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Increasing Water Holding Capacity by 40% with Biochar

Increasing Water Holding Capacity by 40% with Biochar

Back in 2021 we teamed up with Jonathan Mongomery Ph.D. from Humboldt State University to officially test the biochar we were planning to use in our products. Jonathan is one of the top experts on biochar and soil structure in North America and we were thrilled to work with him.

Jonathan knew from his past research that amended soil with biochar increased its ability to retain water, but we wanted to test it ourselves with BC Biocarbon biochar (the biochar used in all our products) so we shipped a 70L to the windy shores of Northern California to get started.

We first set out to measure the water holding capacity increase from the biochar. To do this, we took eighteen 1 L pots and divided them into 6 groups:

  • Control (just soil)
  • 2.5% biochar with soil 
  • 5% biochar with soil 
  • 10% biochar with soil 
  • 15% biochar with soil 
  • 10% biochar + bentonite with soil*

We used soil that was typical of Humboldt, California. Soil in this area tends to be sandy, loamy, and acidic, which is representative of soils in many parts of North America.

Soil characterization of the soil we used in our biochar dry down test.

Our study was simple. We watered each pot until it reached its saturation point, then left the pots in the greenhouse, and measured how well the solid held onto their moisture content. The saturation point being the point at which the pot would no longer accept water (i.e. Any watering past this point was not possible). 

The results can be found here with the following highlights:

  • Water Holding Capacity increased by 40% with an application of biochar
  • Water Holding Capacity decreased with 15% biochar (10% Biochar being the magic number). We found this surprising but consistent with general guidance of a 10% ratio of biochar and soil working the best.
  • The addition of bentonite (at 5% to biochar volume) seems to have increased the water holding capacity even further.
Biochar and soil pots after a couple months of no water.
Water holding capacity of soil based on percentage of biochar

Keeping in mind the limitations of the study:

  • We only tested one type of soil. The structure of your soil, specifically the degree of sandiness, will impact the Water Holding Capacity
  • We tested in pots instead of the field. While this study certainly indicates that biochar increase Water Holding Capacity, it’s possible that the number increases would be different for soil not in pots
  • We only did the study once. We are planning to replicate it in the future which will allow us to publish in scientific journals.

We were impressed with the results, and confirmed exactly what we were seeing in our own trials in our gardens, lawns and our indoor plants. Biochar helps hold onto water longer, which allows you to go more time without watering, and allows the plants to make better use of the water they get.

*We added this group to sanity check the addition of bentonite into some of our products which can help reduce the dustiness of biochar. We used a 5% bentonite 95% biochar mixture in this group.