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FAQs Regarding the IBI Biochar Classification Tool

Questions on the IBI Biochar Classification Tool

Q: What does the IBI Biochar Classification Tool do?

A: The Tool classifies biochars based on four measured physicochemical properties of biochars: carbon storage value, fertilizer value, liming value, and particle size distribution. For simplification purposes, the present classification will not consider N (due to its low availability) or Ca (present in considerable amounts in most biochars and to some extent linked to the liming value).

In addition the Tool provides the fertilizer grade for six nutrients (N, P, K, S, Ca, and Mg) in each biochar based on their available contents. End-users are encouraged to make use of this information together with available information on soil fertility, so that the needs for a specific crop demand are adequately satisfied and balanced with other sources of fertilizer where needed.

Q: Why was the IBI Biochar Classification Tool developed?
A: The Tool was developed to assist biochar consumers when assessing the potential impacts of a specific biochar in a soil or cropping system. For example, if long-term soil carbon sequestration is a goal, then a biochar in a high carbon storage class would be desirable. Or if soil acidity amelioration is a goal, then a biochar in a high liming class would be desirable.

Q: How do I use the IBI Biochar Classification Tool?  
A: To use the Tool you must have laboratory-measured values of: hydrogen to organic carbon ratio (H/Corg) and percent organic carbon (Corg); plant-available concentrations of N, P, K, S, Ca and Mg; calcium carbonate equivalent (% CaCO3-eq); and particle size distribution. Simply enter the values for these measured properties into the input fields and click on “Show Classification” to view the results.

Q: How is the IBI Biochar Classification Tool related to the IBI Biochar Standards?
A: The Tool classifies a subset of the physicochemical properties that are mostly tested for in the IBI Biochar Standards. It is not intended to replace the Standards which provide comprehensive testing and reporting requirements, particularly around safety of the biochar material. Rather, the Tool is complimentary to the Standards in that it provides a way (i) to interpret the test results (for those properties that are classified) to understand the potential soil and crop impact of a specific biochar, (ii) to recognize the widely divergent properties biochars can have, and (iii) to enhance our ability to communicate biochar effects on soil properties using a greater differentiation of these properties.

Q: What is the carbon storage class and how do I interpret it?
A: The carbon storage class is a measure of the estimated long-term (i.e., 100 year) soil carbon storage potential of a biochar. Biochars are classified by the quantity of organic carbon (Corg) in grams per kilogram estimated to remain in soil for at least 100 years (BC+100). For example, a biochar in carbon storage class4 would be estimated to have from 500 – 600 grams of Corg per kilogram of biochar (50 – 60% on a mass basis) after 100 years in the soil.

Q: How is biochar carbon persistence over 100 years (BC+100) estimated?
A: The rationale for the C storage value is provided by the IBI Stable Carbon Protocol1, which proposes a test method to estimate the fraction of organic C (Corg) in biochar that remains in soil for more than 100 years (BC+100). “The methodology is founded on extrapolations from medium-term (3 to 5 years) incubation experiments of biochars carried out under controlled and optimal environmental conditions to support decomposition2, 3, 4 to long-term decay models. Neither the impacts on plant productivity nor the effects on native soil organic C stocks (e.g., positive or negative priming) were considered by the Protocol because of insufficient scientific evidence at this time and variability in the direction and magnitude of these effects depending on soil and biochar type. The effect of soil type and plant roots on biochar C persistence was not considered either. With the availability of more data, this methodology will be updated to include the influence of specific biochar-soil type interactions, as well as the effects of biochar amendments on the decomposition or stabilization of native organic C (‘priming effects’), and plant productivity.”5

Q: What is the fertilizer class and how do I interpret it?
A: The fertilizer class is a measure of the quantity of 4 major plant nutrients in a biochar: P, K, S, and Mg. Each nutrient that is available in quantities that meet the requirements of maize (at a maximum considered biochar application rate of 10 tons per hectare (t/ha)) is assigned an additional class. Further, the amount in t/ha of biochar needed to meet the maize needs of a specific nutrient is listed as a subscript next to that nutrient. For example, a fertilizer classification of P3tMg9t implies that biochar applied at 3 t/ha and 9 t/ha would satisfy the maize requirements for P and Mg, respectively. The fertilizer would be assigned Class 2 because two nutrients (P and Mg) satisfy maize requirements whereas even at 10 t/ha K and S would still be insufficient.

