Activated Charcoal 5: Overall Benefits in Agricultural Soils, and Crops
Published Date: 8th February 2021
It was the discovery of the apparent high fertility of carbon-rich soils on the otherwise poor, and thin soils of Amazonian rainforest in the 1950s that prompted renewed research interests on activated carbon, especially biochar application in crop farming in the last two decades, although char in the form of activated carbon has been in use for different purposes from antiquity. All over the world, intensive agriculture has often led to a decline in soil quality especially its physical, chemical, and biological properties, leading to declining crop yield, and other intrinsic soil functions. In other to address these problems arising from declining soil fertility under intensive agricultural production, intentional, and unintentional deposition of nutrient-rich materials in the form of organic, and inorganic fertilizers or manure on farmlands have become a regular practice. Under traditional farming systems, fresh plant residue materials are applied to the soil, however, their rates of decomposition may be slow, and also their carbon contents are usually lost over time, and may not sustain the soil characteristics needed to maintain the ever-growing food demands of a growing global population. Again, as global agricultural activities increase, carbon-based wastes present new disposal challenges, which could be converted to opportunities through the production of value-added products like biofuel, and biochar.
The importance of carbon balance to soil health, and climate change has gained ground in recent years, and has made the application of biochar in soil amendment, and agro-ecosystems processes important research topics. Carbon dating has shown that the carbon in the Amazonian Terra preta or black earth soils dates as far back as between 1,800 and 2,300 years, therefore implying that it serves as an effective medium for long term sequestration of carbon derived originally from the atmosphere through photosynthesis. Biochar application, therefore, seems like a promising method of increasing the resident time of carbon in the soil for centuries, and even millennia. In addition to its rich charcoal content, Terra preta was also found to be rich in minerals such as potassium, phosphorus, calcium, zinc, and manganese. Several studies have therefore investigated the immediate effects of biochar application to soils on a range of crop production, with increases of 324 percent in yield at different application rates or 0.5 to 135 tons/hectare being reported. Therefore, biomass-derived biochar is increasingly being used in crop farming for the purposes of soil enrichment, and carbon sequestration, which is a potentially large, and long-term carbon sink to agricultural soils.
According to Lehman and coworkers, biochar as a soil conditioner has the potential of sequestering more than 10 percent of the annual carbon emissions from industrial activities, which is significantly higher than the 0.4 - 1.2 Gt per year achievable through other approaches based on organic carbon application to the soils. This is because, the cycling of organic carbon from soils to the atmosphere is fast when compared with the cycling of biochar, which decomposes much more slowly, and has estimated retention times of hundreds of years. The emerging biochar technology, therefore, has the potential of making crop production a carbon-negative industry. The essential criteria for assessing the feasibility of this emerging technology in crop production, therefore, are its effects on crop productivity, and safety, economy, and environment.
Overall benefits of biochar in crop production
Biochar is being utilized in contemporary culture for several agricultural, and ecological purposes in the context of the circular economy. For example, biochar has received a lot of political attention in recent times because of its vital role in the sequestration of carbon, while on the agricultural basis, its ability to improve soil physical, and biological conditions, enhance nutrient uptake, and reduce soil compaction, and emission of nitrous oxide have been highlighted as its benefits. Biochar also acts as soil fertilizers since its application to the soil improves soil fertility status, and helps to safeguard soil ecosystem balance. Therefore, its effects on soil physical, and chemical properties are the essential drivers of its significant effects on crop growth, and production. These biochar effects on agricultural soils, and crop yields have been attributed to enhanced water holding capacity, cation exchange capacity (CEC), improvement of plant nutrients adsorption (nitrogen, and phosphorus availability), and environments for soil micro-organisms, and facilitation of seedling biomass gain. Biochar also has the potential to decrease ammonia volatilization from soil surfaces by acting as a binder for ammonia in the soil. Indeed, due to its resistant nature to microbial decomposition in the soil, biochar has exhibited a high capacity for mitigating greenhouse gases emission from the soil.
