....and I get a 25% employee discount on top of that. And since most of my containers are longterm plantings, I don't have to refill all every year.

    Bookmark   May 4, 2011 at 10:53AM
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gardengal48, the problem we face here are a combo of high shipping costs to a low-population relatively remote area, and the sudden advent of "alternative crops" in containers, the growers of which are very happy to pay $20 a bag for top quality potting soil, what with a different economic matrix than growing a few eggplants.

    Bookmark   May 4, 2011 at 2:46PM
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I found a bag of fir bark fines, have some perlite and some new potting soil, and have the old potting soil spread out outside. Planning to mix it up and transplant some tomato and basil plants this weekend. I got some dynagro foliage pro from the hydro store to fertilize, as I read a few recommendations for it in the container gardening forum. So far my arugula and lettuce that was already planted in the old soil seem to be doing pretty well, but those have always been pretty easy to please. I definitely appreciate all the input here! Potting soil is about 20$ for a cubic foot, and a cubic foot, let alone enough to fill 50+ gallons worth of containers, isn't easy to drag around on public transit. Fertilizer is MUCH easier to transport to try to make up for the loss of nutrients as much as is possible.

    Bookmark   May 5, 2011 at 7:59PM
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I am going to try an experiment with mixing some of my recycled mix with pine cones, of which I have an abundance. That should help with aeration, if I can get the right mix.

    Bookmark   May 6, 2011 at 5:27PM
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I find that if I reuse all the potting soil mix and do not introduce any new material, my plants are not as vigorous. I mix half old and half new. The old contains good water-draining perlite, and the new contains fresh, good organic materials that plants crave. I am incredibly lucky in that I have a large yard that is always wanting more soil, so I put the other half in the garden beds, especially the veggie beds. They seem to like the lighter, perlite soil. I cannot store the soil in the containers, as our winters are incredibly dry and cold. It sounds funny, but I reuse the bags the soil came in, and store it that way. It's easier for me to break up and mix in the spring in the bags. I've never tried to do any decontamination by baking or anything, but I've never had any pest or disease problems.

    Bookmark   September 27, 2013 at 11:45AM
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