Why does ash alkalinize the soil, when wood chips do not?

It is well known that ash is strongly alkaline; why is this so, when the wood or other material it is derived from is not?

If I spread a foot of wood chips over a piece of land, it wouldn't alkalinize it; if anything, it might have a slight acidifying effect. Over time, the  carbon in the wood chips will escape into the air as carbon dioxide, and the minerals (calcium, potassium, etc.) will be left in the soil.

If I burned that same foot of woodchips to ash, the carbon would burn off as carbon dioxide, leaving an ash of minerals; when this ash is applied, it alkalinizes the soil.

The eventual outcome seems to be the same; decomposition is just slow burning. Why does one route alkalinize the soil, and the other does not?

Two related questions: is this because, when people apply ash, they are applying a far higher level of these minerals than they would feasibly be able to apply in wood chips?

Over time, will the alkalinizing effects of the ashes be neutralized in the soil, returning the pH back to what it would have been if the wood was applied without burning?

While I don't actually know as such, here's my best guess: it depends which specific salt the minerals are part of at the moment. For instance, potassium in ash is in the form of potassium carbonate (alkaline) or if the fire was very hot, potassium hydroxide (very alkaline). The corresponding acid of potassium carbonate is carbonic acid, a very weak acid. The corresponding acid of potassium hydroxide is water, which is not really an acid at all. If there are stronger acids (like carboxylic acids) present, the potassium will be found in the form of less alkaline salts. Wood probably contains quite a lot of various carboxylic (and other) acids, so in wood, the potassium will not be very alkaline at all, and the metabolism of the fungi and bacteria decomposing a pile of wood chips will produce a whole bunch more organic acids. Fire, on the other hand, burns away the vast majority of the carbon in the wood, including all the carboxylic acids, leaving the more alkaline salts.

Wood undergoes a chemical reaction when it is burned. Potassium in the wood combines with oxygen and carbon during the burn to produce potassium carbonate, which is left in the ashes along with other minerals, predominantly calcium. Wood ash can be up to 30% calcium. Carbonates neutralize acids, raising ph. When ash is added to the soil, the carbonates will neutralize acids in the soil, but also the calcium in the ash raises ph as well. Sandy acid soils like ours here in New Hampshire always have a calcium deficiency and wood ash is a valuable resource for mitigating this.

Some research needed, but my brain storage collection of "useful" info indicates that wood chips are fine for paths and established trees with good root systems, but not good for mulching veggies, as their breakdown can produce a nitrogen imbalance.

This is an interesting question. My understanding is that organic matters are burnt off so wood ash is composed of inorganic minerals. The alkalinity is mostly from K2CO3 or pot ash, as it is a salt of strong base and weak acid. CaCO3 is alkaline too but of much lower solubility.

When wood chips get decomposed by microbes, the metabolism products are acidic, and Ca and K ions are either incorporated in the microorganisms, precipitated(Ca), bound to soil particles or organic matters. It is also a much slower process so there won't be high concentration of Ca2+ or K+ to ionize soil water as in wood ash solutions.

I have been pondering this a lot as we are getting lots of different kinds of mulch
1. From leguminous trees that have lots of green matter still attached, this seems fine for the veges.
2. From hardwood trees like African mahogany which some people say is allopathic.
3. I read some where recent that wood ash should be mixed with vinegar and left for a few days to enhance its qualities
Any comments would be appreciated

Welcome to the forum, Wendy!

There is a lot that can be said on this subject.

And like many subjects, it depends.  What kind of wood chips?

Maybe putting fire to wood changes its structure.

Here is a similar thread:

https://permies.com/t/75809/woodchips-acidify-soil

Jill Dyer wrote:Some research needed, but my brain storage collection of "useful" info indicates that wood chips are fine for paths and established trees with good root systems, but not good for mulching veggies, as their breakdown can produce a nitrogen imbalance.

