Biochar Trial Beds Fall 2025 — The Results are Finally In!

The crops have been harvested from the four trial beds in the VegHeadz Garden   Get our report on the results here. 

The biochar trial furnished much new information. The most important thing we learned was that there was no indication that the biochar did any harm to the garden.   Please understand this was just an informal test in our garden. A truly scientific trial would include multiple plots with each different amendment.  It would be controlled under  strict guidelines.  Our test consisted of measuring inputs,  observing regularly, and measuring outputs, but it would not qualify as a scientific test.  Still it was worthwhile and we learned quite a bit. 

The plants tested were collards and daikon radishes. The radishes did not do well, possibly for two reasons. The most obvious reason was freezing.  They were much more drastically affected by the winter freezes than the collards. The other reason is most likely the high pH of the soil. The final soil test reflected pH of 7.5 in all beds and daikon radishes prefer a more acidic environment. Perhaps we’ll grow some daikons and add sulfur to the soil to see if the results differ.  Close observation, testing and comparing in a more or less controlled environment, and recording and comparing results, are the best ways to figure out what’s going on in your garden.

The most important thing we learned was that all our fears about the biochar increasing the pH in the garden beds were unfounded. The pH did increase, but it also increased in the control bed, which had no biochar added. Compost tends to have an alkaline pH in any event, and we feel that the increase in pH was caused by the compost. Again, application of sulfur, peat moss, or other acidifying amendments would most likely be beneficial.  

There was a moderate but positive increase in plant survival, production, and plant health in the bed that was amended with compost, first co-composted with biochar for a period of 6 to 8 weeks with no pH adjustment. This was our most successful bed.  Again, repeating the test with each amendment applied to multiple plots would produce a more reliable result and eliminate the possibility of coincidence and other unknown factors.

For those wishing to use biochar in their gardens, we make the following recommendations, based on online research sources and our experience so far:

• Determine the source of the material used to make the biochar you intend to use and the process by which it was created.   Research sources indicate that the best biochar source material for application in vegetable gardens appears to be lignin-rich biomass from hardwood like oak and softwood such as pine. The best processing temperatures for creating biochar from these materials for use in gardens appears to be 400-700°F. 

• Start with a small ratio of biochar to compost, 5% to 10%, and compost them together for a period of at least six weeks to charge before applying to the soil.  This allows time for micronutrients to infiltrate the pores of the biochar and for microorganisms to multiply and populate the biochar. The mixture should be kept moist, but not overly wet as microorganisms need access to oxygen to multiply.

• Keep track of how much biochar you are adding, particularly if adding in more than one application.  5% to 10% of the soil volume is the recommended application rate according to most sources.  It is not recommended to ever add more than 20% biochar to the soil.  Biochar decomposes extremely slowly, and it cannot be removed once it is added to the soil.

• See the entire report on our Fall 2025 biochar trial here and the biochar handout under “soil amendments” in the Gardening Resources section of the left sidebar on this blog.  The handout includes how to calculate how much biochar to add to achieve the desired ratio in the soil and information about pre-treating the biochar, plus a list of many of the biochar research resources we referred to.

A new trial is under way for spring 2026.  The spring trial involves comparing results when adding our organic fertilizer mix and biochar in various combinations to the trial beds.  Test plants will be peppers and green beans.  

We are also trialing the addition of organic fertilizer to half of the tomatoes in the B rotation beds.  The same number and variety of tomato plants have been planted on each side of the bed.  It will be interesting to learn whether this makes a difference, not only in growth and production, but in pest and disease resistance, heat tolerance, and length of bearing season.  So far the unfertilized plants look bigger, but they have not yet set any tomatoes.  The fertilized plants have many small tomatoes.  We’ve also added the fertilizer to the potatoes in the C rotation beds.  So far they look amazing.  We’ll see how many potatoes they produce and whether they are free of blemishes.