Brandstaka and coworkers in their study have summarized the potential benefits of biochar application in crop production to include;
1) carbon sequestration by the natural process of photosynthesis
2) reduction of N2O- JA CH4 --emissions from soils
3) net production of energy in form of bioenergy
4) increase in soil fertility and yields of agricultural crops
5) increase in microbial activity in the soil
6) improvement of water retention capacity in the soil
7) improvement of cation exchange capacity in the soil
8) improvement of durability of soil aggregates and reduction of erosion
9) reduction in need of fertilization
10) reduction of nutrient leaching
However, the application of biochar in small-scale farming, especially by resources of poor farmers have been very low, probably due to lack of adequate information, and apathy for its benefits. There is, therefore, the need for practical interventions in the form of training on biochar production, and access to farm manuals highlighting the benefits of biochar application on crop productivity to small-scale farmers. Overall, biochar may become an essential technology for improving the livelihood of resource-poor farmers who farm mostly marginal lands, and also for developing a circular economy in agriculture, since it could be assimilated into orthodox crop production practices, including organic food production systems. This however requires a thorough understanding of the people, and their location-specific needs, values, and expectations, which is usually achieved through appropriate surveys of the socio-economic characteristics of the farmers in a given region of intervention.
General considerations for biochar application in crop production
Usually, there is a need to understand the characteristics of a biochar material before its application in agricultural soils. This is because of variations in the pH, ash content, surface area, and other characteristics of the biochar, which is dependent on its biomass feedstock, and processing method. For example, acidic soils will usually require the application of high-pH biochars, such as those produced from animal dung, and crop straws, while those of known high carbon content are best suited to sites where carbon sequestration is the major objective. Animal dung-derived biochar on the other hand will usually contain relatively higher levels of ash which ultimately lowers the fixed carbon content of the material. Again, the moisture content of the biochar should be determined or specified when it is purchased from a commercial producer, since biochar can hold a lot of moisture.
Presently in many developing tropical countries, production of biochar or activated charcoal meant for agricultural uses is not regulated, and industrial regulations relating to the health of producers, and minimal products standards are therefore non-existent. Such regulations are however needed to enforce label declarations, and information relevant to the use of biochar in soils is required in other to properly inform, and protect potential users from unethical practices. Again, biochar production can be a source of fire hazards, and environmental pollution when using crude, and inefficient kiln ovens or dug out pits for its production. Biochar dust particles have also been reported to explode when mixed with air in confined environments, and may therefore constitute a danger to the producer. This is usually prevented by increasing the moisture content of the biochar or by adding boric acid or ferrous sulfate under commercial production. Similarly, biochar-manure, and biochar-compost blends or biochar-mineral complexes are much less-flammable, and are therefore safer to handle, and transport.
Biochar is presented in different particle sizes according to the nature of the biomass feedstock, method of carbonization, and whether it was subjected to further size reduction. Biochar made from animal dung, sawdust, rice, and wheat husks, and chaff are usually brittle, and of small particle size, and therefore may not need further size reduction before application to the soil. However, biochar made from wood chips, pellets, sawmill wood saps, and backs are usually large in particle size, and will need to be reduced to the appropriate sizes before the soil application. This can be done manually by crushing the biochar in a mutter on a hard floor, or mechanically with a hammer mill. The crushing process usually generates a lot of dust, which results in substantial loss of the material as well as constituting serious pollution, and health problems to the operator. Therefore, it is recommended that the biochar be moistened before crushing in order to reduce the amount of dust produced during the process.
Strategies for the application of biochar to the soil
The two major factors of considerations during the application of biochar to the soil are the selection of the appropriate biochar, and the application approach. The biochar system is essentially an emerging technology in modern farming when compared with composts or mineral fertilizer management. Therefore, very limited research has been carried out on its optimal application strategies in agricultural soils. Available information however shows that biochar could be applied solely or in blends with animal manure, compost, or mineral fertilizers. Scholz, and co reported that biochar could be applied locally by spreading it on the prepared land, and hoeing or plowing into the topsoil, by top dressing, and allowing natural processes to incorporate it into the soil, in planting holes as a seed coating, in planting tubes, and when soils are used in creating green roofs or leisure lawns.