“As indicated above, these calculations are only for classification purposes, end-users are encouraged to make use of the information provided on the fertilizer grade for these 6 nutrients of a specific biochar (and the corresponding availability) (examples are provided in Table 9.3 of the book chapter) together with available information on soil fertility, so that the needs for a specific crop demand are adequately satisfied and balanced with other sources of fertilizer where needed.”5

Q: Why is maize (corn) chosen as the reference crop and how do I use the results of my classification for other crop nutrient needs?
A: “In order to classify a particular biochar based on its fertilizer value the expected yield and nutrient removal for a specific crop needs to be considered. For the establishment of this classification, it is proposed that the fertilizer value of biochar is expressed in terms of its ability to meet the crop removal demand of an “average” maize crop. Maize was chosen because it is a major crop that is grown worldwide and has relatively high nutrient demands.”5 It can thus be used as a reference crop to estimate the nutrient value of a biochar for other crops. Biochars in a high nutrient class have more available nutrients (P, K, S, and Mg) for plant use.

Q: What is the liming class and how do I interpret it?
A: The liming class is a measure of a biochar’s ability to ameliorate soil acidity, a major constraint to crop productivity in some soils. Biochars in a high liming class have a greater ability to raise soil pH.

Q: What is the particle size class and how do I interpret it?
A: The particle size class is a measure of the distribution of particle sizes in a biochar. Depending on the feedstock and processing conditions, biochars can be comprised of a mix of small (<2 mm), medium (2 – 16 mm), and large (>16 mm) particles. The triangle plot depicts the fraction (by percent weight) of small, medium and large particles as positions in an equilateral triangle. The intersection of the three lines indicates the particle size classification. In the example below, a biochar is comprised of 30, 55, and 15% small, medium and large particles, respectively, and is thus classified as a kernel:














Q: What is the optimal particle size class of biochars?
A: There is no optimal particle class as such. The effect of biochar on soil hydrology depends not only on biochar properties but on soil properties and their interactions over time6. When assessing the particle size classification for a particular end use, biochar users should consult with agronomists, crop consultants or others knowledgeable about biochar use and application.


  1. Budai, A., Zimmerman, A. R., Cowie, A. L., Webber, J. B. W., Singh, B. P., Glaser, B., Masiello, C. A., Andersson, D., Shields, F., Lehmann, J., Camps Arbestain, M., Williams, M., Sohi, S. and Joseph, S. (2013). Biochar Carbon Stability Test Method: An assessment of methods to determine biochar carbon stability. International Biochar Initiative document. Available at accessed on March 20. 2015
  2. Zimmerman, A.R. (2010) Abiotic and microbial Oxidation of Laboratory-Produced Black Carbon (Biochar). Environmental Science & Technology 44(4): 1295–1301.
  3. Harvey, O. R., Kuo, L. J., Zimmerman, A. R., Louchouarn, P., Amonette, J. E., & Herbert, B. E. (2012). An index-based approach to assessing recalcitrance and soil carbon sequestration potential of engineered black carbons (biochars). Environmental Science & Technology, 46(3), 1415-1421.
  4. Singh, B. P., Cowie, A. L., & Smernik, R. J. (2012). Biochar carbon stability in a clayey soil as a function of feedstock and pyrolysis temperature. Environmental Science & Technology, 46(21), 11770-11778.
  5. Camps Arbestain M, J.E. Amonette, B. Singh, T. Wang, H-P. Schmidt. 2015. A Biochar Classification System and Associated Test Methods. In: Biochar for Environmental Management - Science and Technology, 2nd edition. J. Lehmann and S. Joseph (eds.). Routledge, New York.
  6. Masielo, C.A., B. Dugan, C.E. Brewer, K.A. Spokas, J.M. Novak, Z. Liu, G. Sorrenti (2015) Biochar effects on soil hydrology. A biochar classification system and associated test methods. In: Biochar for Environmental Management. Science, Technology and Implementation. 2nd edition. J. Lehmann and S. Joseph (eds.). Routledge, New York.