Biochar as a growth medium and as a substitute for peat: The authors and scientists Steiner and Hartung have tested this in a study (“Biochar as a growing media additive and peat substitute, Solid Earth 2014). Because peat has so far been indispensable in horticulture and landscaping. It is characterised by its high water storage capacity (WHS) and homogeneity, the absence of weed seeds and pathogens, low bulk density, low pH, low microbiological activity and low nutrient content. Especially the low pH and nutrient content facilitate the individual nutrient adaptation of the substrates to the plant-specific requirements. Peat is a finite resource, however, and its extraction releases large quantities of greenhouse gases. The study therefore investigated biochar as a peat substitute and compared it with pearlite and clay granules, which are regarded as similar cultivation substrates. In addition, the performance of biochar and biochar-peat mixtures (25, 50, 75%, by volume) was evaluated.
Peat extraction has serious consequences for the climate
Peat bogs are valuable habitats, contain important carbon reserves and are indispensable in regulating local water quality and water balance (flood protection). As long as peat remains in its natural and undisturbed habitat, peat bogs are usually natural carbon stores. However, if the moor is drained, peat extracted, aerated, limed and fertilized, the peat decomposes rapidly and releases large quantities of greenhouse gases (GHG). The search for peat alternatives turns out to be difficult, often the tested substrates have low structural stability and trigger nitrogen (N) releases, contain too many nutrients (e.g. compost) or have too little water storage capacity.
Biochar has similar properties
Biochar from nutrient-poor raw materials such as wood, like peat, has the advantage of a low nutrient content and also offers exceptional structural stability. Biochar is extremely resistant to microbial decay. In addition, biochar from wood residues is also free of seeds and pathogens and does not provide significant amounts of nutrients. Other studies have also shown that hydraulic conductivity and water availability is improved when biochar is added to a pure peat substrate. The biochar used in the study was produced at 600 °C, the woody biomass was pyrolysed for about 1 hour. The biochar obtained in this way had an average carbon content © of 91% and a very low ash content (1.8%). The electrical conductivity (EC) of the biochar was therefore low.
Same effect with more biochar and less peat
The planting experiments showed that the growth of the miniature sunflower was similar in all substrates, i.e. in the mixture of pearlite, clay granules (seramis), peat and peat-biochar. Of course, not all plant carbons are suitable as peat substitutes. Some raw materials (e.g. poultry litter) are rich in minerals and produce plant carbons with high pH values as well as salt and ash contents. This means that they would cause osmotic stress in plants if used in large quantities. However, biochar made from pure wood has a very low ash content. If this biochar is mixed with peat, the mixture could contain up to 80% biochar without increasing the pH value above 7.