Biochar Classification Tool

A key property of biochars is their persistence in soils compared to the feedstock material from which they were produced. The H/Corg ratio allows estimating the mineralization of the carbon in the biochar in a standardized fashion relative to other biochars. In this classification system, biochars are classified by the quantity of organic carbon (Corg) estimated to remain in soil for at least 100 years (BC+100). This carbon storage value is referred to as stock BC+100 (sBC+100) and can be used when estimating the long-term soil carbon sequestration potential of a specific biochar. Carbon storage value is based on the Corg content and the ratio of hydrogen to organic carbon (H/Corg) of a biochar. H/Corg offers an approximation of the extent of fused aromatic carbon ring structures of biochar, a key indicator of biochar carbon persistence in soils (click here for more information on the test method to determine biochar carbon persistence). The carbon storage value of biochars is divided into five classes as follows:

Plant nutrients are the chemical elements required by plants to sustain growth. Depending on the plant requirement for a given nutrient, this is referred to as either macronutrient or micronutrient. Mineral macronutrients include N, P, K, S, Ca, and Mg (C, O, H contained in plants are obtained from CO2 and H2O and not considered mineral nutrients). When nutrients are combined into plant available forms in mineral or organic compounds they are called fertilizers.

The nutrient content of biochars is largely influenced by feedstock type and processing conditions. Nutrient availability to plants is related to the nature of the chemical compounds in which the nutrient occurs (Camps Arbestain et al 2015). The Tool provides the fertilizer grade for the six nutrients (N, P, K, S, Ca, and Mg), and also a classification system for the levels of four* nutrients (P, K, S and Mg) in a biochar.

The fertilizer class is based on the ability of P, K, S and Mg in a biochar to satisfy the expected yield and nutrient removal demands of maize—one of the main crops grown worldwide. Biochar application rates ranging from 1 to 10 t/ha are used in the classification system. Available levels of P, K, S and Mg, as measured in a laboratory, must be known in order to classify the fertilizer value of the biochar. The fertilizer value of biochars is divided into 4 classes based on levels of P, K, S and Mg as follows:

If a specific nutrient is able to meet the demands of maize the quantity of that nutrient required in tons is written as a subscript next to the 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 (the maximum application rate considered) K and S would still be insufficient. According to the present classification, a biochar will not have any fertilizer value if, when applied at 10t/ha, it cannot completely fulfill the hypothetical demand of an “average” maize crop for at least one of the four nutrients considered in the classification system (P, K, S and Mg).

* In this classification, N and Ca are not considered because of their generally low and high availability, respectively.

The fertilizer grade is an expression referring to the content of plant nutrient expressed as a proportion by weight in a fertilizer. As the total content of a specific nutrient in biochar may differ from its available fraction, the information provided by the fertilizer grade of biochar for that nutrient should refer to its available fraction. Here the fertilizer grade for six nutrients (N, P, K, S, Ca and Mg) will be provided. Those for available P, K, Ca and Mg are expressed as oxides rather than on elemental basis to conform to typical reporting convention for commercially available fertilizers. 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.

Soil acidity can be a major constraint to plant growth. To ameliorate acidic soils, agricultural liming materials are used to raise soil pH to levels optimum for crop growth. Liming materials are typically made from carbonates, oxides or hydroxides of Ca and Mg. Calcitic limestone (pure calcium carbonate (CaCO3)) is a common liming material and is used as a reference for other liming materials; liming values are reported as an equivalent proportion of the liming effect that calcium carbonate would have (% CaCO3-eq).

Water is essential for plant growth. It is taken up via plant roots in soils. Water in soil also facilitates important physical processes such as infiltration, drainage, gas diffusion, and movement of nutrients. Oxygen in soils is also essential for plant and microbial respiration. It diffuses through soils from surface air. In waterlogged soils oxygen is depleted and plant growth for most agricultural crops typically decreases.

Biochar has been shown to improve soil functions related to water retention and soil aeration such as increased water holding capacity and plant available water as well as improved drainage and aeration, as is known for any organic matter additions. These functions are to a certain extent dictated by biochar particle size. Other factors include internal porosity of biochar, properties of the host soil and its interaction with biochar over time3. Biochar particle size distribution is divided into four classes based on a triangle plot as follows:

Biochars that are classified as either powders or lumps can be further classified according to the ternary systems below: