• Plant Growth Trials
    Using Biochar
    At Celignis Biomass Lab


Biochar is a carbon-rich solid material produced by the pyrolysis of biomass. This involves heating the feedstock in the absence of oxygen, which results in the formation of a high-carbon material that can potentially be used in a wide variety of applications, including: as a soil amendment, a component of biobased materials, and in pollution remediation.

Biochar can be made from a variety of feedstocks, including wood, agricultural residues, and municipal solid waste.

Biochar in Soil Amendment

Biochar was used thousands of years ago as a soil amendment in the Amazon Basin, where it is known as "terra preta" or "black earth." However, it has only been the case in recent years that there has been a renewed interest in the use of biochar as a tool for sustainable agriculture and climate change mitigation.

The use of biochar in soil amendment has several benefits. Firstly, it improves soil fertility by increasing the soil's capacity to retain water and nutrients. Biochar has a high surface area, which provides a habitat for microorganisms and increases the soil's ability to hold onto nutrients. Secondly, biochar can help to mitigate climate change by sequestering carbon in the soil. This is because biochar is stable and does not break down easily, which means that the carbon in biochar remains in the soil for a long time.

Biochar can also help to reduce greenhouse gas emissions from agriculture by reducing the need for synthetic fertilizers, which are energy-intensive to produce and release large amounts of nitrous oxide, a potent greenhouse gas.

Biochar for Plant Growth Promotion

Biochar can also be used for plant growth promotion. Research has shown that biochar can improve plant growth and yield by improving soil structure, increasing nutrient availability, and enhancing the activity of beneficial microorganisms in the soil. Biochar can also help to reduce the uptake of heavy metals by plants, which can be a problem in contaminated soils.

The benefits of biochar for plant growth promotion are due to its effects on soil physical, chemical, and biological properties. Biochar improves soil physical properties by increasing soil porosity, which enhances water infiltration and aeration. This is particularly important in compacted soils, where water and air movement are restricted.

Biochar also improves soil structure by promoting the formation of stable aggregates, which helps to prevent soil erosion and increase the stability of soil organic matter.

Biochar also improves soil chemical properties by increasing soil pH and cation exchange capacity (CEC). This means that biochar can help to neutralize acidic soils and increase the availability of essential nutrients such as calcium, magnesium, and potassium. Biochar can also adsorb pollutants and prevent their uptake by plants.

Biochar improves soil biological properties by providing a habitat for beneficial microorganisms. It can act as a substrate for microbial growth and can enhance the activity of soil bacteria and fungi. This can improve nutrient cycling and promote plant growth.

The use of biochar in agriculture is still in its early stages, and there is much to learn about its effectiveness in different soils and under different management practices. However, there is growing interest in the use of biochar as a sustainable tool for improving soil fertility, mitigating climate change, and promoting plant growth. Researchers are exploring new ways to produce biochar and optimize its properties for specific soil types and crops.

Analyses at Celignis to Evaluate Biochar for Soil Amendment

There are a wide variety of variables that will influence the quality and suitability of biochar to be used in soil amendment. These include:
  • The type of feedstock used to produce the biochar.
  • The pyrolysis conditions employed for biochar production.
  • The resulting physical and chemical properties of the char.
  • Type of soil to which the biochar will be amended.
  • The plant(s) to be grown on this amended soil.
At Celignis we are happy to help you evaluate your biochar samples for this end-use. Below we describe a number of the different analytical parameters that we determine which are of relevance to using biochar in soil amendment.

Major and Minor Elements in Biochar

Many of the major elements can be beneficial for plant growth, for example potassium plays a vital role in photosynthesis whilst phosphorus is involved in energy transfer processes and the formation of nucleic acids. Furthermore, the presence of certain elements, such as calcium and magnesium, can affect the soil pH, influencing the availability of nutrients and the activity of soil microorganisms. As a result, elemental analysis can help to predict the impact of biochar on soil pH and inform decisions on its application in specific soil types.

