How much proof do we need that biochar can double food production on some tropical soils?

Experiments around the world are examining the impact of biochar on food production. On poor tropical soils the effect of adding organic matter that has been intensely heated in the absence of air (making biochar) continues to startle researchers. The latest surprise comes from trials in East Africa administered by the US biochar company Re:Char. In Kenya local farmers are showing that soils treated with biochar in year 1 had substantial further yield increases in year 2. An acre of land treated with biochar produced nearly 150% more grain than a similar area using conventional artificial fertiliser. On the biochar-laden soil the only fertility supplement used in both years  was sanitised human urine, which contains copious amounts of phosphorus, potassium and nitrogen.

Biochar isn’t itself a fertiliser. It is a highly stable form of almost pure carbon and can have no direct effect on agricultural productivity. But it does seem to assist soils retain nutrients, and make these nutrients more easily available to crops. The evidence that biochar has profoundly important effects is growing by the week. In addition, it sequesters carbon permanently. Done at large scale across the tropics, biochar may be the lowest cost form of carbon capture and storage.


Wood and agricultural wastes, including dung, can be turned into biochar using very simple kilns. These stoves are cheap and easy to use. Re:Char sells subsidised stoves in Kenya which 1,000 local farmers use to make the biochar for their fields.

Heating organic matter in the absence of air drives off gases and liquids. (This is how charcoal is made). This process is called ‘pyrolysis’ and it leaves just the carbon behind (with traces of minerals). Biochar is sponge-like with a huge surface area. One study suggested that one gram of the substance could contain eight hundred square metres of surface. This is probably the source of its success: it can store nutrients and water better than almost any other material. Microscopic fungi living in the biochar can live off material stuck to the biochar surfaces and then themselves provide food to growing plants. By contrast, many tropical soils struggle to retain nutrients and don’t provide a conducive habitat for beneficial fungus growth.

Re:Char’s work

Re:Char’s business formula, which is increasingly  followed by other social ventures, is to sell biochar kilns at full price in the first world, particularly in the US. (I have one on my allotment garden. I have to admit it cost a fortune in transport costs and import duties). A share of the revenue from each sale is given to its Kenyan partner to subsidise the sale of kilns to small farmers. Researchers work with the agriculturalists to measure the results of biochar application and to help spread the word about successes and failures.

The aim of the company is to help improve tropical soils, and thus food production, by the use of biochar. In addition, if successful, biochar will reduce the need to use very expensive artificial fertiliser. Biochar seems to stay in tropical soils for a very long time, thus permanently storing carbon. By reducing the need to use man-made fertiliser, the use of biochar also cuts the emissions of CO2 in fertiliser manufacturing. It also seems to assist in water retention in dry seasons.

The company has just released summary results from the second year of Kenyan farming operations. Some details are available here. The most important finding is that biochar works best at heavy dosage (about 6 tonnes/acre or 15t/ha) and when supplemented by sanitised urine. Second year yields on land with high concentrations of crushed biochar in the topsoil and a second application of urine were higher than the first year. The clear possible implication – which needs to be tested further – is that some of the good stuff in the first year’s urine was retained in the soil for second year use. Biochar may be functioning as an absorbent sponge that holds useful fertilisers in the soil and stops them being leached by intense rains. (By the way, my own personal observation is that similar techniques work on central England allotment soils).

The measured yield increases are staggering. Jason Aramburu from Re:Char writes

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.

Putting six tonnes of biochar into an acre of soil is not a trivial task. It needs three units of raw material going into the kiln to make one unit of biochar. To get optimum dosages of biochar, the farmer therefore needs to process eighteen tonnes of agricultural wastes or wood. This is the average yearly production of several hectares of land. Opponents have focused on the risk that the increasingly clear yield advances offered by biochar might encourage rapid deforestation as farmers cut down trees to obtain raw materials. At worst, this is a temporary problem because the increased production of food in biochar-rich soils is accompanied by a several fold increase in plant stalks, leaves and grain husks. These wastes can provide the raw material for future biochar kilns.

There is no shortage of urine. One person’s typical production of 500 litres a year provides enough potassium, phosphorus and nitrogen to feed an acre.

The world needs hundreds more experiments like Re:Char’s. The benefits of higher food production, lower fertiliser use and huge amounts of carbon storage should be too obvious to ignore. Unfortunately, the lack of clear commercial incentive means that experimental work is proceeding too slowly and the benefits to subsistence farmers are not being harvested as quickly as they should be.

I’ve argued elsewhere that biochar may be the world’s lowest cost, lowest risk form of carbon capture and storage. Results like those from Kenya show the potential advantages to third world nutrition. Let’s have one hundredth of the proposed UK industrial CCS subsidy to be awarded in the next few weeks devoted instead to biochar in the tropics. I guess that the benefits to humanity just might be a hundred times greater.