When considering the porosity in biochar we tend to classify the pores as micro, meso and macropores. The pore size can have some impact on what the biochar is capable of doing in the soil. For example, there is biochar research that shows water retention focused mostly on the mesopore volume of biochar. A mesopore is between 2 and 50 nanometers. (One nanometer is 1 billionth of a meter.) In contrast to the pore size, the overall surface area of Wakefield’s biochar is approximately 375 meters squared per gram. Therefore, there are billions of pores throughout a gram of biochar.
The surface area of biochar is dependent on a number of factors but it is mostly due to the type of feedstock and the temperature of the pyrolysis. The same wood that is used as a feedstock to make biochar may have a different surface area based on the processing temperature. Additionally, biochar made from biosolids is almost always going to have a lower surface area than biochar made from wood. The surface area is important to the discussion of porosity. The more surface area of the biochar the more that water, nutrients and microbes can interact in the pores.
The particle size of the biochar may have more of an impact on how it integrates with soil or water systems than with the porosity and surface area. Biochar will generally be produced in a range of particle sizes. As long as the biochar isn’t mechanically milled or pulverized, the pores on the surfaces of fines and granules of the biochar are relatively the same. Biochar that is mechanically manipulated will potentially have a reduction of pores available on the surface due to the interaction with the mill.