It is also important to consider that some heavy metals can pose potential risks to plants, soil organisms, and human health. As a result, biochar with high heavy metal concentrations may not be suitable for soil amendment, as it can lead to the accumulation of toxic elements in the soil, plants, and the food chain.

The European Biochar Certificate organisation (EBC), sets an upper threshold concentration for certain heavy metals in biochar, with these limits vaying according to the planned market application. (e.g. lead can not exceed 45ppm when biochar is used in the Agro-Organic sector, rising to 120 ppm the biochar is to be used in consumer materials). At Celignis we check as to whether the values for each heavy metal are above or below the threshold values for each of the EBC application sectors and provide the results in a PASS/FAIL table for each feedstock according to the specific sector and test.

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Electrical Conductivity

The electrical conductivity of a material is a measure of its ability to conduct electric current. Biochar's electrical conductivity is due to the presence of carbon, which is an excellent conductor of electricity. The pyrolysis process, which produces biochar, also results in the formation of various carbonaceous structures, such as graphite-like layers, which further contribute to its electrical conductivity.

The electrical conductivity of biochar depends on several factors, including its feedstock, production temperature, and post-treatment processes. Feedstock type and composition can affect the final properties of biochar, with wood-derived biochars generally having higher electrical conductivities than those derived from agricultural residues. The pyrolysis temperature also influences the electrical conductivity, with higher temperatures (above 500 oC) leading to the formation of more conductive structures.

The electrical conductivity of biochar allows it to effectively retain and exchange nutrients in the soil, particularly cations like potassium, calcium, and magnesium. This property is crucial for soil fertility, as it helps plants absorb essential nutrients more efficiently. The electrical conductivity of biochar can enhance soil microbial activity by facilitating the transfer of electrons between microbes and their environment. This electron transfer is essential for various microbial processes, such as nutrient cycling, and can lead to an overall increase in soil fertility. The conductive properties of biochar can help mitigate the spread of soil-borne pathogens by generating reactive oxygen species (ROS) and inhibiting the growth of harmful microorganisms. This, in turn, can lead to healthier plant growth and reduced disease incidence.

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Water Holding Capacity

The water holding capacity of a material refers to its ability to absorb and retain water. Biochar's water holding capacity is attributed to its porous structure, which provides a large surface area for water absorption. Additionally, biochar's hydrophilic functional groups, such as carboxyl and hydroxyl groups, can form hydrogen bonds with water molecules, further contributing to its water holding capacity.

The water holding capacity of biochar is influenced by several factors, including its feedstock, and the conditions used for pyrolysis. Additionally, post-treatment processes, such as oxidation and activation, can modify the surface chemistry and porosity of biochar, potentially affecting its water holding capacity. For example, oxidizing biochar can introduce hydrophilic functional groups, increasing its affinity for water.

Biochar with good water holding capacity can offer several benefits when used in soil amdendment, including:
  • Improved Soil Water Retention: This can help plants better tolerate water stress and reduce the need for irrigation, conserving water resources.
  • Enhanced Plant Growth and Yield: Resulting from a more consistent water supply to plants, especially during periods of low rainfall.
  • Mitigation of Soil Salinization: By diluting salt concentrations in the soil and promoting the leaching of excess salts.
  • Support for Soil Microorganisms: By maintaining adequate moisture levels, which in turn can promote nutrient cycling and soil fertility.
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Cation Exchange Capacity

Cation exchange capacity (CEC) is a measure of a material's ability to attract and hold positively charged ions (cations) such as potassium, calcium, and magnesium. The cation exchange capacity of biochar is attributed to the presence of negatively charged functional groups, such as carboxyl and phenolic groups, which can form electrostatic bonds with cations in the soil. These functional groups can either be present in the biomass feedstock or formed during the pyrolysis process.

