So that biochar can quickly and effi­cient­ly develop its soil-improv­ing effect in the garden, city park or on the field, biochar must first be “charged”. Mixing with compost is the most common method, but there are many other ways to add nutri­ents to biochar. The article “Wege zu Terra Preta” (Ithaka Journal 2011) by Hans-Peter Schmidt pro­vides a good overview.

No one recipe for success 

If untreat­ed biochar is worked into the garden soil, this leads to an inhi­bi­tion of plant growth at least in the initial period (up to one year accord­ing to Schmidt). Two char­ac­ter­is­tics of biochar are to blame for this. Firstly, their poros­i­ty and thus their ability to absorb up to five times their own weight of water and the nutri­ents dis­solved in it. Another impor­tant prop­er­ty is the high cation exchange capac­i­ty. This pre­vents nutri­ents from being washed out and at the same time ensures that the nutri­ents for the plants and microor­gan­isms are avail­able again under appro­pri­ate con­di­tions. Ulti­mate­ly, accord­ing to the author, there is no catch-all solu­tion for suc­cess­ful charg­ing of biochar, but the fol­low­ing guide­lines should be followed:

  • The charg­ing time should be at least 14 days.
  • There must be enough mois­ture to dis­solve the nutri­ents and recharge the pores of the coal.
  • The great­est pos­si­ble variety of organic (carbon-based) nutri­ents should be avail­able. For example, carbon and nitro­gen are among the most impor­tant nutri­ents for micro­bial colonisation.
  • The stim­u­la­tion of coal with soil microbes by adding humus-rich soil, compost teas, compost or select­ed microorganisms.

Charg­ing the biochar with compost

Accord­ing to Schmidt, charg­ing biochar by mixing it with compost is the best way to produce humus-rich soils. Compost has the highest micro­bial stim­u­la­tion and the nutri­ents are already built into complex organic com­pounds. However, not all compost is the same. Poor compost has to be “digest­ed” by the soil for a long time and leads to the block­ing of nutri­ents and micro­bial imbal­ance. A dif­fer­ent acti­va­tion variant should be used here.
Good compost must have a crumbly struc­ture and must not smell bad. The best way to add the biochar to the compost is by setting the rent at a ratio of 10% to the biomass. Fre­quent turning is impor­tant. If biochar is first added to the ripe compost, both are well mixed in a ratio of 1:1. The mixture should be mixed at least 2 weeks before incor­po­ra­tion into the soil and turned at least twice and thor­ough­ly moist­ened during this time.

Charg­ing the biochar with live­stock manure

A mixture of dif­fer­ent types of manure is prefer­able to a single type of manure. Par­tic­u­lar­ly pos­i­tive effects can also be achieved if the biochar is already used in the barn as bedding for the manure. The ratio of biochar to manure should be about 4:1. If there is not enough straw in the manure, the author rec­om­mends adding at least 10% grass, maize silage or green waste to the mixture. Here too, the mixture should be pre­pared at least 14 days in advance and turned several times before enter­ing the soil.

Charg­ing the biochar with liquid fertiliser

Instead of the con­ven­tion­al NPK (sodium-phos­pho­rus-potas­si­um) fer­til­iz­ers, the author rec­om­mends the use of organic liquid fer­til­iz­ers such as urine slurry. Accord­ing to Schmidt, when cal­cu­lat­ing the amount of nutri­ents nor­mal­ly required, it should be noted that this is halved by mixing with biochar, as the leach­ing and out­gassing of nutri­ents is sig­nif­i­cant­ly reduced. The mineral fer­tilis­er is then dis­solved in suf­fi­cient water and suf­fi­cient biochar is added over a period of 2 days so that the liquid is com­plete­ly absorbed by the biochar. With this variant there is no micro­bial coloni­sa­tion, which only takes place later in the soil.

Charg­ing of the biochar by lactic acid fermentation

The biochar biomass mixture is sprayed with a 3% solu­tion of effec­tive microor­gan­isms (EMA) and 3% sugar cane molasses. Accord­ing to the author, the most suit­able mixture is cattle manure rich in straw with 10% grass cut­tings and 10% biochar and 1% rock flour. The pile is com­pact­ed and covered air­tight, for example in a closed con­tain­er or plastic bag in the garden or on the balcony. After 14–21 days the mixture should smell slight­ly lactic acid. The mixture is now watered and aerated well for a few days, if nec­es­sary further biochar can be added and some soil. The mixture is then worked into the soil from the surface.

Orig­i­nal article: Wege zu Terra Preta – Aktivierung von Pflanzenkohle
Author: Hans-Peter Schmidt
Pub­lished in: Ithaka Journal 1/2011, p. 28–32