Ash vs. biochar

I'm curious how close the two are to each other.

There's a kid in the community who brings me a combination of good stuff for my garden, including spent ash, goat manure, coconut hulls, and charcoal powder.

On initial observation, the charcoal powder is quite dark (unless mixed with our pale local clay), while the spent ash is more . . . Ashy. Fresh ash will be white, but it leaches the white color quickly and leaves behind a dark gray color.

But my question is the makeup. I understand that fresh wood ash consists of a large amount of potassium, and that the potassium leaches out when it rains. So what's left? Is it a form of biochar that's left over? How does spent ash differ from biochar (and from fresh ash), and how can it benefit (or not) the soil?

Priscilla Stilwell wrote:I'm curious how close the two are to each other.

There's a kid in the community who brings me a combination of good stuff for my garden, including spent ash, goat manure, coconut hulls, and charcoal powder.

On initial observation, the charcoal powder is quite dark (unless mixed with our pale local clay), while the spent ash is more . . . Ashy. Fresh ash will be white, but it leaches the white color quickly and leaves behind a dark gray color.

But my question is the makeup. I understand that fresh wood ash consists of a large amount of potassium, and that the potassium leaches out when it rains. So what's left? Is it a form of biochar that's left over? How does spent ash differ from biochar (and from fresh ash), and how can it benefit (or not) the soil?

My understanding was, as a fertilizer it has a NPK make up of 1-2-3. So 1 for the nitrogen, 2 for the phosphorous, and 3 for the potash. Not really "teeming" with potash, but higher of the 3 for sure. Here we use vast amounts of ash, but not as fertilizer, but as a lime alternative to get the PH level of the soil up.

My understanding of the difference is, ash does not last as long in the soil as biochar, but acts quicker. Biochar also absorbs moisture and nutrients better, as well as longer than ash.

Hi Priscilla.  They are completely different materials with different uses.  

The ash will have very little carbon in it, mostly mineral nutrients like Travis mentioned as well as silica.  It will act to increase soil pH and so you should be careful to not apply too much, depending on what kinds of plants you're growing and their pH preferences.

The biochar is usually about 90% carbon/10% oxygen and not only holds water and nutrients in the soil so they don't wash out, but also acts as both an electron donor and receiver.  Soil bacteria have evolved with biochar and dump and accept electrons from it to reduce the energy required for the redox reactions of their metabolic activity, making them more energy efficient.  Think of it a bit like soil humus that doesn't have to be built and maintained.  This means the plants don't have to spend as much energy feeding soil builders and can spend more of that energy growing and fruiting.

Yes, but my question is specifically about aged ash which has lost the majority of it's strongest nutrients.

Priscilla,

Ash is a mixture of a small amount of biochar (the exact proportion varies depending on the condition of combustion) with the rest being the left over dregs of combustion that simply will not burn.  The exact chemical makeup depends highly upon the condition of combustion (moisture, heat of combustion, air flow during combustion, length of combustion and on and on ad nauseum) AND the exact substance being burned.  Typically ash will have a bit of potassium, the only major nutrient of any significant value, and usually a fair amount of alkaline chemicals.  In days past, the ash was collected in barrels and water seeped through.  The water/ash slurry dripped our a small hole at the bottom, was collected.  This slurry was lye used for making soap.  This last anecdote might give an idea of the chemical makeup of wood ash.

Biochar on the other hand is a residual trace of ash along with a LOT of straight pure carbon in a very recalcitrant form (meaning if not set alight, it is extremely stable.  If this is mixed with water and/or soil, the char mixture is not going anywhere easily and is stable on the order of thousands of years).  To be more specific, CHAR is pure carbon left over from oxygen starved combustion.  BIOCHAR is the same char infused with moisture, biology, and often a variety of chemicals that have filtered through.  Optimally, the chemistry run/filtered through the biochar is optimized to support either/both plant grow and/or microbial growth.


Simple version might look something like this

Ash: some potassium and is alkaline

Char alone: no fertility, but does act like a sponge

Biochar:  char infused with biology and chemistry conducive to fertility

Hope this helps,

Eric

Kinda. But it basically tells me that aged ash, or ash with the potassium more or less spent (or what would have been discarded after making potash or lye) is nothing. That doesn't really work for me. Haha.

Perhaps it is unspent carbon, that would make sense. Maybe you said that in a less direct way? :)

Priscilla,

Unfortunately I can only give crude speculation regarding the chemical makeup of ash given I don’t know what wood (I am assuming it was once wood) and knowing exactly how it was burned.  This makes the question a little like asking what the composition of dirt is.

I wish I could be more specific.  Your last point is very likely correct in that at least some of the ash is going to be spent char.

Sorry I cannot be more specific,

Eric

No problem. I'm just speculating :)

Even leached ash has value. Keep in mind that ash is the non oxidized residue of organic material. And there are a number of minerals that don't oxidize off, escaping in the fumes. Plants incorporate small amounts of  minerals, some vital to the plant. By using leached ash, you will be putting those micro-nutrients back into the soil. So I wouldn't discard leached ash. I would gradually incorporate it into my garden soil.

Yes, I'm using it, mixed with my other like-textured amendments like goat manure and compost. I was just curious about the actual makeup and whether it would act at all similar to charcoal once the potassium is mostly leached out.

Priscilla Stilwell wrote:... fresh wood ash consists of a large amount of potassium, and that the potassium leaches out when it rains. ...

Note the term "spent lime" as used in this 2004 paper on CA Apple storage:  
http://nyshs.org/wp-content/uploads/2016/10/Carbon-Dioxide-Control-in-Apple-CA-Storages-Using-Hydrated-Lime.pdf

I believe the term "spent ash", like the term "spent lime" in the linked article, means the causticity has been neutralized, typically by a natural aging process whereby OH and O oxides of calcium and magnesium (and in the case of spent ash, potassium) convert to CO carbonates on exposure to atmospheric humidity and carbon dioxide. Another term that applies is mineral carbon fixation, in that atmospheric carbon is removed in the "spending" process. You can make soap using fresh ash (pH 12 or more) in place of lye, but you can't use spent ash (pH 9 or less) because the original fresh kilned causticity has been partly neutralized.

Spent ash stored outside is likely to lose potassium (K) to leaching prior to losing calcium and magnesium. At pH 9 potassium carbonate is soluble in water, calcium and magnesium carbonate is only slightly soluble. As ash pH decreases further with time, calcium (Ca) and magnesium (Mg) become more readily soluble. Among the other mineral nutrients, you can expect P, Fe, and Mn to resist leaching, and expect B, Cl, and S to be more like K: susceptible to leaching loss during storage.

edit: corrected term

That's essentially what I was hoping for. It would, in theory, be less prone to burning plants or having other averse side effects. It seems spent ash is quite easy to come by here since every house cooks over wood or charcoal. The ash is dumped in a pile and left for the most part. I was hoping to have some positive uses for the stuff in the garden other than essentially just a filler.