"Nitrogen imbalance" as in excess or shortage? IE- would a nitrogen addition facilitate more healthy composting and make their addition beneficial?

This has been an interesting read. I often don't give enough consideration to the science behind things, so it's good to have my thoughts venture off down this road.

We have rather sandy acidic soil, definitely lacking Ca and K. River floodplain in a semi arid environment. We also have a dense stand of invasive acacia trees. Nitrogen fixers, not bad firewood, but way more than we need and would love to manage and utilise them better, and use the much better quality soil underneath them that they've helped create.

We've chopped down a lot, sell and use firewood and chip the small branches. Chips cover roads and pathways, random bare patches left over from flood damage, and also the floors of our animal night shelters, along with straw.

We have refrained from burning a lot of dry biomass from previous felling (before our time) as we have slowly been chipping it and because fires can get pretty risky in our climate/environment, and also because in my mind it's not the best way to clear land if you're trying to build soil.

But, in our situation, perhaps there is value to adding a good amount of ash to the sandy fields we use for pasture, rather than trying to chip it all, even though chips can slowly build pretty good soil, as we've seen on the forest floor under the trees themselves. I'm all for amending things faster if possible!

What would you all do in this situation? Maybe I should start a new thread?

Coydon Wallham wrote:
"Nitrogen imbalance" as in excess or shortage?

Since wood chips are mostly wood (lignin) and are mostly carbon they lack nitrogen when it comes to composting or breaking down. For compost a ratio of roughly 30:1 C:N is ideal. Adding nitrogen rich material (IE manure) can help speed up the break down and prevent the "locking" of nutrients.

Hey Ruth,
I think a new thread could be nice and help you get a lot of input.
I think you're on the right path composting instead of burning. Especially mixed with animal manure you should be making good, rich compost.
I'm sure other permies will have a lot of ideas to help accelerate the process of soil building.

Benjamin Dinkel wrote:Since wood chips are mostly wood (lignin) and are mostly carbon they lack nitrogen when it comes to composting or breaking down. For compost a ratio of roughly 30:1 C:N is ideal. Adding nitrogen rich material (IE manure) can help speed up the break down and prevent the "locking" of nutrients.

I wasn't sure if the consistency somehow affected the process. I've taken my SKIP BB to heart and have been using urine to inoculate the chips around my saplings. Don't know if it would be practical to calculate an actual ratio, but figure that enough to saturate the chips without much draining through, 2 or 3 times a week would be the best bet...

Benjamin Dinkel wrote:

Coydon Wallham wrote:
"Nitrogen imbalance" as in excess or shortage?

Since wood chips are mostly wood (lignin) and are mostly carbon they lack nitrogen when it comes to composting or breaking down. For compost a ratio of roughly 30:1 C:N is ideal. Adding nitrogen rich material (IE manure) can help speed up the break down and prevent the "locking" of nutrients.

This is very much the case in a composting setting where a hefty dose of "green" material is required. It's also what happens in a situation where raw wood chips are mixed into soil and this is why the conventional wisdom always warns gardeners against using them. But a layer of chips on the surface will be colonised by fungi, which break down lignin without needing any N inputs and in doing so create a well-balanced addition to the topsoil.

Phil Stevens wrote:This is very much the case in a composting setting where a hefty dose of "green" material is required. It's also what happens in a situation where raw wood chips are mixed into soil and this is why the conventional wisdom always warns gardeners against using them. But a layer of chips on the surface will be colonised by fungi, which break down lignin without needing any N inputs and in doing so create a well-balanced addition to the topsoil.

So better to not add liquid nitrogen, or would both methods work out okay?

Woodchips mulch adds nitrogen to the soil through bacteria in the soil which causes decomposition.  It's is a very different substance from wood ash.

When woodchip is placed on the surface of the soil, the slow decomposition of the chips  begin forming a microbe-rich ecosystem, Elaine Ingham says this is part of the Soil Food Web. Dcomposition happens when microbial organisms  consuming the chips being to produced organisms high in nitrogen and other nutrients that eventually leaches into the soil.

