Woodchip myths or facts?

[face cord]

Hi trying to find answers to a few questions about woodchip as a mulch or soil improver as have a load of leylandi woodchip to use on vegtable garden round trees etc .

 

Do fresh woodchip rob alot of nitrogen from soil as it decomposes, any proper scientific research trials on this as heard some people say its abit of a myth & others say you shouldn't use it because of this?

 

If so where does the N end up, does it get incorporated into the woodchip so eventually goes back into the soil?

 

Leylandi chip looks a large part is the green leaves so surely it has quite high nitrogen in the C:N ratio in it compared to all wood woodchip anyway?

 

Will it make the soil acidic as leylandi are conifers? Seem to be various opinions on this also. Maybe worth liming soil before using it?

[nepia]

Hi trying to find answers to a few questions about woodchip as a mulch or soil improver as have a load of leylandi woodchip to use on vegtable garden round trees etc .

 

Do fresh woodchip rob alot of nitrogen from soil as it decomposes, any proper scientific research trials on this as heard some people say its abit of a myth & others say you shouldn't use it because of this?

Yes; it's called nitrogen robbery.

 

If so where does the N end up, does it get incorporated into the woodchip so eventually goes back into the soil?

Yes.

 

Leylandi chip looks a large part is the green leaves so surely it has quite high nitrogen in the C:N ratio in it compared to all wood woodchip anyway?

Maybe; don't know that one.

 

Will it make the soil acidic as leylandi are conifers? Seem to be various opinions on this also. Maybe worth liming soil before using it?

Not for any length of time if at all: soil reverts to type quickly if the artificial pH treatment isn't maintained

 

 

Years ago I had a local tree surgeon chip up a dozen 18' leylands for me so I could put the chip on a large newly dug flower bed. I applied it at ~8" thick in early November and by the end of March you could barely tell anything had been put on the soil surface but the texture of the topsoil made it almost good enough to eat!

[agg221]

I'm with Jon on this one - although for preference stack it for a year before applying. That way you get the benefits without the nitrogen robbery.

 

It's a particular range of conditions which result in this. You need the right ratio of carbon to nitrogen for efficient decomposition. To make up the difference, the decomposition organisms take the balance from the surrounding soil. Grass cuttings have too much nitrogen, wood chip has too little (oak leaves are about perfect!) - no specific idea on leylandii chippings but I would guess they wouldn't be too bad.

 

The big advantage of chippings appears to be in soil structure. It adds moisture holding capacity without waterlogging, which is good for both heavy and light soils. It encourages worms, which helps with aeration, and of course it suppresses weeds.

 

There will of course be some direct nutrient addition, but I think this is probably secondary.

 

In truth, the breakdown rate of pure wood/bark chippings is so slow that the benefits of weed suppression and soil structure improvement probably outweigh any minor robbing, but if you need to mix in the green stuff I would let it stand for a while.

 

I do a lot with oak and used to save up the adzings/shavings/sawdust in a pile through the year, then when the leaves fell I'd blow them up into a heap and throw the leaves and oak bits through the shredder together, then stick the lot in builder's bags. By about this time of year I'd have brilliant compost.

 

Alec

[Bob Morris]

A bit more reading on the subject. I think it covers most of the questions from OP.

 

Cheers

Bob

 

If the link does not work just Google Lynda Chalker Scott at WSU

 

http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/magazine%20pdfs/Woodchips.pdf

[Pumpy]

From "Teaming with Microbes - Jeff Lowenfels & Wayne Lewis 2nd edition" (fantastic book)

 

"If there is abundant Carbon in mulch but not much nitrogen, or a ratio of 30:1 or greater, then the decaying microbes use up the nitrogen in the mulch and, once that is gone, will take nitrogen from the soils touching the mulch.

People make a big deal of this nitrogen 'robbing', but it usually only occurs at the thin interface between the soil and the mulch. Although it has a real impact there, it usually doesn't affect the rhizosphere or the bacteria and fungi that reside there.

 

Still, there is no reason to court problems. Experience has taught us that the chances nitrogen will be immobilised in the soils under wood chip mulch can be reduced by making sure the chips are 3/8 inch or larger. This prevents much of the bacterial colonisation you would see in smaller wood chips, and -- where mulches are concerned-- it is primarily the bacteria that tie up the surrounding soils."

 

Seems the idea behind aerobic bacteria dominant/even ratio/fungal dominant soils is down to evolution and how certain plants prefer their Nitrogen in Nitrite form, and some prefer it in the Ammonia form. Most veggies and annuals prefer bacteria dominated soils (nitrite), most grasses prefer an even ratio, most perennials shrubs and trees prefer fungal dominated soil (ammonia).

If you look at a succession of plants from sea shore to mature forest in an evolutionary sense, those at the shore end of the scale prefer more bacterial dominated soil (nitrites), move through the prairies where it is about even, to wood boundary where more fungal dominance is preferred, into mature woodland where a heavily fungal domination is required. But it's not just the preference for a particular form of N, it's also the soil biota the plant has evolved in, it is assumed all plants form symbiotic relationships with at least one form of soil biota, so to get the best from the soil, a plants symbiotic partners must first be present, and also have the correct environment.

 

If you were a shrub with a symbiotic Mycorrhizal fungi, sat in a bacteria dominated soil, your "sole mate" may have been out competed for food by the high bacteria ratio. Conversely a cabbage sat in a fungal dominated soil may find most of the Nitrogen in ammonia form, so will not perform as well as it would sat in the soil biota it prefers, it may not be able to fend off pathogens as well as it might etc.

 

So you would tailor your mulches and aerated compost teas towards the preference of the specific plants. Veggies would be better mulched with a higher ratio of greens than the forest garden, which would be better to mulch with a higher ratio of browns. Mulches dug in a bit will support a bacteria dominant soil, woody/brown dominant mulches left on top will suit fungal dominant soils, taking into account the ratio of greens/browns for preference of the plants.

 

I can't even begin to go into the details of the book, far too much info to do it justice.

 

for more specific (academic) research on woodchip mulch use the term "Ramial chipped wood" (RCW) here's an example Regenerating Soils with Ramial Chipped Wood

 

The problem with conifers/pines is the waxy coating on the leaves/needles takes an awful long time to decompose, I tend to look at it as a "stability" issue, until there is a layer of humus formed at the soil/mulch interface the potential for N robbery is high (unstable) so I would compost pine chip before using it as mulch.

 

There is a good online film all about growing with woodchip, look for "Back to Eden" (if you can get over the heavy Christian promotion in it)

 

HTH.

Edited March 31, 2013 by Pumpy

[Peter]

I do a fair bit of composting wood chip in my garden. All the above info is excellent, the only thing I would add is that you need lignin (woody stuff) to make humus, the rich dark part of the soil that plants love. Lignin needs lots of nitrogen to feed the bacteria to break it down, so adding green (grass clippings etc) or artificial nitrogen fertiliser is a must.

 

You can easily check the pH of your mulch before you use it, if you want to bring it up a bit add garden lime.