Discussions with IRRI gave some simple assumptions which can be used when assessing the impact of biochar addition on the soil.

If biochar analysis is available, then the impact of a known volume of biochar to soil can be calculated. For example, if one square metre of soil is amended with 4 kg of biochar (this is a 40 t/ha application rate), then we can assume that this represents 4% of the soil weight. This is because in 1 m at a 10 cm depth there is 100 litres of soil with an approximate bulk density of 1, which is 100 kg of soil. The properties of the biochar can then be assumed to be diluted in the soil at a 4% rate.

This assumption was then applied to the CEC (Cation Exchange Capacity) where rice husk biochar is added to the research farm soil in Siem Reap, Cambodia.

The soil has a CEC of 18.44 cmol+/kg

Biochar has a CEC of 44.5 cmol+/kg.

Therefore the CEC of the soil can be assumed to be raised to (18.44*0.96)+(44.51*0.04) = 19.48

This is a rough assumption, and should not substitute using actual data, and it assumes that there are no interactions between the soil and the biochar which could influence the CEC of the soil.