Reduce Particle Size
For use as a soil amendment, biochar should be ground into small, sub-millimeter particles. Why? Because the fertility benefit of biochar is largely a function of exposed surface area. Increased surface area "increases the rate of reaction with minerals and soil organic matter".5 Biochar particles made from woody feedstocks will have a large internal surface area because of their pore structure. However, the larger the particle is, the less its internal pore surfaces will be available to the biochemical processes that make nutrients available to plants. While capillary forces during heavy rain may initially draw soil solution into deep pores, free circulation is inhibited, hence oxygen and organic matter.2 A shallow pore will be more available to the circulating soil solution, plant enzymes, root hairs and mycorrhizae.
Another equally important reason to reduce biochar particle size is that sub-millimeter particles, of low temperature biochar containing aliphatic carbon, will be most likely to form soil aggregates.6 Aggregation will enhance the effects of biochar, retaining water and nutrients, often providing islands of habitat for microbial life, and it will make the biochar stable.7 8 9
Current research indicates aggregation is most likely the primary mechanism of soil organic carbon stability. "Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate."10 "Physical protection and interactions with soil minerals play a significant part in black-carbon stability over long periods of time."10 11
A recent study showed that one third of the ground biochar particles added directly to temperate forest soil had formed aggregates within 10 months, demonstrating that aggregation occurs quickly under the right conditions. The biochar used in this study was produced at 450° C, and virtually none of it was lost to decomposition during that time period.7
The biochemical processes plants use to absorb mineral nutrients from soil occur at a microscopic scale. Soil bacteria are from 0.5 to 1 μm (micrometer) in diameter. There are one thousand micrometers in a milimeter, and for reference a human hair is about 70 μm thick. Mycorrhizal hyphae, the thin, hollow tubes of fungi that many plants form a symbiotic relationship with to obtain minerals, are from 2 to 20 μm in diameter. The dilute minerals plants rely on are absorbed as sub-micrometer sized particles.
To a soil bacterium, the chunk of biochar in the hand shown above is roughly as large as the earth is compared to you and me.
While there is speculation and some evidence that biochar pores may be a refuge for beneficial soil bacteria, bacterial populations will only survive where they have access to sufficient nutrients. The soil solution isn't under pressure in such a way that it can continually flush decomposing biomass or root exodates deep into those pores. Research has demonstrated that bacterial populations mineralizing biogenic material can only survive near the surface of a biochar particle, generally in the range of 10 micrometers deep.
Root hairs and mycorrhizal hyphae do at times wander into the pores of a biochar particle, but a given root hair's ability to absorb nutrients will be limited to those that are within the pore it inhabits. It is only via the soil solution surrounding the biochar particle that minerals can be freely exchanged.