Waste potting mix from commercial greenhouse - potential problems?

Here's the basic question: Am I asking for trouble by incorporating used potting mix from a commercial greenhouse in our lasagna garden beds? Or is this a true gift?

The owner of the local garden center has given us permission to use as much as we want from their waste pile. The pile is not really compost, just an accumulation of potting soil and discarded plants that for some reason were not salable. Naturally, they do not use herbicides, but they do occasionally use pesticides as spot treatments. And they use liquid chemical fertilizers as well as starter fertilizers in their mixes.

Knowing all that, I would still like to recover this resource. On our site, we have very, very little soil over slate and shale, so virtually all our gardens have to be raised beds. We build them from whatever organic material we can get our hands on -- from scythed weedy growth to horse manure -- combined with what little soil we have and some that we brought in a few years ago (very sandy loam). We make big lasagna beds, mix them or turn them once or twice, let them sit for a season and then start using them.

My concern is that the residual chemicals in the used potting mix may undermine the development of the soil life so necessary to turning our heaps of organic matter into healthy soil.

What do you all think?

It is tough when you basically need to 'grow-your-own' soil.

And, yes.  Commercial nurseries do use some rather 'exotic' stuff to give themselves a good, quick start.  That being said, the good side is that seeds have everything they need to start, so the nursery most likely uses nothing to start the seeds.  Good plants would be pricked out of the trays, and potted up (once or twice) to the containers that they will eventually be sold in.  That initial soil should be OK.  Now is when they probably begin fertilizing the potted plants.

Home gardeners are likely to over fertilize plants, but commercial nurseries generally use the minimum amount they need.  Fertilizers can be one of their largest costs, and they usually try to optimize this cost.

If you live in an area with many ornamental trees, soon it will be the time when home owners begin raking the leaves for disposal.  (You probably don't want fruit tree leaves if people in your area spray them.)  Keep an eye open in the freebie section of Craig'sList...there will be people offering free bags of leaves.

If you shred, then mix these leaves 50/50 with that potting mix, you should have a nice pile.  Let the pile overwinter.  As early as possible next spring, spread this out and throw on some seeds...grass, or any quick sprouting seed.  See if you get good germination and growth.  If so, stir it back up (to kill the grass), and you should be pretty safe to use this for the summer crops.  If results are poor, pile it back up and wait until next spring.

Good luck.

Thanks for the advice, John. I had not thought of doing a germination test next spring, but that sounds brilliant. I do use spring-seeded cover crops sometimes, so I will do that on these beds.

We are veteran leaf collectors! We even rake neighbors' yards, load them in our trailer and bring home the gold! We're in a small rural community, but people have very urban notions of yard management!

Just to repeat my main concern: will the fertilizers impede the multiplication of the soil organisms that break down organic matter into plant-usable form?

Thanks!

I think it's a good idea.

I used to work in a horticultural nursery. When I created my present garden I incorporated masses of old compost from the nursery that I worked in.
You may very well find that you will get a lot of weeds of a particular type dependent on what has blown into the nursery and where the compost originally came from and possibly the plants that were growing before they died / were discarded.
If you can compost the compost first most anything or everything will have broken down. I was just wary of not growing my veg in this compost because I knew what had been used in the nursery. (at the time I wasn't too bothered about growing veg, just getting a good soil).

I didn't notice any problems with developing a rich soil from the waste compost I used and 16 years down the line I still know where that soil is as it's much better than the areas where I didn't use it. Plenty of worms and beetles and other things in evidence.

Thanks so much for the voice of experience, Abbey! I am feeling reassured.

We went and got a second trailer-load today, so we will be incorporating it into several new compost pile / raised beds. None of them will be used until June next year. Fingers crossed!

Heidi Hoff wrote:Just to repeat my main concern: will the fertilizers impede the multiplication of the soil organisms that break down organic matter into plant-usable form?

Soil organisms eat fertilizers just as readily as anything else does... Nutrients are nutrients. As far as I can tell, more fertilizers equals more soil organisms.

