Ask a Question forum: Sawdust on raspberries for mulch?

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Name: Reid
North Branch, MN (Zone 4b)
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Anderwood
May 10, 2015 3:59 PM CST
How would mulching raspberries with saw dust affect their growth and production?

Name: Rick R.
near Minneapolis, MN zone 4a
I was one of the first 300 contributors to the plant database! Garden Sages The WITWIT Badge Garden Photography Region: Minnesota Plant Identifier
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Leftwood
May 10, 2015 9:09 PM CST
Because of the enormous surface area of sawdust compared to coarse wood mulches, this provides a huge working area for microorganism to get busy breaking down the wood. But while they are working, they need nitrogen for the process, and they will suck up it up from the soil, using it for their purposes, and very little will be available for any plants. (So much so that you can hear the sucking noise... just kidding! Hilarious! ) When the sawdust is done composting, the locked up nitrogen will be released back into the soil. But how long that takes will depend on how much nitrogen is available at the start.

When I was child, in northern Minnesota we use to play around huge sawdust piles from my Granddad's sawmill. They were already decades old, always moist, yet never decayed. There wasn't any nitrogen available for the composting process. Nothing grew in those sawdust piles, not even weeds.

So using sawdust, even with added nitrogen fertilizer, will be a bumpy ride, because estimating how much nitrogen you need is a crap shoot. In the end, you'll be fine; but in between, I don't think so. Best to use the sawdust in a compost pile with ample green material as a nitrogen source.
Name: Tiffany
Opp, AL (Zone 8b)
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purpleinopp
May 11, 2015 12:13 PM CST
Has this spot been mulched with woody material before? If not, I'd be more hesitant as well. With sawdust or any freshly shredded wood, the nitrogen robbing described above can happen in a spot that hasn't yet established a population of microbes that decompose woody material. But one has to start with something, sometime...

DH brings sawdust home from work occasionally, but I've never used it as a first or only mulch material. It's spread/slung around beds where decomposition of a myriad of organic matter is already happening on the surface.

If you're able to add grass from mower bag, equal amount of grass or more, I would be confident about it going well. You would want to mow before grass (or other lawn plants) has seed heads. Add more to the surface if it disappears while the sawdust still looks fresh (light in color.) Another option for mixing with the sawdust would be moist produce scraps. Pureรฉ first if you don't want to recognize the bits (which would also greatly speed the decomposition process, and aid in mixing with sawdust.)

If you choose to use the plain sawdust with nothing added, adding a few successive additions of thinner layers might be your best bet, vs. adding everything you have in one thicker layer. More than an inch or so of any single kind of OM at a single time can form into an at least temporarily unproductive mat, that can sometimes shed water instead of absorb it, until the decomposition process gets going well.

I try to maintain a cover of constantly decomposing stuff, from compost to leaves, to grass clippings, whatever presents itself that's OK to look at in a given spot (or sometimes slightly buried if not.) Mulch doesn't have to come from a bag at all.

๐Ÿ‘€๐Ÿ˜๐Ÿ˜‚ - SMILE! -โ˜บ๐Ÿ˜Žโ˜ปโ˜ฎ๐Ÿ‘ŒโœŒโˆžโ˜ฏ๐Ÿฃ๐Ÿฆ๐Ÿ”๐Ÿ๐Ÿฏ๐Ÿพ
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โ˜•๐Ÿ‘“ The only way to succeed is to try.
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
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RickCorey
May 14, 2015 12:20 PM CST
I think that nitrogen deficit can only affect the plants much if the sawdust is mixed into the soil. If they are mixed together, the microbes eating the sawdust can steal N from the soil they are mixed with.

If the sawdust is only laid ON TOP OF the soil, as a top-dressed mulch, I think that the only place the N deficit can occur is in the top few millimeters of soil - the interface between N-starved sawdust and C-hungry soil. I don't think that microbes can reach out thousands of times farther than their own size to steal N from soil an inch away.