Biochars derived from plant materials with higher lignin content, such as wood, tend to have a higher CEC than those produced from agricultural residues. Lower pyrolysis temperatures (below 500 oC) typically result in biochars with a higher CEC, as higher temperatures can lead to the degradation of negatively charged functional groups responsible for cation exchange.

Biochar with a good CEC can offer several benefits when used in soil amdendment, including:
  • Enhanced Nutrient Use Efficiency: By retaining and exchanging cations in the soil, allowing plants to absorb essential nutrients more effectively.
  • Amelioration of Soil Acidity: Through the biochar buffering soil acidity and promoting the availability of essential nutrients, particularly in acidic soils.
  • Mitigation of Soil Salinization: By diluting salt concentrations in the soil and promoting the leaching of excess salts.
  • Mitigation of Heavy Metal Contamination: High CEC biochar can contribute to the immobilization of heavy metals in contaminated soils, reducing their bioavailability and potential toxicity to plants.
Request a QuoteCation Exchange Capacity

Polycyclic Aromatic Hydrocarbons (PAH)

Polycyclic aromatic hydrocarbons (PAHs), are a class of organic compounds comprising multiple aromatic rings. They can be formed during the pyrolysis of biomass and accumulate in biochar with their concentrations and types dependent on several factors, including the type of feedstock, pyrolysis temperature, residence time, and post-treatment processes.

The presence of PAHs in biochar is an important concern when using it as a soil amendment, as these compounds can pose potential risks to the environment, including:
  • Toxicity to Plants and Soil Microorganisms.
  • Bioaccumulation and Transfer to the Food Chain: The consumption of crops grown in biochar-amended soils with high PAH concentrations can lead to the ingestion of these harmful compounds.
  • Environmental Persistence: PAHs are known to be persistent in the environment and can accumulate in soil over time, leading to long-term ecological risks.
As for heavy metals (see above) the European Biochar Certificate organisation (EBC), sets upper threshold concentrations for PAHs in biochar. At Celignis we check as to whether these thresholds are exceeded for each of the EBC application sectors and provide the results in a PASS/FAIL table for each feedstock according to the specific sector and test.

Click here to read our dedicated webpage covering our analyses of PAHs in biochar.

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Surface Area and Porosity

The surface area and porosity of biochar are critical properties that influence its effectiveness as a soil amendment for several reasons:
  • Water Holding Capacity: A biochar with a high surface area and porosity can more water, improving the soil's water holding capacity.
  • Nutrient Retention and Release: Biochars with high surface areas and porosity can provide more active sites for nutrient adsorption, improving soil fertility and plant nutrient uptake.
  • Cation Exchange Capacity: CEC increases with surface area.
  • Microbial Habitat: A biochar with high surface area and porosity can promote the establishment of diverse microbial communities, which can enhance nutrient cycling and improve soil health.
  • Pollutant Adsorption: Biochars with high surface areas and porosity can adsorb and immobilize pollutants, such as heavy metals and organic contaminants, reducing their bioavailability and potential toxicity to plants and soil organisms.
Click here for our dedicated page on surface area analysis of biochar.

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Scanning Electron Microscopy (SEM)

Scanning electron microscopy is a widely used technique for characterizing the morphology, structure, and composition of materials at the micro- and nanometer scales. In the case of biochar, SEM analysis can provide valuable insights into its surface properties, porosity, and elemental composition, which are crucial factors that influence its performance as a soil amendment.

SEM analysis is a very useful companion analysis to the surface area and porosity analysis of biochar as it helps to visualize the size, shape, and distribution of pores in biochar.

SEM analysis can provide valuable information on the effects of various production parameters, such as feedstock, pyrolysis temperature, and post-treatment processes, on the properties of biochar. This can help optimize the production process to create biochars with desired characteristics for specific soil amendment applications.