I would say that if you are at all concerned, plant a cover crop on that soil with a multi-species mix of seeds.  It would give you peace of mind and give the micro-organisms a full growing season to break down any thing of concern, add additional organic material to the soil, as well as build up the fungal network.  Mix a healthy dose of compost into the soil before planting so that there are plenty of microbes populating the soil.

After a year, the nutrient cycle would be fully functioning, the microbial herd will be healthy, and your plants will be happy.

TO:  Heidi Hoff
FROM:  Eric Koperek = erickoperek@gmail.com
SUBJECT:  Recycled Commercial Potting Mixes
DATE:  PM 4:09 Wednesday 7 September 2016
TEXT:

(1)  My family started "dirt poor" (literally without soil) 800 years ago.  We learned to farm with rocks and our bare hands.  So I know something about creating soil "from scratch".  We have learned a few things over the past 8 centuries.

(2)  RULE:  Never look a gift horse in the mouth.  Take all the free potting soil you can get.  Smile sweetly, thank your patron politely, and make sure you give him baskets of vegetables.  Offer free labor in exchange for waste potting soil.  YOU ARE SITTING ON A HORTICULTURAL GOLD MINE.

(3)  Commercial potting mix is very expensive.  Peat and perlite are not cheap.  Many American greenhouse operators have switched to composted hardwood bark.  Grab whatever you can get, in any quantity.  Commercial potting mixes have excellent aeration and drainage = prime qualities for growing horticultural crops.

(4)  Trace amounts of chemical fertilizers or pesticides will have no lingering effect on your crops.  You need have no concern about "contaminated soil" unless you are trying to obtain organic certification.  Most certification programs require "chemical free" fields for 3 years prior to licensing.  1 year of "bacterial cleansing" = natural microbial decomposition will reduce any chemicals present to statistically meaningless amounts far below detection ranges commonly used in water quality reports.  (We test everything that comes onto our farms).

(5)  Mix whatever organic matter you have with your free potting mix, or simply dump it into raised beds and use it "as is".  

(6)  Seed Common Lawn Clover = Dutch White Clover (Trifolium repens) over raised beds filled with free potting soil.  Inoculate clover seed with nitrogen fixing rhizobia bacteria before planting.  Fill powdered sugar shaker with clover seed and sprinkle liberally over beds.  Dutch White Clover only grows 6 inches high so it makes ideal living mulch for horticultural crops, especially anything that can be transplanted (such as tomatoes, peppers, eggplant, melons, squash, pumpkins, gourds, cucumbers, cabbage, broccoli, Brussels sprouts, cauliflower, and sweet potatoes).  Simply plant and water.  No mowing or weeding necessary.  Mix soluble fertilizer with irrigation water or sprinkle dry fertilizers directly into standing clover.  No tillage necessary.  You can run entire commercial vegetable farms by transplanting into Dutch White Clover.

(7)  Do not overlook cemeteries and graveyards as sources of free potting soil.  Most graveyards have great, heaping piles of abandoned flower pots, grass clippings, tree leaves, and other organic wastes.  Buy or rent a pick-up truck and grab these resources before anyone else gets smart and claims these treasures for their own gardens.

(  I managed my first farm when I was 12 years old:  10 hectares = 25 acres of raised beds (vegetables, herbs, flowers, and raspberries).  Market gardeners are absolutely vicious when it comes to finding and protecting sources of organic matter for their crops.  Visit every commercial greenhouse and graveyard in your area.  Bribe them with fresh baked bread.  Do whatever is necessary to lock up these prime sources of organic wastes.  Call local tree trimming companies or talk to any arboreal crews working in your neighborhood.  Give them cider and cookies.  They will deliver all the wood chips you want.  Mix wood chips with free potting soil and a generous portion of nitrogen fertilizer (to speed decomposition of high carbon wood chips).  Fill raised beds then plant immediately with any cover crop of choice (ideally a nitrogen-fixing legume like peas, beans, or clover).  It will take 6 to 12 months for wood chips to decompose sufficiently to grow commercial horticultural crops.  Irrigate raised beds to speed decomposition.  Moist (not wet) soil favors rapid growth of fungi which are necessary to decompose wood chips.  Never let your raised beds dry out.

