Benefits of biochar

Lately, I’ve seen many people asking: “How is biochar different from regular charcoal?” So in this post, I’ll try to answer that question in detail.

In fact, both biochar and regular charcoal are made by burning wood (or other carbon-rich materials) in a low-oxygen environment, a process called pyrolysis. This creates a stable form of carbon with an extremely high surface area (up to 2,000 m² per gram, depending on the method of production). This process is known as carbonization, which is different from oxidation that occurs when wood burns in the presence of ample oxygen and releases carbon as CO₂. This stable carbon form does not break down easily in soil and can last for thousands of years.

So, what’s the difference between biochar and regular charcoal?

There are two main differences:

1. Burning Temperature

Regular charcoal is typically made at relatively low temperatures (300–500°C), meaning the oils in the wood are not fully burned during pyrolysis. This makes the charcoal easy to ignite—ideal for cooking fires.
In contrast, biochar is made at higher temperatures (500–800°C), which burns off all the oils in the wood, leaving behind a purer, more porous form of carbon that no longer contains tar/oils that might harm plant roots. That’s why biochar is more suitable as a soil amendment.

2. The “Activation” Process

After carbonization, biochar needs to be “activated” before use in soil improvement. Activation involves charging the biochar with microbes and nutrients (such as fish emulsion), which promotes microbial colonization in the porous structure.
If biochar is applied to soil without activation, it can absorb nitrogen from the surrounding environment, leading to nutrient deficiencies in plants. Conversely, when properly charged, biochar enhances soil vitality.

Benefits of Biochar

Increases Soil Cation Exchange Capacity (CEC):

CEC is the soil’s ability to hold and exchange nutrients. High CEC helps retain essential nutrients like K⁺ (Potassium), Ca²⁺ (Calcium), and NH₄⁺ (Ammonium), making them more available to plants and preventing them from leaching.
CEC depends on two main factors:

1. Clay particles

2. Organic matter

Clay particles carry a negative charge that attracts and holds positively charged nutrients (cations). Organic matter is even better—it can retain both cations (like K⁺, Ca²⁺) and anions (like NO₃⁻, PO₄³⁻).

In Vietnam, many soils have low CEC due to intense weathering from heavy rainfall, which washes nutrients away and accumulates acidic ions like H⁺, Al³⁺, and Fe³⁺ (hydrogen, aluminum, and iron). Modern agriculture has further reduced soil organic matter and increased acidity, worsening CEC over time.

Adding biochar to soil can boost CEC, especially in acidic clays and sandy soils—both of which typically have low CEC. This is due to biochar’s large surface area, which serves as an ion exchange surface and helps retain nutrients. The effectiveness depends on the feedstock and pyrolysis temperature.

Raises Soil pH:

Biochar is usually alkaline, so it can help reduce soil acidity by replacing H⁺ ions in the soil. A higher pH allows important nutrients like potassium (K⁺) and phosphorus (PO₄³⁻) to bind more easily to the soil, making them more accessible to plants.

Improves Soil Water Retention:

Because of its porous structure and high surface area, biochar improves the soil's ability to retain water. This is especially helpful for sandy or low water-retention soils. Better water retention helps plants grow more vigorously and reduces drought stress.

Increases Organic Matter in Soil:

Biochar is a stable form of carbon that doesn’t easily decompose. When mixed into soil, it increases the total organic matter content. More organic matter means healthier plants and a more diverse microbial community.

My Biochar-Making Method

There are many ways to make biochar. I use a horizontally laid steel drum with air holes cut into it (see photo). I stack a small amount of wood inside, prepare a water source nearby, then light the fire. While the fire is burning, I watch the top layer of wood. Whenever it starts turning white (a sign of oxidation), I add another layer of wood.

The fire stays in the top layer and produces heat, which evaporates oil and water from the wood below. I keep adding layers until I run out of wood or the drum is full. Once the top layer starts turning white, I extinguish the fire by spraying or dousing with cold water. The sudden cooling increases cracking in the char, which increases surface area.

I leave the char soaking in water until it cools, then strain out the soaking liquid to water plants—this “biochar tea” is rich in minerals. The char itself is soaked in fish emulsion for a few days, then added to compost before being spread in the garden.

Things to Note When Making Biochar

To apply this method effectively, keep these tips in mind:

1. Burn completely dry wood at high heat, while maintaining low oxygen conditions

2. Use wood pieces of similar size

3. Crush large chunks into small pieces (around 0.5 cm is ideal)

4. Activate the biochar by:

   • Soaking it in a nitrogen-rich solution (e.g. fish emulsion), or

   • Mixing it with compost (or both)

5. Mix into soil at a ratio of 1 part biochar to 10 parts soil