What would you say, Rick R? Is there any N deficit with top-dressing mulches, a little, a lot, or what? I assume we agree that sawdust mixed with soil causes more N deficit (like, a huge amount) than it does if layered on top.

I guess someone would have to do a study to be sure that fungi with long mycelia or hyphae can't digest cellulose at one end of the hyphae, and steal N with the other end, then transport energy compounds down and N up. My belief is that acti8ve internal transport like that is very "expensive" for a fungus to perform, so not very much of it happens.

But I've read many times that wood CHIPS cause N deficit only if mixed INTO soil (and seen it happen myself). A top-dressing of lots of wood CHIPS doesn't cause N deficit. Or so I've read in multiple places.

If a top-dressing of saw DUST were to decompose very fast (for example, if dusted with hydrated lime or maybe if dusted with some N source), organic compounds might leach downwards with rain. If enough of those leached into soil that was already N-starved, it might make enough difference to be detectable.

My general approach is to top-dress with wood CHIPS, not sawdust, so that air and rainwater can perk downwards unobstructed. Also, I can easily rake away a mulch of wood CHIPS when I need to. And few weeds will root in fast-draining wood CHIPS.

But any weed would love to root in sawdust, IF it can reach any N with its roots before it starves.

P.S. It might help avoid top-dress-N-deficit if you add any N fertilizer into the soil, and mix it in, BEFORE adding sawdust on top. If you add N on top of a top-dressing, the top-dressing is likely to catch and hold lots of the N, and accelerate the growth of anything breaking down the sawdust.

Name: Rick R.
near Minneapolis, MN zone 4a
I was one of the first 300 contributors to the plant database! Garden Sages The WITWIT Badge Garden Photography Region: Minnesota Plant Identifier
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Leftwood
May 14, 2015 6:20 PM CST
There is little to argue about with your statements, RickCorey. Of course, sawdust mixed into the soil has a "thousand" times greater effect. Also a "thousand" times more temporary (and the much much smaller effect of the top mulch a "thousand" times longer lasting) . Due to the fantastic surface area of sawdust compared to wood chips, the "interface" between sawdust mulch and soil is a "thousand" times greater, and is in fact a "thousand" times more blurred. Such an interface is thus more of a transition zone, rather than a line of demarcation, as with wood chips. Such a zone has more ramifications than the simple (and logical) physical aspects of your interface scenario.

If soil microorganisms weren't so darn efficient compared to plant roots in gobbling up nutrients, the admittedly small physical reach of nitrogen sucking microorganisms would have little effect, long or short term. But because of the "thousand" times effect (I can't say if it's really a thousand), natural sources of adding nitrogen to the soil(that occurs with other mulches) is essentially void, resulting in an increasing nitrogen deficiency as plants struggle to remove the remaining soluble nitrates. Remember too, that a portion of these nitrates move down the soil column with the drainage of excess moisture, and out of the reach of roots. The soil becomes increasingly nitrogen deficient, though not in the way you might expect.

As for your other tangential statements(which I generally agree with), If you could explain the relevance to the subject at hand, I'd be happy to engage..... Or, perhaps they were just comments.....
---- But any weed would love to root in sawdust, IF it can reach any N with its roots before it starves.

---- But I've read many times that wood CHIPS cause N deficit only if mixed INTO soil (and seen it happen myself). A top-dressing of lots of wood CHIPS doesn't cause N deficit. Or so I've read in multiple places.

---- C [carbon]-hungry soil

---- I guess someone would have to do a study to be sure that fungi with long mycelia or hyphae can't digest cellulose at one end of the hyphae, and steal N with the other end, then transport energy compounds down and N up. My belief is that active internal transport like that is very "expensive" for a fungus to perform, so not very much of it happens.
I'm not aware of any internal "pumps" that require direct energy input to work. The mechanism(s) are there or not, and work as needed on their own. Indeed hyphae do transport nitrogen (and other compounds) regularly, but for your stated purpose, I have no idea.
[Last edited by Leftwood - May 14, 2015 7:09 PM (+)]
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