(9)  Don't be picky.  If you don't have soil you have to use anything close to hand.  We started on old quarry land with nothing but rocks, gravel, and sand.  Now we have 5 to 15 FEET of topsoil.  Grab any organic matter anywhere you can get it.  Nature will do the rest.

(10)  Some of our farms have hundreds of acres of raised beds.  And we build more beds every year to increase vegetable production.  A good practice is to use your oldest beds to provide "bacterial culture" for new beds.  Dig out old beds and mix soil with raw organic matter to fill new beds.  This will "jump start" the composting process so beds might be ready in half the time (6 months) depending on availability of nitrogen (which is the most limiting factor in composting).  Alternatively, you can build a compost pile and turn it every 2 or 3 days for 1 full month = 31 days.  Turning compost piles is hard work so we use mechanical compost turners to produce hundreds of tons of compost yearly.  You can do the same thing if you have a rototiller with rear tines.  Just spread materials (not more than 8 inches deep) on soil surface then set rototiller depth to 1 inch.  Run tiller over compost ingredients.  Rototiller will mix organic materials just like a commercial compost turner.  Leave wastes to cold compost = sheet compost in shallow windrows or fork into raised beds to finish decomposition.  Space your raised beds to fit your rototiller then you can compost right in the paths.  Throw all agricultural wastes from raised beds into paths.  Fork finished compost back into beds as needed.  This greatly minimizes labor for making and spreading compost.

ERIC KOPEREK = erickoperek@gmail.com

end comment

What about persistent systemic neonicitinoid pesticides? I'm in the same boat, would love to grab this stuff, but that is the one set of compounds that give me pause. They are persistent, dangerous (to insects) and widely used in the nursery industry in the USA.

Gilbert Fritz wrote:What about persistent systemic neonicitinoid pesticides?  

https://en.wikipedia.org/wiki/Neonicotinoid wrote:As of 2013 neonicotinoids have been used In the U.S. on about 95 percent of corn and canola crops, the majority of cotton, sorghum, and sugar beets and about half of all soybeans. They have been used on the vast majority of fruit and vegetables, including apples, cherries, peaches, oranges, berries, leafy greens, tomatoes, and potatoes, to cereal grains, rice, nuts, and wine grapes.

Seems to me, like the bigger danger than having them in compost, would be eating any food from The Corporation.

Wow! Thanks, Eric, for the short course in building soil on rock! I'm already applying many of those techniques and principles, but I'll be bolder in the future. I had really never considered visiting the local cemeteries...

I do share Gilbert's concern about persistent pesticides, but the owner assured me that they use only spot treatments, and the ratio of potting mix to plant matter indicates that very few pots with larger plants have been tossed on the pile.

So I'm going to assume that any pesticides are quite diluted by the uncontaminated materials in their pile. Also, most of the pile has been there for several seasons already and is completely unprotected from the elements, so the effects of time, rain, snow-melt, sunshine and some minimal microbial activity should have further diluted any pesticide residues. And then, in our compost/lasagna/sheet mulch beds, we have so much other organic material that any residue will be even further diluted. We add wood chips to all the beds, using chips that have smoking hot  fungal/bacterial activity going (literally, we have to wear dust masks to avoid toxic organic dust syndrome!!).

So, all things considered, I think we'll soon reduce the garden center's pile significantly. We've already picked up three trailer loads. The owner is delighted to see the pile shrinking and we're delighted to see our garden beds growing!

TO:  Heidi Hoff
FROM:  Eric Koperek = erickoperek@gmail.com
SUBJECT:  "Smoking Hot Fungal/Bacterial Activity"
DATE:  PM 3:46 Thursday 8 September 2016
TEXT:

(1)  WARNING!  "Smoking hot fungal/bacterial activity" in your wood chip piles is DANGEROUS = BAD FOR PLANTS!  THIS MATERIAL IS TOXIC AND WILL RETARD CROP GROWTH OR KILL PLANTS OUTRIGHT.

