Plant productivity

Plant Productivity with Biochar Applications to Soils

Biochar is not a fertilizer. It does not supply large amounts of plant nutrients on its own. But in many soils—especially those with chemical or structural constraints—it can significantly influence yield. The benefits come from biochar’s capacity to modify the soil environment: improving nutrient retention, buffering acidity, enhancing microbial activity, and reducing losses through leaching or gas emissions. This chapter summarizes what the data show about how and where biochar can increase plant productivity.

Yield Effects: Context-Dependent but Often Positive

The clearest and most consistent yield improvements are seen in highly weathered, acidic, or degraded soils. In these cases, biochar often delivers rapid gains by:

Neutralizing soil acidity and reducing aluminum toxicity.
Increasing cation exchange capacity (CEC), allowing the soil to hold nutrients more effectively.
Improving soil physical structure and water retention.

These effects can translate into substantial yield gains, particularly for cereal crops like maize and wheat. Field trials in tropical and subtropical regions have demonstrated that biochar produced from feedstocks such as poultry litter or green waste can boost yields significantly when applied to soils such as Ferralsols and Acrisols.

Synergy with Fertilizers

On its own, biochar rarely matches the performance of mineral fertilizers in supplying plant nutrients. But when combined with fertilizers, biochar often enhances nutrient use efficiency. This happens in part because biochar reduces nutrient leaching—particularly of nitrogen and phosphorus—and buffers against nutrient losses due to volatilization or denitrification.

Trials consistently show that crops grown with both biochar and fertilizer perform better than those grown with fertilizer alone, even when the total applied nutrient levels are the same. The combination is especially effective in coarse-textured soils and in regions with high rainfall, where nutrient loss pathways are more active.

Application Rate and Form Matter

Field studies use a wide range of application rates, from as low as 2 t/ha up to 50 t/ha. Typical rates that produce yield responses fall between 5–20 t/ha. In degraded soils, even lower rates can produce measurable improvements. In fertile soils or those under high-input management, higher rates may be necessary to observe an effect.

Particle size and incorporation depth are also important. Finely ground biochar is more reactive but presents handling challenges, including dust risks and wind losses. Surface application is easier but tends to produce weaker short-term responses than incorporation into the topsoil.

Soil Type Influences Outcomes

Biochar’s effects are shaped strongly by soil mineralogy and texture. Sandy soils benefit from improved water retention and nutrient holding. Acidic soils see gains through pH correction and aluminum immobilization. Clay-rich soils often show less immediate response, since they already have high CEC and inherent buffering capacity.

In phosphorus-limited soils, biochar may enhance availability indirectly, either by altering microbial dynamics or through changes in sorption equilibria. In phosphorus-rich soils, such effects are typically minimal.

Crop Type and System Management

Not all crops respond the same way. Fast-growing cereals and nutrient-demanding annuals tend to show the greatest response. Root crops may also benefit, particularly if biochar reduces subsoil constraints like acidity or compaction. In contrast, leguminous crops or species grown under high fertility regimes may show limited response to biochar alone.

Yield benefits are generally stronger in low-input systems where nutrients are limiting or poorly retained. Under high-input conditions, the incremental benefits of biochar are often smaller, though improvements in fertilizer efficiency and soil health may still justify its use over the long term.

Summary of Field Trial Evidence

More than 40 field trials have been reviewed across diverse environments. General patterns include:

Biochar combined with fertilizer consistently outperforms fertilizer-only treatments in low-fertility soils.
Biochar alone gives variable results; yield increases occur mainly in degraded soils.
Responses are most pronounced in tropical regions, especially in acidic soils.
In temperate or high-input systems, responses are more variable and often modest.
Effects may persist or grow over time as biochar integrates into the soil matrix.

Mechanisms Beyond Nutrients

In addition to nutrient effects, biochar can influence productivity through other pathways:

Improved root growth due to better soil structure.
Increased microbial activity, particularly in nutrient-limited soils.
Changes in soil moisture dynamics, reducing drought stress.
Altered microbial symbioses, including enhanced mycorrhizal colonization in some cases.

These secondary effects can contribute to yield gains even when biochar contributes few direct nutrients.

Limitations and Knowledge Gaps

While the evidence is promising, results vary widely depending on site conditions, biochar properties, and crop management. Key limitations include:

Lack of standardization across studies in terms of biochar type, application method, and soil management.
Uncertainty about long-term impacts beyond 2–3 growing seasons.
Limited economic assessments that integrate transport, processing, and labor costs with yield benefits.
Gaps in understanding interactions with soil biota and root-microbe dynamics.

Practical Takeaways

Start with your soil. Biochar performs best in degraded, acidic, or low-buffer soils. If your soil already holds nutrients well and has a neutral pH, benefits may be marginal.
Use it with fertilizers. The greatest gains come from combining biochar with other nutrient sources, not using it as a substitute.
Incorporate it. Mixing biochar into the root zone is generally more effective than leaving it on the surface.
Expect variable results. Trial on a small area before scaling up, especially in high-input or temperate systems.