#5 Clara: Biochars & ashes as bio-based fertilisers

How to enhance the plant availability of phosphorus of biowaste-derived biochars and ashes?

Clara is a Marie-Curie PhD student at the University of Copenhagen and part of the EU project FertiCycle. She conducts research on biochars and ashes and on how to increase their fertiliser value.

Department and Section: Department of Plant and Environmental Science, Soil Fertility Group

1st supervisor: Dorette Müller-Stöver

2nd supervisor: Lars Stoumann Jensen

Dissertation title (as for now :D)

Enhancement of plant phosphorus availability from biowaste biochars and ashes

Topic – in brief

Bio-based fertilisers : Enhancing the phosphorous availability from biochar and ashes

What is biochar?

  • Similar to coal production: Organic compounds are heated up to very high temperatures without any oxygen (= which is typically referred to as ‘under anaerobic conditions’). The process is called ‘pyroloysis’.
  • Captures carbon –> when applied to soil it can increase the carbon content of the soil and serve as carbon sink

What are ashes?

  • Burnt organic waste materials (this time oxygen is available, which is typically referred to as ‘under aerobic conditions’)


Wet-chemical extraction:

  • materials are mixed with water or other chemicals, which should mimic the soil solution (i.e. the water in the soil, which is available to plants). Whatever ends up in that solution, is assumed to be available to plants

Pot experiments:

  • Fertilisers + Soil are put into pots and plants are grown in those pots
  • In the ‘control’ no bio-based fertiliser is applied to the soil and the plants in the control are compared to the ones which grew in pots with bio-based fertiliser addition

Clara’s findings so far

  • Initially not too many nutrients are released into the soil
  • Nutrient release depends on soil pH à low pH: better release but still low release
  • Acidifying the materials can enhance nutrient release
  • For sewage sludge-based materials increasing the pH also works (one or the other extreme works)
  • Plants don’t seem to like it when acidified fertiliser is placed in concentrated form – they won’t grow their roots in such areas
  • Biochar doesn’t release a lot of P if it’s not acidified

Previous studies

How does long term fertilisation with compost change the soil in terms of soil organic matter & nutrient availability?

  • Since compost contains a lot of carbon, the carbon content in the soil increased
  • Compost cannot replace fertiliser treatments because content and availability of for instance phosphorous and nitrogen is rather low

If you would like to connect with Clara check out LinkedIn, or Twitter @ClaraKopp21.

Clara’s funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No.860127.

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