The Longterm Impact of Biochar in Soil (Season 2)

After posting our results from our second season test plots in Kenya, we received a wide range of commentary. While it was nearly all positive, some people did not quite understand the impact of these results. We’ve tried to clarify the information below, and present what we feel are the most important results from this experiment. This is not intended to be published as a peer-reviewed scientific document, it’s only a statement of our findings, and the conclusions we’ve drawn. If you’d like to draw your own conclusions from our raw data, please send an email request to

Our test farm is managed by our team of local farmers in Bungoma County, Western Kenya. All test plots receive similar sunlight and rainfall.

1) Baseline yields of staple grain in Kenya are below subsistence levels

Nearly 70% of Kenyan smallholders use some amount of chemical fertilizer. This stands in contrast to the rest of Subsaharan Africa (SSA), where use of chemical fertilizer is far less common. For those farmers who do not use any chemical fertilizer (or organic amendment), we found they could only produce around 70kg of dry sorghum per acre. By adding 50kg of DAP/acre– the Kenyan Government’s recommended allowance of chemical fertilizer– farmers can increase their yield of sorghum to 420 kg/acre. One 50kg bag of fertilizer currently costs $45 at retail, and must be applied each season. Most of our customers in Kenya typically spend $90-200/year on chemical fertilizer.

2) Organic composted manure is an effective fertilizer replacement, but not a solution

In our tests, we found that by applying 6,000kg/acre of composted cow manure, farmers can produce 810 kg of dry sorghum/acre. This represents a 48% increase in yield over plots treated with recommended allowance of chemical fertilizer. However, we believe that compost is not a longterm solution to improving smallholder yields, as there is simply not enough available feedstock. In Kenya, cattle are open-grazed and are rarely confined to pens or feed lots. As a result, manure is distributed around the countryside, and difficult to collect. Manure is also already in use throughout the developing world as a fuel.

3) Biochar + sanitized human urine produced the highest yields of sorghum 

Many of our customers experiment with using diluted human urine as a replacement for chemical fertilizer. In plain soil, we found that a 15% solution of sanitized urine and water added to soil produces a sorghum yield of 205kg/acre, 51% less than the yields observed with chemical fertilizer. On its own, sanitized urine is not a viable replacement for chemical fertilizer.

When this same urine treatment was applied to soils amended with biochar (at a rate of 6,000 kg/acre), we recorded a sorghum yield of 1,025 kg/acre. This represents a 144% increase relative to chemical fertilizer. Biochar+ human urine outperformed any other treatments (organic and chemical) that we tested.  We conclude that biochar somehow ‘activates’ urine-based fertilizers. The most logical explanation is that biochar improves the soil’s ability to capture and retain the nutrients found in urine.

4) The effects of biochar persist, and may increase over time

This effect is perhaps the most interesting of all that we have observed. As previously mentioned, this is our second season of cultivating these particular test plots. Biochar was applied in season 1 and then not reapplied in season 2. In season 1, our urine + biochar plots outperformed chemical fertilizers by 27%. In season 2, urine+biochar outperformed chemical fertilizers by 144%, without adding any additional biochar. These results indicate that biochar produced with our proprietary kiln has an impact on soil beyond the first season. Unlike chemical fertilizer, our biochar is not consumed in one season. It is possible that after 1 season of application, biochar becomes more effective in soils due to increased weathering. It is also possible that biochar acts as a longterm nutrient sponge, retaining nutrients throughout the year.


Based on our findings, we conclude that the most effective staple-crop fertilization strategy for smallholders is to combine biochar with dilute human urine solution. With over 1,000 farmers in Western Kenya now using our technology, we find these test results are consistent with user reports.

Urine is readily available, and free to collect. While biochar must be applied at high concentrations (6,000 kg/acre) to see the greatest impacts, it can be added progressively over time, to sustainably increase yields. With each seasonal application, farmers will see an improvement in their yield. By eliminating chemical fertilizer consumption, a farmer can save up to $200/year.

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