Sandy acidic soils with low CEC in high rainfall areas have low Ca. Presumably this is because millennia of precip has leached what calcium was left by the glaciers. Nevertheless trees grow, and when one cuts the hardwoods and burns them as we know the ash has fairly large amounts of calcium which is a crop nutrient, and through some reaction wood ash is well known to raise soil ph also.

My question is why do the trees have plenty of Ca in them? One would have to think that they root deeply enough to get to soil zones where the eons of Ca has perked downward, perhaps caught up in denser layers below the sandy subsoils. If this were true then trees could be thought of as calcium pumps for acidic areas, perhaps. The same might be true for K as well.

I am thinking about this as a speculation about perpetual methods for food production in a situation where mined calcium sources might be unavailable.

I agree that trees with their deeper roots will search out calcium that shallow rooted crops cannot reach. Alfalfa and sweet clover are wonderful deep rooters too, but they will not thrive on acid soils.

Actually, I believe that limestone for calcium is about the most abundant mineral amendment we have. Yes, I suppose in certain circumstances of war disruption, it might not be available.

In the early 1900s constant cropping wrecked productivity of soils (especially K) in northern parts of New York.

Red pines (and other trees) were planted which reclaimed some of the deep soil nutrients, returning them to the surface with leaf/plant litter.

It's not a quick process, but it works.

The wood could be charcoaled in state of the art cookers to produce heat which would displace other fuels from that purpose, and the other minerals driven off in the process could be captured rather than released into the atmosphere and used to rapidly charge the bio-char before introducing to the soil to grow crops.

Or, in the absence of such advanced technology, the wood could be radially chipped and spread on the ground for a new cycle of permaculture. Huge amounts of area is kept cleared in the humid east of north america that would happily grow trees if left alone.

I could be out of my gourd, and I'm certainly no expert on this, just thinking out loud.

I do remember hearing about how tropical rainforests often have very poor soil. All the nutrients are in the plants themselves, and anything that drops to the ground is either sucked back up into the food chain or washed away by the heavy rains. Yet the rainforest is prolific.

This is a key statement regarding calcium availability for plant nutrition in those situations.

As to the burnt plants decreasing the pH, that isn't a function of calcium. It's a function of the carbonates created when the wood burns. Remember that it's the carbonate in lime that decrease the pH, not calcium. That's why calcium sulfate does not decrease soil pH but magnesium carbonate does.

(If you want a more detailed explanation, see my post about half way down in this thread. For an explanation regarding the confusion around the term "base cations" see my last post in the same thread.

The reason you have so much carbonate when wood burns is that wood (being a truly organic material) is stuffed full of carbon and fire is an oxidation-reduction reaction. Carbon plus oxidation = lots of CO3 (aka: carbonate).

As to high rainfall washing out base forming cations, resulting in an acid conditions...that's an exceedingly popular misconception. You'll even it see it show up in lots of university extension publications. However, if you sit down with a pen and a piece of paper, the chemistry simply doesn't work out.

Don't take this the wrong way. County extension folks are great but they, just like anyone else, can't be experts in everything and very few are chemists by training and the chemistry involved in soils can be very complex. Soil acidification happens to be a little stickier, chemistry wise, than most want to deal with. Rather than re-inventing the wheel, they'll typically read stuff published by extension offices in the past and repeat it when putting together info for general public consumption. Remember, they aren't doing research, they're compiling information. If the info they refer to is off kilter, the problem is self perpetuating.

When you get lot of rainfall, carbonates leach out of the soil and are replaced by hydrogen ions. They hydrogen also is not as prone to leaching because it is a cation (having a positive charge) and can be held by exchange sites (anything with negative charge) in the soil. Things that have a negative charge include clay particles and organic matter. When things like lime go into solution, the carbonate leaches out readily and is not held to these sites because it is an anion (negatively charged). That leaves a net decrease in the base forming carbonates and an abundance of hydrogen; pH, of course, being a measurement of the hydrogen in a solution.

So this is quite interesting that you say Ca does not leach, but rather carbonate (which raises the possibility that Ca can be raised to excessive levels, perhaps). I might posit that even so, the natural level of calcium in a low CEC sandy soil is very inadequate for most crop production. Last winter I added goodly amounts of lime and gypsum to a sandy soil pasture before turning the sod, and the increase in production over past years was very dramatic.

I suppose I might be mixing apples and oranges ( so to speak) in comparing the role of available Ca in plant growth between annual crops and trees. Still, all plants use the nutrients in roughly the same proportions, and we find trees growing well in situations where annual crops would do poorly.

I would say that there are some differences between annual crops and trees. But, I think a lot of it comes down to what Tox mentioned. Another thing to take into consideration is how much of the plant is harvested and removed each year when comparing something like lettuce to something like fruit trees. Also, who knows? Even in a situation in which the trees look to be doing pretty while, if you put some gypsum (or nitrogen, or magnesium, etc.) on those trees you might see them go gangbusters. It just depends on all the combined specifics of each individual situation.

This works in the South-East for a variety of reasons, though it's not all "washing out" as much as it is pH influenced. In many parts of the South-East, thanks to air "pollution" from coal power + animal (especially chicken) production rainfall can acidify soil a lot easier than many other parts of the US.