Different application rates have been reported. However, in order to optimize the benefits of biochar use in crop farming, any recommended application rate must be based on actual soil, and the biochar analysis results, and according crop. Several studies done in different regions of the world have reported positive effects with 5 - 50 tons of biochar application per hectare, on crop yields under typical farming conditions. This range is however too wide for specific local recommendations, and may lead to varied outcomes, which may be positive or negative depending on the source of the biochar, and its carbon content. For example, a 10 ton per hectare application of poultry manure derived biochar will supply less carbon than an equivalent application of wood waste-derived biochar. The poultry manure derived biochar, however, has a much higher ash content which serves as a source of additional nutrients of critical importance in the management of marginal soils. Therefore, in order to improve the precision in biochar application rates, several researchers have adopted application rates in tons of biochar-carbon per hectare, instead of the conventional tons per hectare of bulk biochar material. Again, the frequency of application is a very important consideration, because of the characteristic resistance of biochar to decomposition in the soil. Therefore, the beneficial, and residual effects of a single biochar application could be observed for several planting seasons. Some researchers have however suggested incremental application of biochar, depending on the target application rate, the availability of the biochar supply, and the soil management system, since the beneficial effects also improve with time.
Biochar has been applied in mixtures or blends with animal manure, compost, or mineral fertilizers in several studies. This is usually done to augment the low nitrogen, and other nutrient contents in the biochar, and therefore balance the nutrient supply to the crop. Biochar should however not be considered a source of nutrients unless when it contains high levels of ash as seen in animal manure derived biochars. For example, researchers at the UTA-TOSOLY, Socorro, Colombia investigated the effects of biodigester effluent, rich in ammonium-nitrogen, combined with biochar derived from the gasification of sugar cane bagasse on soil fertility as a function of the growth of maize plants over a 30 - 40-day period, following seeding. Two soil types, fertile soil or sub-soil, with or without biochar at 50 g/kg soil were prepared as well as with or without biodigester effluent at 100 kg nitrogen /hectare. The study showed that biochar application increased the green biomass growth of the maize on the fertile soil in the absence or presence of biodigester effluent, and in the sub-soil when effluent was applied, but had no effect on heavily leached soil without effluent. Similarly, the effluent increased green biomass growth when biochar was applied to the sub-soil but had no effect in the absence of biochar. The effects on root growth were similar to those on the green biomass except in the case where unilateral biochar application also increased root growth, while Soil pH was increased from 4-4.5 to 6.0-6.5 due to the biochar application. These results, therefore, highlight the significant synergistic effects of biochar, and biodigester effluents on plant growth in poor acidic soils.
The effect of biochar on soil properties
Several studies have detailed the various positive impacts of biochar application on soil physical, chemical, and biological properties as well as some of the negative impacts. The positive impacts on the soil’s physical properties include increased resistance of aggregates of soil structure, porosity, and moisture filtration rate, enhanced water holding capacity, and decreased soil density. The soil also becomes darker in color. These impacts are attributable to the various physical characteristics of biochar such as high porosity, large surface area, low density, darker color, and the binding of the biochar particles to organo-mineral complexes in the soil. The essential positive impacts on the chemical properties of the soils include an increase in pH through its high pH buffer capacity, increase in soil organic matter through the addition of highly recalcitrant carbon, and mineral or nutrient recycling by increasing the pool size, and turnover of important organic nutrients, surface adsorption, and effects on soil microorganisms. Other chemical impacts include the enhancement of nutrient availability through the direct supply of nutrients, and surface adsorption of elements, and enhancement of cation exchange capacity, by means of its porous structure, large, and mainly negative surface as well as by the biotic, and abiotic oxidation of its organic functional groups.
