Differences in Biochar Quality based on types of wood or materials used to make it?

Are there differences in the quality of biochar based on what it is made from? Are there certain types of wood or plant materials that make superior biochar?
I would think the density of the wood could make a difference. The vascular structures of different plants could effect how "open" the biochar would be.
If there are large differences it could be beneficial to use certain plant materials for biochar while saving the other types for things like composting or hugelkultur.  

Excellent idea, JYoungman.
I have read many say that you should only burn things that are extra, leftover, not needed for something else.
I have also read that wood produces more carbon material for the char than other organic materials.
I guess it's a balance of what you could get for free or is leftover with how much carbon they will make.
Where I live, trees grow super fast and there are always lots of people giving away wood.
I have to space my wood apart in my 55 gallon barrel so that it burns evenly and it's not too dense.
Making sure it's dry and of similar sizes is also key in having a good even burn.
I'm a baseball player and sometimes the guys will break a bat and it will become my biochar.  
I definitely have to chop it in at least halves and burn it toward the first half because the wood is so dense.
I use very rotten wood for planting in the hole of the next baby tree or woody bush, because it will help more there than in biochar.

John S
PDX OR

My divergent view (extremely minority opinion) is that biochar is best understood as an analog for the black carbon bits that remain after natural wildfires. Plants and soil biology evolved with and are adapted to naturally occurring charcoal.  My view is that the biochar is ideally made from plant materials native to the region and ecosystem within which the biochar will be used. Because biochar interacts with biology, and has since the beginning of plant life on the planet, presenting them with biochar in a form closest to what they are adapted to makes deep sense to me, and this practice allows me to show respect for biochar's connection with my local natural systems, this despite my ignorance of how exactly (scientifically) biochar works to increase soil health, plant disease suppression, and plant productivity in the ways that I see it working. Again, I don't get a lot of love for this apparently crazy idea that biochar is an analog for naturally occurring charcoal in soil, my lonely idea that biochar wouldn't work the way it does absent millions of years of evolution.

In practice I will use the complete available range of plant materials to make biochar for my garden: commercial biochar made from my region's softwood, hardwoods from foreign lands made from broken pallets, nutshells, almond pits. I would use bamboo, if it was handy. Because I see it working. Beyond avoiding excessive ash, woodsmoke stink, and causticity, I have loose standards in actual real life, otherwise, my soil's hunger for biochar cannot otherwise be satisfied. But I do value the locally sourced biochar the highest. There's a local outfit that sells biochar made from local grain stubble. Biochar made from grain stubble works, and it works well. Maybe that relates to the soil biology carried on the local winds is well adapted to grasses-based soil charcoal. I like to think so anyway.

I suppose there is some science on char quality somewhere. I haven't come across it yet. But I do think the method of "cooking" the char needs to be adjusted for the feed material.

My char comes from a mix of hardwood and softwood branches. It's waste material that most others consider rubbish and burn to ash on an open pile. I avoid leaves as much as possible. I find that with active management, my semi-open kiln* creates decent quality char.

I find that non-woody material (grass, leaves, weeds) tends to create a lot more ash than I would like. If these were my primary source materials, I would use a closed kiln (giant pot with lid in a fire).

*BTW, "semi-open kiln" is my fancy name for a barrel with the top removed and an access hole chopped in halfway up the side. It's much more impressive to people at cocktail parties than "hillbilly brush burner."

Philip Small- I think a lot of people would agree with parts of what you're saying.  Since the documentation of Terra Preta in Brazil, researchers have found many instances of indigenous people harnessing fire intentionally in many different parts of the world,  creating better soils.  The debates aren't all in to determine what the process was, what the intention was, or how an individual making biochar saw him or herself relating that action to others presently or over time.  Our own Bryant Redhawk has written about this, as have others.  There was a guy from Brazil in this forum a short while back talking about indigenous world views of the people living in the Amazon Basin, and how they didn't really separate themselves from the life around them the same way that modern man does.  

I believe that indigenous people worldwide observed that many plant processes were helped by natural forest fires.  I think they may have intentionally participated in the fires when they saw that abundance was helped by the fires in some ways.

There is a lot that we don't know, but it is interesting to wonder about, and consider, as we experiment with biochar.

John S
PDX OR

It would make sense that most plants have some adaptation to biochar when you consider the black mat layers, such as the younger dryas black mat layer, found around the world. Also any ecosystems specifically dependent on seasonal wildfires would be an even greater selection pressure.

I belive any biochar that goes through composting will be of good quality and we should use whatever is available and not needed somewhere else.

Having said that, I also belive small diameter of wood is better at least for flame courtain kilns becouse it allows the creation of oxigen groups in biochar surfaces and thus increases Cation Exchange Capacity