Due to its huge inner surface area and porous structure, charcoal has a high nutrient storage capacity. A practical trial was conducted to test how the addition of plant charcoal in the composting of meadow cuttings and clover grass plus soil affects the composting process and the carbon and nutrient contents of the finished compost. Three variants were tested, with 3 % and 6 % charcoal addition, and without charcoal.

The results show that the addition of charcoal led to increased respiration and higher carbon losses during composting. For nitrogen, between 44 and 48 % of the initial amount was retained after composting. The addition of coal had no significant effect on N losses during the composting process. For potassium, the addition of charcoal significantly increased the plant-available K content. The higher availability is probably due to the increase in cation exchange capacity by the coal.

The machinery and transport processes produced a total of 278 kg of CO2e emissions per 1,000 kg of carbon in the form of finished, spread compost. Turning the compost 34 times was both the most time-consuming and cost-intensive sub-step. Calculated for an application of 100 kg nitrogen with coal-compost, the cost was €795; that is €95 higher than the cost for 100 kg N in the form of organic commercial fertiliser.