(2)  Pull apart wood chip piles IMMEDIATELY.  HOSE down wood chips generously then TURN VIGOROUSLY TO AERATE WOOD CHIPS.

(3)  Turn wood chip piles every 2 or 3 days to maintain HIGH OXYGEN LEVELS.

(4)  Alternatively, spread wood chips 1 foot = 12 inches deep over soil surface and SHEET COMPOST = COLD COMPOST at ambient (air) temperature.

(5)  Do NOT pile wood chips unless you are willing and able to TURN PILE FREQUENTLY = every 2 or 3 days until primary decomposition is completed in 31 days.  Keep wood chip pile CONSTANTLY MOIST during this critical period.

(6)  For best results, INOCULATE WOOD CHIPS WITH COMPOST OR TOPSOIL to speed decomposition.  Spread 1/4 inch of topsoil or compost for every 6 inch layer of MOIST wood chips then mix thoroughly.  You can use more "seed" material at your discretion.  For example, mix equal parts by volume of topsoil and wood chips to obtain maximum composting rate.

(7)  Be very careful = cautious = conservative when tending compost piles.  Otherwise, you can propagate the wrong "critters" --  then, you will stand around wondering why a whole bed of tomatoes died.

(  Spread wood chips in paths between raised beds.  Make certain that wood chip DEPTH DOES NOT EXCEED 12 INCHES.  Chips will cold compost slowly without further attention.  If you wish to speed decomposition, add some extra nitrogen.  Keep wood chips constantly moist.

(9)  Fungal decomposition of wood wastes is best accomplished at relatively low temperatures.  Ambient air temperature is most convenient because cold composting does not require human intervention = less work for you.

(10)  Think like a fungus.  Fungus likes COOL and MOIST.  You can work horticultural miracles simply by keeping soil CONSTANTLY MOIST (not wet or saturated).  Constant moisture, excellent aeration, and moderate temperatures favor ENORMOUS growth of fungus and plant roots.  You can double or triple crop yields by very careful irrigation.  Equally true, you can destroy entire fields by over watering.  This requires close management and a highly accurate, very low flow irrigation system.  Conventional drip irrigation is too much.  You want "seep" not drip.

(11)  Be very careful how much "dirt" especially clay that you put into your raised beds.  Clay or silt exceeding 10% will greatly decrease aeration and drainage.  Equally important, don't chop plant materials too finely.  1 or 2 inch pieces are ideal.  Anything smaller restricts aeration.  Remember that plant roots need oxygen in order to absorb water and nutrients.  Most agricultural soils are oxygen deficient.  This is one reason why raised beds are so productive; eliminating soil compaction increases soil oxygen which stimulates root growth.  There is a reason why many commercial greenhouse owners have switched to composted hardwood bark potting soils; the texture is amazingly loose so roots get maximum possible oxygen.

ERIC KOPEREK = erickoperek@gmail.com

end comment

What toxins exactly will be created in a hot pile of wood chips?

Seems to me that a pile initially gets hot as the included green leaves start breaking down, the cools to a slow fungal crawl.

The Jean Paine method is predicated on hot piles of wood chips.

In fact, every pile of freshly chipped wood I've ever seen has heated up within a day or so, due to the fresh sugars and proteins on the surface of the chips, and the leaves, then cooled off.

Maybe we have a language misinterpretation going on? I'm guessing Heidi's pile is not actually "smoking."

Thanks for all the timely warnings, Eric. We had learned (the hard way) that those hot wood chips were toxic for us, but I didn't count on them being toxic for plants as well. To be clear, we had originally simply "closed" our new raised compost beds with a layer of these wood chips, with the intention of having the wood chips serve as protective layer over the fall and winter months, letting the microbial activity seep into the bed over the next few months. It was only after we had all the beds covered in this way that we learned that we could use the material from the garden center. At that point, the beds had all sat for one to three weeks with their wood-chip covering. We then added several inches of the greenhouse waste and gave the beds a light mixing with a pitchfork (mixing in place, not turning the piles). And now we are in the process of covering them once again with a thin layer of wood chips. We are using fresher chips now, that are not fermenting at all.

