Biochar Application Rates: Finding the Optimum for Crop Production
Biochar is increasingly being investigated as a way to improve crop production while reducing agriculture's dependence on conventional fertilisers. By adding stable, porous carbon to soil, biochar can improve nutrient retention, soil structure, water availability and biological activity. Depending on its feedstock, it can also supply plant nutrients directly.
But more biochar does not necessarily mean better crop performance.
For agricultural deployment, it is therefore necessary to determine the ideal biochar application rate, which is influenced by both crop species and biochar characteristics.
Research increasingly suggests that, when biochar is used alongside fertiliser rather than as a standalone input, relatively low application rates can produce meaningful agronomic benefits. For potato production in particular, recent field studies indicate that rates in the region of 1–2 tonnes of biochar per hectare give a high crop yield 1, 2.
How does biochar reduce fertiliser requirements?
Conventional fertilisers supply crops with readily available nutrients, particularly nitrogen (N), phosphorus (P) and potassium (K). Biochar works differently.
Its porous structure and surface chemistry can change the environment in which these nutrients are supplied to plants. Depending on the biochar and soil, it can:
increase nutrient retention and reduce losses through leaching;
increase soil organic carbon;
improve soil porosity and water retention;
influence soil pH and cation exchange capacity;
provide habitat for soil microorganisms; and
improve the availability of nutrients such as phosphorus and potassium.
A global meta-analysis found that biochar application increased soil organic carbon, cation exchange capacity and porosity, although the response varied substantially according to soil texture, feedstock, pyrolysis conditions and application rate 3. Crop productivity responses were similarly dependent on these factors.
This means biochar should not simply be considered another fertiliser. Instead, it can help modify the soil environment in ways that improve nutrient availability and potentially increase the efficiency with which conventional fertilisers are used.
That creates the possibility of maintaining crop productivity while reducing conventional fertiliser inputs.
Biochar application to potato fields
Potatoes provide a useful case study because several experiments have tested biochar alongside conventional or organic fertilisation.
0.66 t/ha biochar with organic fertiliser
Hou et al. (2024) investigated combinations of corn-straw biochar and sheep-manure organic fertiliser in potato production in China 1.
The treatment identified as producing the strongest overall effect combined 0.66 t/ha biochar with 4.46 t/ha organic fertiliser. This treatment improved soil fertility, rhizosphere bacterial diversity and potato yield compared with the untreated control. They concluded that the combination made the largest contribution to increasing potato yield among the treatments tested.
1.5 t/ha biochar with reduced compound fertiliser
A 2025 study by Shen et al. evaluated biochar alongside different combinations of calcium-magnesium phosphate fertiliser and compound fertiliser in potato production in Yunnan Province, China 2. Treatments included full and 50% compound-fertiliser rates, both with and without biochar.
One of the treatments combined:
3.0 t/ha calcium-magnesium phosphate fertiliser
1.5 t/ha compound fertiliser
1.5 t/ha biochar
This represented a 50% reduction in compound fertiliser relative to the full compound-fertiliser treatment, and reported favourable effects on potato yield and soil quality 2.
This shows the ability of biochar to reduce fertiliser inputs into agricultural fields, lowering both emissions and costs for farmers.
Higher application rates
Higher rates of biochar have also produced positive responses.
Mollick et al. (2020) tested potato production using biochar rates of 2.5, 5.0 and 7.5 t/ha alongside either the recommended inorganic fertiliser rate or half of that rate, as well as a 10 t/ha biochar-only treatment 4. Biochar significantly affected potato growth, tuber yield, quality and post-harvest soil organic carbon.
Their results demonstrate that higher biochar rates can be agronomically effective. However, the fact that a high rate can improve crop performance does not necessarily mean it represents the most efficient rate for commercial deployment.
Ideal application rate?
There is no universal optimum biochar application rate.
Biochars differ considerably according to feedstock, pyrolysis conditions, nutrient content, pH, particle size, ash content and surface chemistry. Soils likewise differ in texture, organic matter, pH, nutrient status and water availability. Crop requirements further influence the ideal application.
For this reason, application rates of 10, 20, up to 50 t/ha sometimes used in biochar research should not automatically be translated into commercial recommendations.
For biochar used alongside fertiliser in potato production, the available evidence seems to recommend the 1-10 t/ha range as most effective for growth 1, 2, 4.
On the evidence currently available, approximately 1–2 t/ha therefore represents a reasonable starting range for field evaluation where the objective is to investigate improved nutrient-use efficiency and reductions in conventional fertiliser use. This should be regarded as a trial range rather than a universal agronomic prescription.
The optimum rate for a particular farm should ultimately be determined by:
biochar composition and nutrient content;
soil analysis;
crop nutrient requirements;
soil type and existing organic matter;
application method;
economic cost; and
replicated field performance.
Application method
Application rate is only one part of the problem.
Biochar needs to interact with soil, roots, water and nutrients. How it is applied can therefore influence its performance.
Biochar studies on potato crops typically spread biochar across the plot and subsequently incorporated it uniformly into the upper layer of soil (e.g 20 cm) 5.
Biochar can also be combined or "charged" with nutrient-rich materials before application. This may involve mixing biochar with compost or exposing it to a fertiliser solution so that nutrients are associated with its pore structure and surfaces before incorporation into the soil.
Particle size may also matter – it has been found that reducing particle size was associated with stronger plant-growth responses overall, with the analysed response peaking at approximately 0.5–1.0 mm 5. However, practical agricultural deployment must also consider dust generation, handling, spreading equipment and product stability.
The ideal physical form therefore needs to balance agronomic performance with practical farm application.
Conclusion
The evidence shows that biochar can alter soil nutrient availability, organic carbon, microbial communities and crop performance. Potato-specific studies also show that useful responses can occur at relatively modest application rates when biochar is combined with other nutrient inputs 1, 2, 3, 4.
For potatoes, the current evidence suggests that 1–2 t/ha is a strong starting range for further field testing, with future customisation in response to research findings and on different crops and field conditions.
References
1 Hou, J., Xing, C., Zhang, J., Wang, Z., Liu, M., Duan, Y. and Zhao, H. (2024). Increase in potato yield by the combined application of biochar and organic fertilizer: key role of rhizosphere microbial diversity. Frontiers in Plant Science, 15, 1389864.
https://doi.org/10.3389/fpls.2024.1389864
2 Shen, Y., Zhu, W., Zhu, X., Fan, D., Zhang, S., Hu, D., Yan, C., Tian, M., Shang, Y. and Ma, L. (2025). Effect of biochar with fertilizer on potato yield and soil quality. Plant and Soil, 516, 1975–1992.
https://doi.org/10.1007/s11104-025-07839-z
3 Tesfaye, F., Liu, X., Zheng, J., Cheng, K., Bian, R., Zhang, X., Li, L., Drosos, M., Joseph, S. and Pan, G. (2021). Could biochar amendment be a tool to improve soil availability and plant uptake of phosphorus? A meta-analysis of published experiments. Environmental Science and Pollution Research, 28, 34108–34120.
https://doi.org/10.1007/s11356-021-14119-7
4 Mollick, O.A., Paul, A.K., Alam, I. and Sumon, M.M. (2020). Effect of biochar on yield and quality of potato (Solanum tuberosum) tuber. International Journal of Bio-resource and Stress Management, 11(5), 445–450.
https://doi.org/10.23910/1.2020.2140
5 Thomas, S.C. (2021). Post-processing of biochars to enhance plant growth responses: a review and meta-analysis. Biochar, 3, 437–455.
https://doi.org/10.1007/s42773-021-00115-0
