Disagree man, I wrote a paper on biochar in college. Oxygen is a bitch chemically and if it’s around, certain necessary things just don’t happen.
Discussion
Obviously oxygen has to be eliminated from the burn, that is why the fire is built layer by layer, as soon as ash appears on the surface, you add more fuel; this creates a lack of oxygen in the material below as all oxygen is being consumed at the surface. If you add more material before ash appears, then you will not eliminate the oxygen from the lower layer, if you let too much ash appear then you are losing char quantity. Once you have filled the pit or exhausted your material supply, you can extinguish the fire thoroughly with the aid of a hose. Some people bury the pile (like the indigenous method), but then you have to wait a long time for it to cool completely and when using water, the char structure can be helped supposedly. The way to ascertain if your char is good (without equipment) is firstly if it makes a satisfactory tinkly noise when you drop it, and secondly if when you rub it on your hands, that any residue left behind is minimal and rinses off with just water.
There is, obviously an exact science to this and frankly, that technique is not it. The chances of you reaching the necessary temperature throughout the substrate without disambiguating the carbon chain is practically zero. You’re playing a percentage game and it’s definitely better than nothing, but why not do it right and have amazing results? I used infrared spectroscopy to verify quality with various techniques and there is a huge difference.
Oh, also they say to optimise for the cleanest burn, the pit should be as circular as you can, and about the same depth as the radius (like a semi sphere I suppose), this helps to direct the flames back over the fire surface to combust the syngas or whatnot. But I wouldn't obsess over that, I have an oval pit and the dimensions are not accurate at all, seems to work fine.