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Liming tests are used to determine the ability of biochar to neutralize soil acidity, a key property that can influence its performance as a soil amendment. There are a number of reasons why it is important to undertake liming tests of biochar:
  • Soil pH Modification: The ability of biochar to modify soil pH can influence nutrient availability, as certain nutrients are more readily available at specific pH levels.
  • Enhancement of Soil Microbial Activity: Soil pH can also affect the activity of soil microorganisms, which play a crucial role in nutrient cycling and decomposition of organic matter.
  • Alleviation of Aluminum Toxicity: In acidic soils, high levels of soluble aluminum can be toxic to plants and inhibit root growth. By neutralizing soil acidity, biochar can reduce aluminum solubility and alleviate its toxic effects on plants.
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Tests at Celignis to Assess the Effects of Biochar on Plant Growth

We can undertake experiments to see the effect of adding biochar to soil on the germination and growth of plants. Within these experiments we can consider various variables and their effect on growth including:
  • Type of biochar used (e.g. compare between biochars from different feedstocks or between biochars produced from the same feedstock but using different pyrolysis conditions or different upgrading approaches).
  • Biochar loading rate (i.e. ratio of soil to biochar).
  • Soil used.
  • Plant type (examples of plants we have used include potato, tomato, corn, and lettuce).
These plant growth trials can be run at various scales, ranging from small pots in the laboratory to large trays in a dedicated greenhouse. Each set of conditions is run in triplicate and compared against a control where no biochar is used.

We can collect various data associated with these trials, ranging from crop yields to detailed analyses of the physical and chemical properties of the plant and soil, as detailed below.

Germination and Plant Growth

In our standard Biochar Plant Growth Trials package (P388) we collect the following data:

Plant Health Analysis

Relevant analyses that we can undertake to see the effect of biochar supplementation on plant health include: pigments (highly dependent on the nutrient uptake efficiency of the plant), leaf lamella size, and nutrient uptake. We can also perform stomata count, if the leaf type allows such analysis (which can be correlated to water stress resistance)

These results can then be correlated with the physical and chemical characteristics of biochar and plant growth results.

Soil Biology Analysis

Soil biology analysis can include: soil respiration test, bacterial count, fungal count (fungi to bacteria ratio), and soil enzymes (C-degrading, N mineralisation and P solubilisation enzymes).

These results can then be correlated with the physical and chemical characteristics of biochar and plant growth results.

Germination Inhibition

This is a separate test that we can undertake to evaluate the potential phytotoxicity of biochar when applied as a soil amendment. We use seeds of a fast-growing and sensitive crop (e.g. lettuce or radish), and expose them to different concentrations of biochar or biochar-amended soil under controlled laboratory conditions. The seeds are typically placed in Petri dishes containing a moistened filter paper or a mixture of the tested biochar and a growth medium, such as sand or soil. The dishes are then incubated in a growth chamber or greenhouse with controlled temperature and light conditions.

After a predetermined period (usually 3-7 days), the germination rate and the growth of the seedlings (e.g., root and shoot length) are assessed. The results are compared to a control group of seeds exposed to the unamended growth medium or a non-toxic reference material.

If the germination inhibition test indicates that biochar has a negative impact on seed germination or seedling growth, we can potentially undertake further investigations to determine the cause of the phytotoxicity and identify possible mitigation strategies.

Additional Information on Evaluating Biochar for Soil Amendment Applications and Undertaking Plant Growth Trials

Feel free to get in touch with us if you have any questions about our analytical services for assessing the potential for using biochar in soil amendment or for plant growth promotion. We can also provide assistance if you need to screen biochar for potential phytotoxicity issues or to understand how variations in feedstock and/or pyrolysis conditions may affect the value of biochar for soil amendment. Relevant members of the Celignis biochar team will be happy to assist. Those team members with the most experience with undertaking these tests and interpreting the resulting data are listed below.

Sajna KV

Bioanalysis Developer


Our Biomass Detective! Designs, tests, optimizes and validates robust analytical methods for properties of relevance to the various biochar market applications.

