# Constructed Technosols: A Strategy toward a Circular Economy
## 4 Waste Materials in Technosols Design
### Growing Material from Wastes  
Organic waste materials could be a potential source of nutrients that could be reintroduced to natural cycles [99]. Recycling organic wastes, which allows the organic matter and the nutrients to be returned to the soil, has already been introduced in agricultural practices to save the costs of fertilizers and amendments and to avoid waste disposal [42]. Various types of organic materials are added to the inorganic matrix (Table 1) and (Figure 3), mostly compost from aerobic digestion of urban or green wastes [42; 47; 63; 88; 90; 91; 93], but also sewage or paper mill sludge [47; 90; 94], street sweeping wastes [47], green waste [42], furfural residues [92], peat [100], and biochar [101; 102]. As for the inorganic structural materials, it is difficult to assess the a priori availability of each material but the recent legislative actions leading to greener industrial practices will probably tend to increase the available feedstocks for compost and digestate, as an example. Compost is now estimated to be the second most important organic waste, with estimated quantities of 15 million ton recycled each year [67; 68]. Digestate, both from agriculture and from bio-waste and sewage sludge is probably the most available material, although mostly used directly in agricultural practices, with estimated productions of 120 and 60 million tons, respectively, in agricultural and urban settings [68]. The characterization of waste materials is fundamental and has to be conducted a priori before realizing constructed soil in order to control the suitability of these substrates, excluding heavily polluted materials, and to predict agronomic quality of final mixture. Despite the benefits of adding organic materials are widely recognized, the optimal percentage of organic matter cannot be established in general terms for constructed soils, as it strictly depends on the type of organic waste, soil matrix, and plant species selected for the site. The research by [PERSON] et al. [91] focused on the constructed soil mixtures obtained with fine coal waste and compost from urban wastes for _Medicago sativa_ growth, and in particularly the mixtures with 2.5% and 5% of organic matter showed a biomass production equivalent and superior, respectively, compared to the agricultural soil. On the basis of these results, they suggested the addition of 550 t of compost per hectare of constructed soil. The type of organic matter, which is provided to the soil, plays an important role, because it can interact with inorganic particles and leads to the formation of long-term stable aggregates [103; 104]. The presence and stability of aggregates affect significantly soil fertility, e.g., the availability of phosphorus [42], but it is not easy to predict these physical parameters for constructed soils, because the characteristics of technogenic starting materials, which influence soil formation and evolution, are often unknown. Furthermore, many other factors influence the aggregation of soil particles, such as the activity of plant roots and macrofauna [105]. To increase aggregates stability, biochar can also be used as binding agent between minerals and organic matter, with the formation of stable macroaggregates [69]. The nature of the organic matter affects the delivery and transfer of nutrients, in constructed soils as demonstrated by [PERSON] et al. [42]. Four mixtures obtained from the combination of excavated subsoil or bricks with green waste or compost were investigated and the biomass growth on these matrices was evaluated comparing it to that obtained from an agricultural soil. The characteristics of investigated materials affected the constructed soils significantly: compost contain mature organic matter, mostly mineralized, whereas green waste is constituted of vegetable residues subject to fast degradation. Therefore, the mineralization rate in mixtures with compost was higher than in soils with green  
Figure 3: Schematic representation of the organic and inorganic materials mostly used in constructed soils.