Friday, November 8, 2013

Biochar nutrient content

     The composition of biochars depends upon the nature of the feedstocks and the operating conditions of pyrolysis. A review of the literature has revealed that only scant information is available on the nutrient properties of biochars.


     Most of the research on pyrolysis of biomass has focused on energy and fuel quality (Horne and Williams, 1996; Tsai et al., 2006) rather than on biochar as a soil amendment. Biochars can be produced at almost any pH between 4 and 12 (Lehmann, 2007) and can decrease to a pH value of 2.5 after short-term incubation of four months at 70°C (Cheng et al., 2006).


The above graphic shows the basic process of how Biochar becomes a safe habitat for micro-organisms and storage of nutrients and moisture, the symbiotic relationship between plants, fungus and micro-organisms, how plants feed this micro-life carbohydrates in return for moisture and nutrients. Source:

1. Biochar applied around a trees root zone or drip line.
2. Plant roots (white) linking up with common stringy soil fungus (pink). The plant carbohydrates produced from photosynthesis are shared with the fungus in return for nutrients and moisture.
3. Fungus linking up with Biochar full of moisture, nutrients and micro-organisms
4. Micro-organisms embedded into the porous structure of Biochar share nutrients with fungus in return for plant produced carbohydrates.

     Carbon contents range between 172g kg–1 and 905g kg–1 (coefficient of variation, CV = 106.5 per cent).The ranges are even larger in the case of total N (1.8g kg–1 to 56.4g kg–1), total P (2.7g kg–1 to 480g kg–1) and total K (1.0g kg–1 to 58g kg–1), all with CV-100 per cent (see Table 1).

Table 1

      The variability can be attributed to different feedstocks and different conditions under which the various biochars were manufactured. The influence of feedstocks is particularly evident in the case of total P where higher contents were found in biochars produced from feedstocks of animal origin – namely, sewage sludge and broiler litter – than those from plants (e.g. wood). Similarly, total N contents of biochars from sewage sludge (64g kg–1; Bridle and Pritchard, 2004) and soybean cake (78.2g kg–1; Uzun et al., 2006) were much higher than those from pure plant origins (e.g. green wastes) (1.7g kg–1; Chan et al., 2007a).

Moss growing in a piece of biochar

     Compared to other forms of organic amendments commonly used in agriculture both total N and P contents of biochars cover ranges that are wider than those reported for the whole spectrum of typical organic fertilizers. It is important to note that the same type of feedstock can produce very different biochars.

 FTIR samples of bamboo biochar at different pyrolysis temperatures. Source: Odette Varela

Different temperatures of bamboo biochar and coconut biochar. Source: Odette Varela

      For example, Chan et al. (2007b) reported total N contents of 20g kg–1 for biochar produced from poultry litter compared to 7.5g kg–1 and 6.0g kg–1 for 2 biochars made from different poultry litter reported by Lima and Marshall (2005). Such large differences in total N are a result of either different poultry litter qualities or different pyrolysis conditions.

     This cannot be ascertained, as information is typically not given to the extent that allows such conclusions to be drawn. A much higher temperature (700°C) was used by Lima and Marshall (2005) compared to the 450°C reported by Chan et al. (2007b).


  1. Great site. Thanks

    1. Thanks for your comment, more post are coming this week.

    2. Wonderful blog. Please add the references you have cited in this post, too. Have you published this material elsewhere?

    3. Wonderful blog. Please add the references you have cited in this post, too. Have you published this material elsewhere?