The soils, themselves, can be full of Ca+, K+, etc, but because of the quality of clays it's tied up in extremely plant unavailable forms...not just chemically, but physically cleated into the clay's structures (especially inner layers). It's there, it just needs to be set free.

pH (and acids residually formed when lowered) are a lot more important in making these cations available to plants. It's a form of weathering and moisture content does play a role, but without the acids weathering on the material you can dump all the "pure" water in the world into it without it making much of a difference.

When we lime soils in the South-East we're semi-halting this process, but the main advantage is that the soils are already loaded with cations which are naturally weathering out anyway...plus halting/slowing Aluminum toxicity.

It's a semi-regional thing, but important in a lot of Utisols heavy in "red clay" type soil structures found all over the world.

This works in the South-East for a variety of reasons"...

I hope I wasn't giving the impression that high rain fall does not result in acid conditions because....it does. The "misconception" statement was in reference to the "how" it happens, which has broad implications in regards to calcium status relative to soil pH.

That said, anions and cations have to stay together, so I'm not quite sure I understand how carbonate (CO3--) or bicarbonate (HCO3-) can leach out and be 'replaced' by H+. Ca++ can be replaced by sodium (Na+) on cation exchange surfaces (such as clay), but you can't add or subtract ions in a solution except in equal + and - amounts, nor can anions be replaced with cations. I'm probably missing something here so don't feel obligated to learn me basic soil chemistry. :-D

One more thing about burning wood and how it leads to alkalinity: K, Mg and Ca can also become oxides at high temps (rather than carbonates), and when mixed with water will form highly alkaline hydroxides. This (I think) is how you can make soap by leaching them out of wood ash. I'm not sure what the decomposition temp of CaC03 is, but it would not surprise me if it was hot enough in the average wood fire to drive off the CO2 and leave only the oxide.

So there is probably little to no carbon or carbonate in fly ash, the carbon is in the char. Generally any wood stove or fireplace produces ashes with a fair amount of char in it, because enough wood gets buried in the fire to undergo pyrolysis.

As to "That said, anions and cations have to stay together"
Sort of. Remember how dynamic things get in a solution.

Calcium and sulfate (as an example) won't bond and then say "Well, we're gypsum forever". They associate and disassociate when in solution.

If a gram of gypsum is put completely into solution in however much water that would take you will always have X percent CaSO4 and the remainder will indeed be Ca floating around on its lonesome and sulfate doing the same. Then they will associate and disassociate freely in that solution. That's why reactions in a solution are often shown with the reactants and products connected by an arrow going both ways rather than just pushing in one direction.

The cations that are not currently part of an ionic bond will be free to stick themselves to something with a negative charge, whether that's finding an anion (i.e. a sulfate that is floating around) or an exchange site on a colloid.

Some anions will hook up with cations, some will move down freely (and probably hook up with something else along the way, disassociate, hook up with something else, etc.) Some will become part of highly insoluble materials and just hang out pretty much for ever...etc. Some won't make as many associations/disassociations on the way down and move a little faster. And these things all happen to different degrees depending on a myriad of factors.

Cations does the same sort of stuff and will indeed leach out but because they're positively charged they have all the opportunities to stop moving through the soil that anions do in addition to all of the negatively charged exchange sites on colloids, significantly increasing their ability to stay higher in the soil profile longer.

pnbrown: I was just leaching some ash the other day. I have tried this previously and the liquid came out very dark brown, so I thought if I removed more char from the ash, it might have less of that. So I sifted it down to window screen size (harvesting much char in the process for the garden!). It's *still* dark brown to black. Kinda leery of using it to make soap, I'd hate to be washing myself in carcinogenic PAHs. :-\ But I can still spread the leached ash on the lawn. :-]

Right on the money.

I've started experimenting with pine bark, of which I have a relative abundance because I burn pine for firewood, first in containers and now in the garden. Pine bark has an acidic pH, *seems* to have a reasonable amount of nutrients because the plant roots just love to go into every little hole and cranny. I"ve used it as a mulch quite a bit, but now I'm staring to turn it under in the vegetable beds. It lasts 3-5 years.

So anyway, something along the lines of your wood idea. In containers, I can mix it 50:50 with with rich soil, some perlite, and it does surprisingly well.

to answer the question, it might make a great topic for discussion, and the extension of the question might make for interesting remediation studies work, but as far as practicality, using tree species to act as nutrient pumps in remote locations, my guess is the time frame involved is against you. and if you do have the luxury of decades to effect a change, can you lay the reason for the change to the plantings used or just the inevitable weathering that would have occurred anyway.

interesting question. and the extension, for use in remediation work, is even more interesting.

in the uptake of nutrients, it's hard to overlook the role of soil solution. would there be a difference in the ability of different plants to absorb different nutrients based on minute differences in a particular species associated soil solution characteristics (pH, EC, CEC, etc)? and if so, how do you manipulate those characteristics to produce the desire results?

Ideally after careful cropping for 5-10 years the plot would be allowed to re-grow to brush and then trees for 25-50 years, again bringing up the leached minerals and repeat the process.

So I think your idea has considerable merit.

Of course, there are some plants that are far better than others at getting stuff from low in the soil and pumping it up (like grapes). To be honest, I don't know how good your oaks would be at that but any way you slice it, you'd certainly have better nutrient status if you put the material back in the ground rather than carting it off.

Albrecht stated many times that forest is a sign of excessive precip and leaching, desert a sign of inadequate precip to wash away salts and support plant life, while prarie signaled the "goldilocks" zone, so to speak.