An important nutrient impacted by the soil application of biochar is nitrogen, a very important element in the soil ecosystem. Although biochar has low available nitrogen, it, however, plays vital roles directly or indirectly in determining the availability of nitrogen in its various forms, and in processes involved in the nitrogen cycle as shown in Figure 1. Usually, biochar is able to decrease soil nitrogen losses in different soil types, by aiding organic nitrogen dissolution, immobilization, mineralization, nitrification, N2O emission, ammonia volatilization, and nitrogen fixation. Animal manure-derived biochar usually contains higher levels of nitrogen, and other minerals than plant-derived biochars.
Biochar is also reported to directly, and indirectly impact the soil phosphorus functions by adding extra phosphorus to the soil, altering the soil pH, and changing the soil microbial compositions. However, since animal manure, and crop residues derived biochars usually contain higher phosphorus levels than wood-derived biochars, they are used more in amending soils low in phosphorus content. Again, most biochars are high in their potassium content, due to the fact that the mineral volatilizes at relatively higher temperatures than other elements. Biochar application, therefore, usually serves as a direct source of potassium amendment for K-deficient soils. Availability of potassium to plants is however influenced by its solubility in water, and other ionic solutions, the extent, and rate of dissolution, and of course certain soil properties like consistency, exchange capacity, pH, and moisture content. Usually, the more stable, and less-soluble potassium ions embedded in the carbon skeleton dissolves slowly over time, especially during the aging of biochar, and are made available to the biota.
The positive biological impacts of soil biochar application include increases in soil microbial abundance, diversity (more bacteria than fungi), and activity through increased CO2 and enzyme activity, as well as creating favorable habitat for microbial refuge against predators. Furthermore, soil biochar application promotes plant growth through an increase in root biomass in response to the increased elaboration of plant growth-promoting hormones, and also plant disease resistance. Biochar application has also been reported to have some negative impacts on the soil chemical properties, especially immobilization of nutrients like phosphorus, nitrogen, iron, and boron when its pH is too high or when nitrogen is immobilized by the increased microbial population induced by the biochar application. Similarly, the biochar may contain high levels of heavy metals, and metalloids which may increase its pH, and enhance its mobility. This may also result in the decline of the microbial population of the biochar amended soil.
Environmental benefits of soil biochar application
Among the environmental benefits linked to biochar application, carbon sequestration, rehabilitation of degraded lands, reduced greenhouse gas emissions, adsorption of contaminants to counterbalance streams, and groundwater pollution have been severally reported. Carbon sequestration is essentially the extended storage of CO2 or other forms of carbon in order to reduce global warming. Soil biochar application has particularly been promoted as the strategy for increased soil carbon sequestration since it has the added benefit of positively influencing crop yield. However, the increase in soil carbon content stock, with the potential of capturing up to 20 percent of atmospheric CO2, and the estimated residence time of 1000 years are the most pronounced effects of biochar soil application, that has elicited both environmental, and political interests. This resistance to natural decomposition processes that contribute to the stability of biochar in the soil has been attributed to its intrinsic recalcitrant nature, spatial partitioning of the substrate, and decomposers, and the development of interactions between mineral surfaces. Again, biochar induced reduction in soil mineralization rates has been reported to result in lesser amounts of mineral nitrogen in the soil for conversion into N2O, and consequently reduced emission. Similarly, some studies have shown that biochar amendment reduces methane emission from rice paddy field soils as a result of the increased populations of the methanotrophic proteobacterial organisms, against those of the methanogenic organisms.
The benefits of biochar in mitigating both soils, and water pollution through adsorption of agrochemicals, and immobilization of heavy metals has also been advocated as a cost-effective, and eco-friendly approach to the management of polluted environments. For example, biochar has been found to be 400 – 2500 times more effective than the natural soil in sorbing agrochemicals, and other pollutants, thereby reducing plant toxicity. Again, biochar has been reported to be an effective adsorbent of anthropogenic chemicals such as steroid hormones, and heavy metals pollutants. Biochar amendment is effective in adsorbing heavy metals such as copper, lead, cadmium, zinc, chromium, and nickel in soils, and aqueous media, through cation exchange mechanisms, and by forming complexes with the surface functional groups on the biochar carbon skeleton. Phosphates in the biochars have also been reported to form precipitates with copper, and lead in order to enhance their fixation in the soil. Biochar application, therefore, reduces the activity, and supply of these heavy metals to plants by enhancing their adsorption mechanisms, in soils, and other media.
