how can nitrogen be "tied up"

njitgradMay 21, 2013

I was considering putting down mini bark nuggets on my raised veggie beds as a mulch but I've been reading here and there that bark nuggets and wood chips tie up nitrogen.

What does tying up nitrogen really mean? If I don't mix the mulch into the soil (by using it only as a top layer) I won't have to worry about it breaking down. If I ever want to ammend my beds, I would simply rake the nuggets off, place them into a wheelbarrow, make my ammendments and reapply the nuggets.

Just (still) trying to figure out the best option for mulching my beds before the heat kicks in a few weeks from now.

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toxcrusadr Clay Soil(Zone 6a - MO)

Your plan will work fine. Wood only affects nitrogen if mixed into the soil. On top, it only contacts the very top surface and won't have a noticeable effect.

The tying-up is related to microbes growing and using nitrogen from their surroundings as they digest the high-carbon, low-nitrogen wood. Eventually when the microbes die off they release the N again so the effect is temporary.

    Bookmark   May 21, 2013 at 10:58AM
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^^^ Correct.

    Bookmark   May 21, 2013 at 11:13AM
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There are only two ways I know that N gets "tied up" in soils. The Tox-man covered one, the other is in the formation of humic/fulvic acid and humates in general, which are about 3% nitrogen by weight, and are pretty resistant to giving it up in the short term. Neither should be an issue for mulching. Put fresh wood chips into your soil, and then you get what Tox noted.

    Bookmark   May 21, 2013 at 11:46AM
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Leaves worked into the soil can do it as well. Been there, done that.


Here is a link that might be useful: N deficiency due to leaves

    Bookmark   May 21, 2013 at 7:07PM
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Any high carbon material worked in to the soil can cause a temporary "tie up" of available Nitrogen as the Soil Food Web uses that N to digest that high carbon material. However, that same high Carbon material laid on the soil surface as mulch will not cause the Soil Food Web to get that busy digesting it so very little available N is "tied up.
Numerous times I have mulched plants using fresh wood chips, years old wood chips, shredded bark, or shredded leaves and have not seen any indication of N deficiency in the plants but have seen those plants grow faster and greener then other, similar, plants that I did not mulch.

    Bookmark   May 22, 2013 at 12:00PM
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dchall_san_antonio(8 San Antonio)

Many years ago a guy here on this forum reported applying pounds and pounds of sawdust to his garden from his cabinet business. He said at first it took forever for the sawdust to decompose, but after 12 years the soil microbes could decompose it in just a few weeks.

Generally the wood rot fungus requires access to the air to decompose. When it does not get that, it will stop decomposing while it tries to gather nitrogen from all other sources.

    Bookmark   May 26, 2013 at 1:12AM
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so then is there a point to 'composting' wood chips?Obviously no one is making compost out of wood chips, but I mean, does it make a difference whether they are fresh from a saw mill or they have been in a large pile getting turned periodically? I have read that they need to be piled and slightly 'composted' before applying.

    Bookmark   May 26, 2013 at 3:25AM
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There is a point to composting any vegetative waste and that is to make more available the nutrients that waste contains. There are people that do compost wood chips, not many, but it can be done. The concept that wood chips need to be composted, or slightly composted, or will "tie up" Nitrogen in your soil comes from people using wood chips, or any high carbon material, as a soil additive and not as mulch.
As I stated above in my experience when used as a mulch, not a soil amendment, it does not matter whether the wood chips are fresh or have been aged the plants growing in the soil they mulch grow better because of the retained soil moisture that helps make necessary nutrients more readily avialable. if those same wood chips were worked into the soil, a soil amendment and not a mulch, then the Soil Food Web would gather all avialable resources, Nitrogen, to digest them tieing up the N plants need to grow temporarily until those wood chips have been digested, or the life span of the members of the SFW is complete at which time the N they consumed would again be available for other uses.

    Bookmark   May 26, 2013 at 6:24AM
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