Lalitha Gottumukkala

Chief Innovation Officer


A serial innovator managing multiple projects. Has particular expertise related to the upgrading of biochar and on the assessment of its impact on plant productivity and soil health.

Edgar Ramirez Huerta

Biochar Project Developer


Has taken a major role in developing Celignis's capabilities for biochar analysis and project development. His thesis covered the evaluation of high value applications for high-carbon materials.

Other Celignis Tests and Services for Biochar

Global Recognition as Biomass and Biochar Experts

Celignis provides valued services to over 1000 clients. We understand how the focus of biochar projects can differ between countries and have advised a global network of clients. We also have customs-exemptions for samples sent to us allowing us to quickly get to work no matter where our clients are based.

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Feedstock Evaluation

Our analysis packages can screen biochar feedstocks. We can estimate biochar yield and quality using feedstock chemical composition and can estimate biochar composition using the ultimate and major/minor elements analyses of the feedstock. With TGA analysis we can also monitor feedstock behaviour under pyrolysis conditions.

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Biochar Production

We can produce biochar samples from your feedstocks using a wide range of temperatures, heating rates, and residence times. We can formulate a Design of Experiments (DoE) to study the effects of varying process parameters on biochar yield and quality and can optimise these outputs according to your desired biochar market applications.

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Biochar Analysis

We have an extensive array of analysis packages to evaluate the suitability of biochar for a range of applications. These analyses cover properties relevant to combustion, soil amendment, feed, and biomaterials. Our reports compare the results against internationally-recognised limits for using the biochar in specific end-products.

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Biochar Combustion Properties

Biochar can be a superior fuel versus virgin biomass due to its greater carbon content and energy density. We offer a wide array of analysis packages to fully evaluate biochar as a fuel. For example, we can determine both organic and inorganic carbon and can monitor the behaviour of the biochar ash over wide temperature ranges.

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Analysis of PAHs in Biochar

Polycyclic aromatic hydrocarbons can be formed during the pyrolysis of biomass and accumulate in biochar, leading to potential risks to the environment. We can accurately quantify a range of different PAHs and determine if their concentrations exceed regulatory limits. We can also develop strategies to reduce the amount of PAHs in biochar.

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Surface Area and Porosity of Biochar

The suitable markets for a biochar are often greatly dependent on its surface area and pore size-distrubtion. We provide detailed reports on biochar surface area and porosity and can provide guidance on the implications of the results. We can also work on strategies to increase the surface area and modify the pore-size distribution of biochar.

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Thermogravimetric Analysis of Biochar

TGA is a powerful analytical technique for the study of biochars because it allows us to examine the thermal stability of the material as a function of temperature. The thermal stability of biochars is an important factor to consider when evaluating their potential use as a soil amendment or for carbon sequestration.

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Biochar Upgrading

There are several different methods (covering physical, chemical and biologial routes) by which we can upgrade your biochar in order to increase its value and make it more suitable for the desired market applications. We are able to fully characterise the changes in physicochemical properties associated with upgrading.

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Biochar for Carbon Sequestration

Biochar's efficacy as a means for sequestering carbon depends on a range of factors (e.g. feedstock and pyrolysis conditions). We can undertake a range of analytical tests to help you determine the stability of your biochar's carbon. We can also suggest alternative approaches to improve carbon sequestration potential.

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Technoeconomic Analyses of Biochar Projects

Our TEA experts work with you to evaluate the economic prospects of your biochar facility, considering various scale, technology, and feedstock options. We apply accurate costing models to determine CAPEX/OPEX of simulated and pilot scale processes which are then used to determine key economic indicators (e.g. IRR, NPV).

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Research Project Collaborations

Celignis is active in a number of important research projects focused on biomass valorisation. Biochar is a key component in some of these ongoing projects as well as in several prior projects. We are open to participating in future collaborative research projects where our extensive infrastructure and expertise in biochar can be leveraged.

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