Characteristics of the optimum combination of synthetic soils by plant and soil properties used for rock slope restoration

Bare rock slopes often deteriorate microclimates and drive water erosion and landslides. Moreover, they impose hidden troubles on humans and have increasingly been brought to the attention of the public. In order to optimize synthetic soils (SS) for improving the outside soil spray seeding (OSSS) technique used for the restoration of rock slopes, we investigated physico-chemical properties and plant growth of a series of SS designed with orthogonal array OA25 (55). Five additive components, cement, inorganic compound fertilizers, peat soil, wood chip and water–absorbent polymers at five levels, were applied to optimize the synthetic soils. In the present study, we only investigated the effects of a single additive component on all the indicators measured. Primarily, results showed that the contributions of cement to plant biomass, pH and water holding content; the contributions of inorganic compound fertilizers to plant biomass and total nitrogen, available nitrogen, total phosphorus and available phosphorus; the contributions of both peat soil and wood chip to plant biomass, water holding content and organic carbon; and the contributions of water absorbent polymers to the plant biomass, water holding content and available nitrogen were all significant. According to the plant growth and physico-chemical properties of the synthetic soils, Treatment-12 consisting of 1.5% cement, 0.4% inorganic compound fertilizers, 1.5% peat soil, 0.3% water absorbent agent and 2.0% wood chip, was the optimum combination among the 25 treatments for the restoration of rock slopes. The properties of the optimized SS to support growth of plants and to resist erosion were verified by an experiment using an actual rock slope.