Thousands of academic papers have been published on the manufacture, properties and uses of biochar in agriculture and horticulture. Taken together they present a complex if not bewildering picture of biochar. So many different starting materials and ways of making biochar do not help those who would like to see its widespread adoption. There’s a similar picture when it comes to field and pot trials with different species, soils and growing conditions. There are some very handy review papers that try to cut through the maze.

In this post we shall try to summarise those: a brief meta study of the meta studies! But first what is a meta study?

Meta Study

It is a statistical analysis that combines the results from many studies where those studies have sources of error. By doing the analysis a better understanding of the common truth behind the topic of study can be obtained. The technique(s) require that the investigator makes judgements on the techniques, which data to include and exclude, how to handle missing data etc.

In the case of a broad study covering many variables there may be a tendency to assume that there is a common truth. Perhaps that is not always the case.

The Studies

In 2011 Jeffery et al1 showed an average effect of biochar to be a modest 10% improvement in yield. They found that the best yield improvements were on acidic (+14%) and neutral soils (+13%) or soils with coarse or medium texture (+10 and +13% resp.). They surmised “that two of the main mechanisms for yield increase may be a liming effect and an improved water holding capacity of the soil, along with improved crop nutrient availability.”

Their headline figure of 10% was a grand mean statistic illustrating what had been achieved to date. It did not highlight where the best results had been achieved.

Crane-Droesch et al2 reviewed 84 studies in 2013 with “a dataset comprised of 365 observations from 40 studies which compared crop yields in biochar-using treatments to biochar-free controls”. They estimated an average crop yield increase of approximately 10% for 3t/ha biochar addition in the first year after application. Notably, they found that crop yield improvements, after a single biochar application, continued for several years.

They found that the CEC (cation exchange capacity) and organic carbon content of soil (soil organic matter SOM) were strong predictors of crop yield with “low CEC and low carbon being associated with a positive response”. Going on to say, “Our estimates suggest that biochar has a substantial and specific agro-ecological niche” under these conditions “and perhaps (in soils) with lower pH and heavier textures.”. These characteristics describe many of the world’s lowest-potential agricultural areas, predominantly found in the humid tropics.

Surprisingly they found that the biochar parameters of pH, carbon content and temperature of pyrolysis were not significant predictors of yield. They “found little evidence that plant response to biochar is mediated by nitrogen additions to soil”.

They did note the lack of complete data sets, full range of soils types and growing conditions. The short term nature of the 2011 studies, only 1-2 years at the time of the review limited their analysis.

Similar results with overall 11% increase in yield were noted by Xiaoyu Liu et al3. They carried out a meta analysis on the data from 103 studies. “At current prices and C-trading schemes, however, BSA (Biochar Soil Amendment) would not be cost-effective unless persistent soil improvement and crop response can be demonstrated.”

Lili Ye et al4 in a 2019 meta-study on field trials from 46 sites across the globe found an increase in yield in many situations. They included not only biochar applied on it own but also used in conjunction with inorganic fertiliser.

Yield Change %
IF vs Control +26
BC+IF vs Control +48
IF only vs BC only +15
BC only vs Control 0

(IF = Inorganic Fertiliser, BC = Biochar-mostly wood derived)

Following the initial application biochar continued to give an improved performance averaging 31% >= 3 yrs. The researchers interpreted this as an increase in CEC and beyond the effect of liming from the biochar.
Results suggest there is no improvement with biochar application rates >10t/ha or 1kg/m2.

Nitrogen in Soils amended with Biochar

Nguyen et al5 carried out a meta study focused to the availability of soil inorganic nitrogen (SIN) in biochar dressed soils. They used statistical methods to review the output of 55 reviewed and published studies together with one unpublished manuscript covering 914 cases. These studies covered 12 crop types and non-cropped trials, though crops were only a modest part of their overall study. They showed that ca 70% of the effect of the biochar on availability of SIN was related to the length of time the biochar had been in the soil, use of fertiliser, rate of application of biochar, soil texture and pH and crop type. The pyrolysis type and temperature with feedstock type had ca 17 & 22% influence on SIN respectively ammonium and nitrate N.