Application of biochar in other agricultural media
Biochar has been applied to different types of growing media other than soil in which plants are produced. These media are made from different types of materials, and blends are usually used for potting flowers, vegetables, herbs, spices, and ornamental plants. The commonly used materials include peat, and its blends, wood bark, coir pith, green compost, pine bark, rice hulls, wood fibers, perlite, and mineral wool among others. The continued use of some of these growth media, especially peat has recently raised some environmental concerns because of its non-sustainability, and the tendency to emit greenhouse gasses. Similar to biochar, peat is characterized by high water, and air holding capacity, low bulk density, and high cation exchange capacity (CEC), therefore it has been hypothesized that biochar, and its blends could serve as an alternative to peat for growing potted plants. For example, emerging research evidence suggests that the application of biochar to peat-based substrates improves the porosity, microbial biomass, and biological activity in the medium, thereby improving the air, and water-holding capacities, and nutrient availability to plants.
Several studies have reported the impact of adding blends of different biomass-derived biochars, and different types of growth media on the growth of herbaceous, and woody plant species. These studies reported that at about 50 percent by volume of biochar application rates some improvement in terms of plant growth was recorded, while at 25 percent application, the results were generally similar or higher in plant growth when compared to the control commercial substrate. Biochar could therefore serve as an additive for improving the characteristics of many commercial growing media. Several drawbacks to the used of biochar in growing media have also been highlighted. These include its high salinity, and alkalinity, which may require prewashing, and the addition of natural acids to the biochar in order to reduce the salt content and the pH respectively. High levels of heavy metals, and other pollutants have also been assayed in biochars, necessitating careful selection of biomass feedstock, and carbonization conditions during the production of biochar targeting growing media application.
Biochar or activated charcoal is frequently used in plant tissue culture production to enhance cell growth, and development. According to Thomas in his review of the subject, the major benefits derived from adding activated charcoal to culture media are the adsorption of inhibitory substances in the culture medium, a significant decrease in the phenolic exudates, alteration of medium pH to an optimum level for morphogenesis, and establishment of a darkened environment in medium, which simulates the culture medium conditions. However, the most significant impact of activated charcoal on plant culture media is a drastic decrease in the concentrations of plant growth regulators, and other organic supplements, due to the adsorption of these chemicals by the activated charcoal. Although some drawbacks such as adsorption of essential hormones, and nutrients, have been identified in the use of activated charcoal in plant culture media, a larger number of reports however affirm its positive role in medium promoting growth, and development of plant tissues.
The commercial production of crops in nutrient-rich solutions, instead of soil, known as the hydroponic system offers several benefits such as water conservation, improved yield, good-quality products, precise nutrient and disease management, short cultivation times, and safe food, and growth environments. However, a major problem encountered in hydroponic systems is the invasion of microalgae, which has adverse effects on the water supply system, and nutrient uptake by plants, leading to a remarkable reduction in crop yield. Furthermore, algae growth has been shown to compromise the quality of the crops through the toxins they secrete, which may also be harmful to the consumers. Biochar has been used as a substrate in vegetable hydroponics systems to overcome these problems due to its stability, and high resistance to microbial degradation. Awad and coworkers have recently recommended the use of rice husk biochar alone or in combination with perlite as substrates to decreased algal growth, and ensure the high yield of safe, and healthy products during hydroponic production of cabbage, mallow, and red lettuce vegetable. Yu and coworkers also studied the value of biochar in hydroponic cultures for growing vegetables like tomatoes, and reported that the addition of activated charcoal to the nutrient solution for such cultures resulted in a significant decrease in carbon concentration in the solution, and increases in the dry weight of both the plant, and the fruit yield. These results highlight the potential value of activated charcoal in hydroponic cultures.
