**5. Soil amendments**

The pyrolysis of biomass under low oxygen conditions produces biochar, which is a carbonaceous material with agricultural applications due to its porous surface and nutrient content. The estimated annual residual biomass from the distillation of *Mentha arvensis*, *Mentha citrata*, and *M*. *piperita* is 10.5 thousand million tons [31]. These wastes are burned or composted, but both approaches face implementation problems. The use of aromatic plant biomass has been associated with antigerminating attributes upon composting [32]. Proper burning to secure complete combustion requires a considerable initial investment. An alternative to burning and composting is a two-step sequential approach in which solvent treatment of the residual biomass is used to obtain an extract with antioxidant capacity, and the plant material is

#### *Essential Oils and the Circular Bioeconomy DOI: http://dx.doi.org/10.5772/intechopen.112958*

subsequently heated under an inert atmosphere to obtain a biochar. The biochar is used for soil amendment [33].

An interesting alternative approach that reduces investment and operating costs is to involve the distillation plants themselves in biochar production. The ash pit of a distillation plant with 500 g plant material capacity was used to hold stainless steel boxes (25 L) that contained biomass waste (10 kg) and was provided with eight holes for the release of volatile compounds. A residence time of 2 h was suitable to produce biochar in up to 60% yield and 0.26–1.6 g/m3 density [34].

Crop cultivation leftovers from pruning and plant selection at a commercial production of fresh basil, rosemary, and sage were used for essential oil and hydrosol production and the distillation residue was employed in on-farm composting. Temperature monitoring showed that 30 days was sufficient to obtain usable compost [35].