In light of the information you provided, I hope we did not go too wrong with all this. We do water our compost beds during and after establishment. We water now and then if we have an extended dry spell, but in our cool coastal region in Quebec, we typically get a good rain once or twice a week. Right now, we're getting a deep soaking rain that is more than we need, but the piles will all be thoroughly wetted. They are perpendicular to a slope, so they catch some water from uphill, but they also easily drain the excess downhill. Also, almost all the beds contain a fair amount of quite rough, freshly scythed plant material (green and dry stems), specifically to provide plenty of air channels.

I guess we'll find out how much luck we have in the spring! I may sow some clover on the piles this weekend, if you think that would be a good strategy this late in the season (first frost is in about a month, 6 weeks).

Now, if a huge pile of compacted wood chips is left for a while, it will go anaerobic and generate lactic acid and other problematic compounds. But a hot compost pile of wood chips will probably have plenty of air in it.

And yes, spores can be toxic to people; don't breath too many.

Yep, my husband learned that the hard way, Gilbert. He's got a touch of asthma anyway, so he ended up with bronchitis-like symptoms, with a bit of a fever, for about four days. I made him wear a mask the next time, but he did not keep it on the whole time he was working with that pile. Another three or four days of misery. Now I do the wood chip work, with a mask. No problems.

For anyone who comes across this later, here's an early reference on an incident investigation: organic dust toxic syndrome

Heidi,

I deal with a lot of wood chips. Often times, the powdery moldy stage is followed by the damp blackish stage relatively soon, at least with my piles. I try to just wait.

I've also tried hosing down the piles, but this does not work. Each pile can absorb so much water that a hose doesn't make a dent.

I'm glad other people are catching on to this potential hazard!

TO:  Heidi Hoff & Gilbert Fritz
FROM:  Eric Koperek = erickoperek@gmail.com
SUBJECT:  Toxins in Hot Composted Wood Chips
DATE:  PM 7:33 Tuesday 20 September 2016
TEXT:

(1)  Get a load of wood chips from a tree trimming company.  Let the pile sit for several days or weeks until it gets hot.  Dig into the pile and you will most often see a white or gray-white (gris-blanc), powdery, stringy mess.  Surprise!  What looks like fungus is ACTINOBACTERIA = facultative anaerobes that appear when compost piles turn ANAEROBIC = oxygen deficient.  Anaerobic compost is BAD compost = it contains all sorts of "critters" that you don't want around plant roots.  Actinobacteria are especially harmful because they produce natural toxins = antibiotics that kill fungi, especially symbiotic mycorrhizal fungi.

(2)  When you see actinobacteria it is a sure sign that a COMPOST PILE IS OUT OF CONTROL = severely oxygen deficient.  Turn compost pile IMMEDIATELY and keep turning every 2 days (or more frequently) every time the temperature spikes.  Translation:  Turn compost when thermometer reads 160 degrees Fahrenheit.  When in doubt, turn sooner rather than later.  High temperatures rapidly deplete oxygen so your compost pile becomes anaerobic overnight.

(3)  Anaerobic compost filled with actinobacteria can kill entire greenhouses full of plants.  It can also destroy crops covering raised beds hundreds of meters in length.  I grow vegetables for a living.  I know what financial havoc toxic wood chips create.

(4)  To safely decompose wood chips mix equal parts chips and manure by volume then compost 31 days or until temperature drops to ambient levels.  Monitor compost temperature constantly and turn frequently = any time temperature exceeds 131 degrees Fahrenheit but not higher than 170 degrees Fahrenheit.  

