Back­ground

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.

Orig­i­nal article: Biochar as a growing media addi­tive and peat sub­sti­tute
Author: C. Steiner, T. Hart­tung
Pub­lished in: Solid Earth, 5, 2014, p. 995–999.