In a study on the impacts of biochar on soil properties and crop productivity, published in the Journal of Pharmacognosy and Phytochemistry, authors Naresh Kumar Yadav, Vijay Kumar, KR Sharma, Raj Singh Choudhary, Tejbir Singh Butter, Gobinder Singh, Manoj Kumar, and Rakesh Kumar reviewed the multifaceted roles of biochar as a soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More. The review highlights that in sandy soil conditions, applying cow manure biochar at a rate of 15 tons per hectare significantly boosted maize grain yield to over 3.0 tons per hectare, representing the highest gain compared to other tested application rates, which ranged from 0 to 20 tons per hectare.

Biochar’s primary goals as a soil amendment are to improve soil properties, sequester carbon, and reduce the reliance on conventional fertilizers. Biochar’s highly porous structure retains the cell wall structure of the original biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More. This intrinsic feature allows biochar to alter soil physical properties such as density, pore size distribution, and structure, thereby impacting soil aeration, water-holding capacity, and workability. The porous nature is also key to its role in carbon sequestration; the stable carbon in biochar remains sequestered for much longer periods in the soil than in the original biomass. This property is central to biochar’s potential role in climate change mitigation.

The addition of biochar can significantly improve various soil properties, including increasing soil pH (acting as a liming agent) , lowering bulk density , and enhancing water-holding capacity. Furthermore, it increases the soil’s cation exchange capacity (CEC) , allowing it to adsorb and retain essential cations in an exchangeable form more effectively than other types of soil organic matter. These improved conditions enhance nutrient availability and uptake for plants, which has been frequently demonstrated in acidic and highly weathered tropical field soils. For example, studies showed biochar improving the soil fertility status in sandy loam soils by increasing organic carbon, CEC, available Phosphorus (P), and exchangeable Potassium (K), Calcium (Ca), and Magnesium (Mg). Biochar application has also been found to decrease nutrient leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More, further enhancing nutrient availability for plants.

The positive influence of biochar on crop productivity is variable and highly dependent on factors such as the initial soil properties, environmental conditions, and the specific characteristics of the biochar used (feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More and pyrolysis conditionsThe conditions under which pyrolysis takes place, such as temperature, heating rate, and residence time, can significantly affect the properties of the biochar produced.   More). A meta-analysis of over 371 independent studies showed that biochar generally increased aboveground productivity, crop yield, and the concentrations of nutrients like Phosphorus and Potassium in plant tissue. An earlier review of field and pot experiments reported an average crop productivity increase of 11%. However, the yield benefits vary substantially across crop types, showing greater increases for legumes (30%) and vegetables (29%) than for cereal crops like corn (8%), wheat (11%), and rice (7%).

The optimal application rate of biochar is critical and must be determined through extensive field testing. Most positive effects on crop yields are reported within a broad range of 5 to 50 tons of biochar per hectare, provided appropriate nutrient management is also implemented. For instance, cow manure biochar in sandy soil dramatically increased water use efficiency by 139% at a 15 tons per hectare rate compared to the control. However, the benefits are not limitless. Experiments have shown instances of decreasing yields or no difference from the control at very high application rates, such as 165 tons/ha in a pot experiment or 8-16 tons/ha for upland rice. In a sandy loam soil, the application of peanut hull and pine chip biochar at 11 and 22 tons per hectare could even reduce corn yields below control levels under standard fertilizer management.

Biochar is generally not a substitute for fertilizer; it must be used in conjunction with necessary amounts of nitrogen and other key nutrients to achieve yield improvements. Its main function related to fertility is enhancing fertilizer use efficiency (FUE) by increasing nutrient retention and decreasing nutrient loss through leaching or gaseous emissions. Due to its recalcitrance to decomposition, biochar’s beneficial effects can last over several growing seasons, meaning it does not need to be applied with every crop cycle. While biochar shows great promise as a sustainable soil amendment, further clarity on optimizing its application for various crop systems is needed for it to gain widespread acceptance.

Source: Yadav, N. K., Kumar, V., Sharma, K. R., Choudhary, R. S., Butter, T. S., Singh, G., Kumar, M., & Kumar, R. (2018). Biochar and their impacts on soil properties and crop productivity: a review. Journal of Pharmacognosy and Phytochemistry, 7(4), 49–54.