Biochar has been used to improve the quality of organic fertilizers derived from composting processes. Generally, biochar as an additive in the compost provides suitable habitat for microorganisms, and also improves the environmental conditions for microbial growth, thereby increasing the abundance of bacterial communities colonizing the composting matrix. These microbial effects have been shown to critically impact the overall outcome of the composting process by reducing the processing time, and influencing the nutrient cycles to enhance the quality of the end products. The composting process also causes the biochar to undergo biological weathering, which increases its cation exchange, and sorption capacities, and therefore its functionality.
Biochar has several positive impacts on soil physical, chemical, and biological properties, which have been attributed to enhanced water holding capacity, cation exchange capacity, improvement of plant nutrients adsorption, and environments for soil micro-organisms, and facilitation of seedling biomass gain. These benefits have been exploited in crop production, and mitigation of environmental pollution, especially in carbon sequestration, rehabilitation of degraded lands, reduction in greenhouse gas emissions, and adsorption of contaminants to counterbalance streams, and groundwater pollution. However, biochar application in small-scale farming, especially by resources poor farmers have been very low, probably due to lack of adequate information, and apathy on its benefits. Overall, biochar remains an essential technology for improving the livelihood of resource-poor farmers who farm mostly on marginal lands, and also for developing a circular economy in agriculture, since it could be assimilated into orthodox crop production practices, and organic food production systems.
Ali, S. (2018). Biochar: A novel approach for sustainable crop production and soil health. Acta Scientific Agriculture, 2(11): 113.
Awad, Y.M., Lee, S., Ahmed, M.B.M., Vu, N.T., Farooq, M., Kim, S., Kim, H.S., Vithanage, M., Usman, A.R.A., Al-Wabel, M., Meers, E., Kwon, E.E. and Ok, Y.S. (2017). Biochar, a potential hydroponic growth substrate, enhances the nutritional status and growth of leafy vegetables. Journal of Cleaner Production, 156: 581e588
Brandstaka, T., Helenius, J., Hovi, J., Kivelä, J., Koppelmäki, K., Simojoki, A., Soinne, H., and Tammeorg, P. (2010). Biochar filter: use of biochar in agriculture as soil conditioner. Report for Baltic Sea Action Summit (BSAS) Commitment, 2010, 22 pp.
Jindo, K., Audette, Y., Higashikawa, F.S., Silva, C.A., Akashi, K., Mastrolonardo, G., Sanchez‑Monedero, M.A., and Mondini, C. (2020). Role of biochar in promoting circular economy in the agriculture sector. Part 1: A review of the biochar roles in soil N, P, and K cycles. Chemical and Biological Technologies in Agriculture, 7:15 https://doi.org/10.1186/s40538-020-00182-8
Major, J. (2010). Guidelines on practical aspects of biochar application to field soil in various soil management systems. Document Version Information: Ver. 1.0, International Biochar Initiative. www.biochar-international.org
Hussain, M., Farooq, M., Nawaz, A., Al-Sadi, A.M., Solaiman, Z.M., Alghamdi, S.S., Ammara, U., Ok, Y.S. and Siddique, K.H.M. (2016). Biochar for crop production: potential benefits and risks. Journal of Soils and Sediments, DOI 10.1007/s11368-016-1360-2
Rodriguez, L. (2010). Integrated Farming Systems for Food and Energy in a Warming, Resource-depleting World. Doctoral Dissertation, Humboldt-Universität Zu Berlin, Germany.
Scholz, S.M., Sembres, T., Roberts, K., Whitman, T., Wilson, K. and Lehmann, J. (2014). Biochar systems for smallholders in developing countries: Leveraging current knowledge and exploring future potential for climate-smart agriculture. International Bank for Reconstruction and Development / The World Bank, Washington, DC.
Thomas, T.D. (2008). The role of activated charcoal in plant tissue culture. Biotechnology Advances, 26: 618–631