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.