Wood ask is not microbial, it's no longer a living produce. The wood ash question has been answerd.

Coydon Wallham wrote:So better to not add liquid nitrogen, or would both methods work out okay?

I think the takeaway is that wood chips on their own, as long as they stay on the surface and get broken down with fungi, become incredibly high-quality topsoil. You can always add other things, but I like to think of it in terms of a system where the deep mulch layer is your "fertility engine" and any tweaks you make are on the order of changing the fuel blend or swapping out a gearbox. The engine is doing the bulk of the work and you can always pee on the mulch or bury kitchen scraps in it to enhance the finished product (the healthy soil that's building up underneath).

Phil Stevens wrote:This is very much the case in a composting setting where a hefty dose of "green" material is required.

I'm wondering now if allowing the woodchips to decompose via a fungal route ends in a different pH soil amendment than if urine (or another high nitrogen substance) was added so composting the chips bacteriologically.
(another permie with acid soil and (now) trees for mulching)

Technical comments:
Wood is a composite material made from cellulose (long chains of
glucose, pure carbohydrate).  The cellulose fibrils are cross linked by a mesh of lignin.
The cellulose:lignin ratio is major determinant of decomposition rate.
Celery: no lignin
Poplar: some lignin
Oak: lots of lignin.
The lignin itself is complex and diverse across species; a swarm of cyclic structures that are cross linked.
The lignin rings are derived from amino acid metabolism but the N groups are mostly removed during lignin synthesis.

There are other carbohydrates, resins etc depending upon species.
During decomposition the most resistant fractions of the lignins transform to humins and humic acids which are stable and accumulate as soil carbon.
The carbohydrate fractions turn over to CO2 faster.
There is very little N in most wood, but some nitrogen fixing bacteria and directly or indirectly access  the wood carbohydrates to fuel fixation of N2 to available N compounds.
So adding woodchips will tend to cause a temporary drop  in available N as decomposers take up soil N to build biomass to breakdown the wood.
Others have answered the alkalinization question.
cheers Doug

If I understand correctly, younger tree branches generally have more N than older branches & mature trunk wood.  

This is where "ramial" chipped branch wood comes in: with a high enough N content that you don't get lock-up from the surrounding soil as they decompose.

The timber industry creates a lot of racial chips in some areas, hence it's popularity as a mulch.

The cut-off between 'low' & 'high' N content is said to roughly fall around 2-3 in diameter of branch, I believe?

I learned all this from the late Dave of Darlington, whose writings have now been published in collection by the Vegan Organic Network: https://veganorganic.net/2012/04/organic-box-scheme-pioneers-writings-published/

Nancy Reading wrote:
I'm wondering now if allowing the woodchips to decompose via a fungal route ends in a different pH soil amendment than if urine (or another high nitrogen substance) was added so composting the chips bacteriologically.
(another permie with acid soil and (now) trees for mulching)

The answer seems to be, "it depends?!

"Intriguingly, the competitive outcome appears to depend on the ambient pH: fungi prevail at low pH, bacteria at higher pH (Rousk, Brookes and Bââth 2010).

"This is salient in light of the major pH modification—typically lowering—that fungi effect in wood (de Boer et al. 2010)."

_Bacteria in decomposing wood and their interactions with wood-decay fungi_ Johnson, Boddy & Weightman 2016

They conclude: "the microbiota of dead wood remains a lively and underexplored area of ecological research"

Sarah R. Johnston, Lynne Boddy, Andrew J. Weightman, Bacteria in decomposing wood and their interactions with wood-decay fungi, FEMS Microbiology Ecology, Volume 92, Issue 11, November 2016, fiw179, https://doi.org/10.1093/femsec/fiw179
https://academic.oup.com/femsec/article/92/11/fiw179/2403112

Younger, smaller trunks amd branches have alot more bark, which is metabolically active and higher in N.