(5)  Alternatively, mix equal volumes of wood chips and manure then spread in a shallow layer 1 foot = 12 inches deep.  This is a form of cold decomposition called "sheet composting".

(6)  If manure is not available or too costly use any convenient nitrogen source in the following proportion:  1% AVAILABLE nitrogen by wood chip WEIGHT.  Thus, 1 ton = 2,000 pounds of wood chips require NOT LESS THAN 20 pounds of available nitrogen for composting.  When in doubt add more nitrogen.

(7)  Wood chips require large amounts of water for composting.  Irrigate compost every time pile is turned.  In dry weather you might have to water compost daily.  For most effective composting keep wood chips constantly moist.

(  To dispose "Bad" compost mix with equal or greater parts by volume of fresh material and re-compost  -- or --  spread "bad" compost in paths between raised beds or greenhouse benches and let "ripen" 6 months or longer.  Alternatively, throw actinobacteria-infested wood chips into cattle pen or chicken yard and let rot at least 6 months.

ERIC KOPEREK = erickoperek@gmail.com

end comment

Eric;

How could a loosely stacked wood chip pile go aerobic? Especially since the powdery layer is often near the surface.

Chip piles seem better areated then normal compost piles.

Here is from the extension service:

We had a Douglas fir cut down recently since it appeared dead. We kept the wood chips, plus some chips that were in the chipper's wagon from a previous job. Today we were about to spread the fiber and found mold close to the top of the pile and maybe throughout. Can we use it without transferring disease or mold throughout our yard. Is it safe to inhale it? Will it dry out and be useable? What do we do? Thank you for your help on this.

Lane County Oregon mulch wood chips

2 Responses

The mold is normal on rotting wood. It will not be a problem for any of your plants, but you should probably wear a mask when spreading it, especially if the material is dry, to avoid inhalation. Also, do not pile wood chip mulches next to your house's structure.

Clearly there is an art and a science to using wood chips that goes far beyond "wood chips = good." In our case, the human health effects of a pile gone hot are bad enough to avoid repeating the experience. In the past, we had inadvertently done as Eric describes: used the chips promptly or let the pile go for over a year without messing with it. The chips in the neglected pile were used for paths (over cardboard), and the spot where the chips were piled was turned into a garden.

As I had not really planned on that compacted clay spot becoming a garden, I did not do much to it. I just broadforked it, spread the remaining wood chips, and broadcast peas all over it, then seeded some squash and corn. I'm guessing that you already know the results: the peas were fine, the corn and squash are pretty miserable. Between the clay, the damage done by "toxic" wood chips and other disadvantages of the spot (shade, proximity to the road and road salt), it will take a lot of reconditioning (and better planning of what I plant there) to make that spot productive. First step will be to sheet mulch it with everything that is currently growing there, other organic matter and, yes, a layer of wood chips.

Gilbert Fritz wrote:Heidi,

I deal with a lot of wood chips. Often times, the powdery moldy stage is followed by the damp blackish stage relatively soon, at least with my piles. I try to just wait.

I've also tried hosing down the piles, but this does not work. Each pile can absorb so much water that a hose doesn't make a dent.

I'm glad other people are catching on to this potential hazard!

^^^ THIS ^^^


I've noticed that if I don't move a load of chips within the first couple of days, they can become moldy, and that stuff is pretty toxic to breath.  If you get a really wet pile, all the worse.  As Eric stated, if you spread them out to a foot thick or so, they don't seem to develop that black mold and then you aren't dealing with breathing the spores when you move them around.

If there is a steam cloud coming off your fresh chip pile, don't breath it.  You'd be smart to wear a resperator mask.  In effect, the team becomes the conveyor to carry those spores from the pile right up to your nose and mouth.  

Like almost anything else in life, too much of a good thing can quickly become a bad thing.  Wood chips = good.   A large pile of moist and molding wood chips = not so good.  A thick layer of chips under your trees and in your pathways or between your raised beds = very good.  As Eric stated, keeping them consistently moist = very very good, as widespread fungal growth in your chip mulch layer is garden gold goodness.