Rice husk biochar: big gains on poor acid soil, little on good soil

Biochar is charcoal made from plant waste for putting into soil. The black rice husk (charred husk) Thai growers already use is a biochar. Recent research is clear that it is not a miracle: yields rose several-fold on poor, acid soil, but on soil that was already good they did not rise at all.

This lesson sums up what the research found, which chars you can use and which you can’t, and how to charge biochar with compost safely before it goes into the ground.

How biochar works

Biomass charred with little air leaves a carbon skeleton full of tiny pores. Those pores hold water and hang on to nutrients so they don’t wash away as easily. The char is also alkaline, so it can lift the pH of acid soil.

The rice husk biochar used in the Timor-Leste study measured:

• pH 8.9, fairly alkaline
• 27.9% total carbon and only 0.27% nitrogen
• some phosphorus and potassium
• made in an open retort at a maximum of about 413-450 °C, a range the authors considered close to ideal for soil use

With almost no nitrogen, biochar is not a fertiliser. It helps the soil keep what you add to it.

What the research says: poor soil gains a lot, good soil little

A 2023 study tested rice husk biochar on vegetable plots at three sites in Timor-Leste. Every site was short of phosphorus and zinc. An Iowa State University project tested biochar under peppers on loam and sand in better condition. The results were very different.

Trial	Starting soil pH	Biochar applied	Effect on yield
Timor-Leste, acid site	5.2	20 t/ha	chili, tomato, soybean up about 230% on average
Timor-Leste, neutral site	7.3	30 t/ha	tomato up 76% (5.9 to 10.4 t/ha)
Timor-Leste, alkaline site	7.9	30 t/ha	four vegetables up 183% on average
Iowa, USA, loam and sand	not given in the abstract	1.1-4.5 kg/m²	no change in marketable yield; fewer culls such as blossom end rot

Read the table with care. Timor-Leste gained strongly even on alkaline soil. The authors concluded that the real limits on yield were phosphorus and zinc shortages; the rice husk char supplied some of both and helped the soil hold them. As for pH, in an incubation of acid soil, 20 t/ha of biochar lifted pH from 4.5 to about 6.2-6.3 over 26 weeks, which helps only soil that is acid to begin with.

One more acid-soil finding: biochar plus phosphorus fertiliser beat biochar alone by 34-92%, depending on the crop.

Biochar helps most when the soil is missing something the char can fix. If your soil is already crumbly and gets regular compost, don’t expect your vegetables to double.

On soil that is already alkaline, the University of Arizona guide warns that biochar tends to add salts and raise the pH further at first. If you don’t know whether your soil is acid or alkaline, test the pH first.

Barbecue charcoal is not biochar

Plain lump charcoal from clean wood is char too, but what is sold for grilling varies:

• Briquettes. Some makers’ safety data sheets list a starch binder, limestone and borax. Borax supplies boron, which plants need only in tiny amounts.
• Self-lighting or pre-soaked charcoal carries ignition fluid. Keep it out of the vegetable bed.
• Ash from the grill is what remains after full burning. It has no porous char structure and is strongly alkaline.

The Arizona guide advises buying only biochar sold specifically for soil, because char made for other uses may carry contaminants. In Thailand the easiest option is bagged black rice husk sold for potting mixes.

Charge it with compost first

Fresh char is like a dry sponge. Put it straight into the ground and it soaks up some nutrients for itself first. The Arizona guide’s answer is called charging:

1. Mix the char with compost. Start at 1 part char to 10 parts compost. Up to 1 to 1 works, but most gardeners start at 1 to 10 to be sure plants tolerate it.
2. Sprinkle with water until evenly moist, not soggy.
3. Leave it for several weeks in the shade, keeping it moist.
4. Work it into the soil. Don’t leave it on the surface; char dust is light and blows away.

If the char came from plants grown on salty soil, soak it overnight and drain it two or three times first. Soaking washes out salts and lowers the char’s pH a little.

Worked example: a 1 x 2 m bed on acid soil

Say the bed is 1 m wide and 2 m long, so 2 m², and the pH reads 5.2.

1. Convert units first: 1 t/ha equals 0.1 kg/m².
2. The Timor-Leste rate of 20 t/ha is 2 kg/m², so this bed would take 2 × 2 = 4 kg.
3. The authors suggest 5-6 t/ha may be more practical for growers. That is 0.5-0.6 kg/m², or about 1-1.2 kg for this bed.
4. Start with 1 kg, mix with 10 times its volume of compost, charge for several weeks, then dig it into the top 15 cm.
5. Grow one crop, then test pH again. If it is still below 6 and the vegetables did better, add more next round.

If the same bed read pH 7.5, skip the char and add compost instead.

Making rice husk char safely

Sources disagree. Washington State University (WSU) Extension says to use commercial biochar with known properties, because air is hard to control at home and the process creates pollutants. The Arizona guide says home production is fine if you follow basic fire safety. If you make your own:

• Work outdoors, away from buildings and fuel, and check local burning rules first.
• Use one clean material, such as husk alone. No plastic, painted wood or toxic plants such as oleander.
• When the flames die down, smother the fire with a lid or quench it with water, then let it cool for several hours.
• Char dust is hazardous to breathe. Wet it before scooping or crushing, wear a dust mask, and store it in sealed bags.

Common mistakes

• Adding char to soil that is already alkaline. The pH climbs past what most vegetables like, and the char may add salts as well.
• Using ash instead of char. Ash is strongly alkaline and has no pores. It is a different material.
• Cutting fertiliser after adding plain char. Char is not fertiliser; plants may run short of nitrogen early on.
• Spreading char as a mulch. The dust blows away. Mix it in.
• Crushing dry char without a mask. Fine dust reaches the lungs easily.

Sources

• Productivity gains in vegetables from rice husk biochar application in nutrient-poor soils in Timor-Leste — Scientific Reports via PMC
• Effects of Biochar on Soil Nutrient Retention and Microbial Communities in Vegetable Cropping Systems — SARE
• Guide to Making and Using Biochar for Gardens in Southern Arizona (AZ1752) — University of Arizona Cooperative Extension
• Biochar: A Gardener’s Primer (FS147E) — Washington State University Extension