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biochar and clay have distinctly different water hold strategies.
my impression from field observations is that they complement and need each other. the idea they compete is an error of insight; they form a partnership that can boost water retention beyond either alone.
biochar retains water in its micropore sponge structure, sucking water in largely thru simple capillary action. biochar's electric charge sites on its surfaces enhance this capillary action by increasing the forces that pull water inwards, and then hang onto it once inside, but loosely.
biochar has far fewer electric charges on its surfaces than clay, and thus, the water is not so tightly held. thus, biochar rather rapidly dries out once exposed to atmosphere or heated. a pile of biochar will develop a thin dry surface layer, while remaining moist inside. and being black, char exposed to sunshine & atmosphere will heat up and dry out even faster.
however, biochar inside soil, in full contact with soil particles, will safely retain the water it absorbs into its hollow sponge spaces.
clay forms much more intimate bonds with water molecules. its thick abundance of negative charge sites grab and hold those water molecules more tenaciously, and assembles water into clay-water complexes – hydrates. thus, clay will stay wet longer than biochar. however, clay also absorbs and retains water much more slowly, and clay is slower to allow water to pass thru and percolate deeper into it.
on the other side, clay resist water absorption, in part because it is so fine, with tightly stacked & packed microplates, and narrow spaces in between. further, as clay absorbs water and forms its hydrate complexes, it swells up, further reducing those spaces between its tiny particles. so clay tends to shed water after its surface layer gets wet, and water is slow to penetrate deeply. fortunately, as clay dries out , it shrinks and opens large cracks that will allow water to rush in deeper.
my impression is if biochar – or many other ultrafine carbon, like humic & fulvic acid – is added to clay, the carbon swarms & surrounds clay's strong electric charges, to isolate & insulate them, reducing their polar attractions and reducing stickiness. this allows the mass of clay to relax and become more open. this happens, too, in a different way, with larger pieces of biochar, further loosening, relaxing and opening the clay. this means water can into penetrate into clay easier, faster, deeper. so, while biochar slows water movement thru sand, biochar increases water movement thru & into clay.
so, what i like to do it blend biochar and clay together. usually this means adding fine clay powder to biochar, so the grains of biochar are coated by the dust, and deployed into soil already in an intimate association, ready to work together to manage water and nutrients. the biochar & clay don't just fall together, they become organized into complex structures that seem to serve functional purposes. likely stephen joseph et al have research and microscopic views that reveal this structured partnership.
below is my slide for talking about water & biochar:
On Wed, Oct 21, 2020 at 12:46 PM Charles Hegberg <chegberg@...> wrote: