February 23, 2024
minute read

Biochar Basics: Soil Water

Biochar is highly porous at multiple scales, giving it a vast pore volume. This structure can absorb significant amounts of water and dissolved nutrients within the pore structure and between particles. Using biochar as a soil amendment can improve the water-holding capacity of soils and, eventually, plant available soil water by up to 28.5%.

The effects of biochar on soil water properties can improve crop resilience to water-related stresses, such as drought and flooding, which are becoming increasingly frequent as a result of climate change.

How does it work?

Sponge-like Properties

Biochar is highly porous on multiple scales. This structure enables it to act as a sponge and improve soil water retention.

Microscope image of porous structure in woody biochar.
Microscope image of porous structure in woody biochar.

Improves Root Structure

Biochar can increase root biomass and improve crop root structure. Healthier root systems mean the plant is more effective at accessing available water in the soil. This improves the resilience of crops, especially in drought conditions. Root-zone application can improve local soil water properties, even with low-dose biochar applications.

Biochar held within the crop roots.
Biochar held within the crop roots.

High Porosity

The porous structure of biochar can benefit hydraulic conductivity (how easily water moves through pores in the soil), helping crops uptake water.

This is especially when applied to clay soils, which have poor hydraulic conductivity. In sandy soils, biochar decreases hydraulic conductivity which improves water retention. Both these effects improve plant available water.

Biochar in different soil types

Clay soils: Biochar improves water flow and reduces wilting point in clay soils.
Sandy soils: Biochar improves field capacity and water retention in sandy soils.
Dense soils: High dose biochar application lowers soil bulk density, increasing field capacity.

💡Using biochar effectively

The targeted application of tailored biochar to soils can increase the effectiveness of biochar on crop water accessibility and drought resilience. This is dependent on soil type. To achieve effects on soil water availability, large amounts of biochar (>10 tha−1) would have to be applied. Low-dose biochar application (<1t ha-1) at the root zone provides locally increased available water to the roots during the early stages of plant growth, when crops are especially vulnerable to drought.

  • In sandy soils, small biochar particle size biochar with high specific surface area and porosity are more effective.
  • In clayey soils, biochars with larger particle sizes and with high specific surface area are recommended.
  • To improve bulk soil water properties, biochar application must achieve rates >10 t biochar ha-1.
  • Low-dose targeted biochar application (<1 t biochar ha-1) without soil cultivation can improve plant available soil water if applied to the crop root zone.
Biochar and soil water
Effects of applying biochar at the root zone, for example through co-application with slurry or direct application.
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