In other studies it has been shown that pyrolysis temperature and CEC are related6, CEC increases as the pyrolysis temperature increases to approximately 600C. CEC then decreases, believed to be due to melting and pore collapse. They also measured extractable crop nutrients from biochar (wood chip and straw pellets) they showed the extractables reached a maximum at ca 450C. Taken together with other findings they projected that a pyrolysis temperature of ca 550C would yield a good balance between CEC, high extractables, high intractable carbon, low labile carbon and high pH. This laboratory study did not look at how these features translate into plant growth.

influence of variables on nitrogen
Taken from Table 2 Ref5

The take away is, that initial application of biochar to soil does depress the availability of inorganic nitrogen in the soil to a statistically significant but modest extent ca 10%. Woody derived biochar however, shows smaller effects than biochar from other sources.

The paper carried a review of the mechanisms effecting soil inorganic nitrogen after biochar application. These included; abiotic, biotic mechanisms (mineralisation, immobilisation, nitrification, denitrification, nitrogen fixation) as well as plant assimilation and the interaction of fertilisers. They comment that the habitat provided by biochar, on its surface and within its pores facilitates microbial activity including nitrogen fixation and denitrification. They note that biochar reduces denitrification of nitrate (NO3) to nitrous oxide (N2O) by up to 50%! Nitrous oxide is a powerful greenhouse gas.

Biochar improves plant uptake of SIN through:

1) improved soil moisture and (lower) pH which stimulates N mineralisation and nitrification,

2) the liming effect creates a more favourable root zone environment reducing the availability of toxic metals ions such as Al3+,

3) increased retention of SIN and reduced leaching.

Some authors report these effects after “priming” of the biochar by prior treatment with inorganic or organic fertiliser.


The first three papers (2011 &13) noted a similar improvement in crop yield averaging to 10%. They did find that biochar works best where the soil shows low CEC and has low SOM.

The third review4 from 2019 found much more definitive increases in yield. The greatest effects, +48% were seen when biochar was used with inorganic fertiliser. Surprisingly, they found no effect when biochar was used on its own. They also showed sustained performance +31% of biochar up to 3 years from the initial and single application. This finding also echoes the earlier work of Crane-Droesch et al2.

The study on how biochar effects Soil Inorganic Nitrogen showed that there is a small decrease ca 10% in SIN initially. Some results though suggest that the biochar may take up ammonium and nitrate ions initially and release them more slowly. In turn this may offer better nitrogen utilisation, less need for ongoing fertiliser applications and less leaching into ground water.

All the studies noted that much data was missing notably regarding the longer term effects.

These studies have gone some way to glimpse into the substantial matrix of variables associated with biochar. What is perhaps remarkable is the more limited suite of variables that seem to have real impact on crop yield and there is a real increase in yield if biochar is pre-prepared with nutrients or used with fertilisers.


  1. A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis Simon Jeffery, Frank GA Verheijen, Marijn van der Velde, Ana Catarina Bastos
    Agriculture, ecosystems & environment, 2011, vol144, pp175-187
  1. Heterogeneous global crop yield response to biochar: a meta-regression analysis
    Andrew Crane-Droesch, Samuel Abiven, Simon Jeffery, Margaret S Torn Journal Environmental Research Letters, Volume 8, Issue 4, pp 044 049, pub 2013/12/16
  2. Biochar’s effect on crop productivity and the dependence on experimental conditions—a meta-analysis of literature data
    Xiaoyu Liu, Afeng Zhang, Chunying Ji, Stephen Joseph, Rongjun Bian, Lianqing Li, Genxing Pan, Jorge Paz-Ferreiro
    Plant Soil (2013) 373:583–594 Springer Science+Business Media Dordrecht 2013
  3. Biochar effects on crop yields with and without fertilizer: A meta‐analysis of field studies using separate controls
    Lili Ye, Marta Camps‐Arbestain, Qinhua Shen, Johannes Lehmann, Balwant Singh,
    Muhammad Sabir
    British Society of Soil Science, pub 16 September 2019
  4. Effects of biochar on soil available inorganic nitrogen: A review and meta-analysis
    TTN Nguyen, CY Xu, I Tahmasbian, R Che, Z Xu
    Geoderma 288 February 2017
  5. Biochar – synergies and trade‐offs between soil enhancing properties and C sequestration potential Kyle Crombie Ondřej Mašek Andrew Cross Saran Sohi, Bioenergy, Volume 7, Issue 5 September 2015