## 1 Introduction
[PERSON] hypothesized that the friction between the root system and the soil contact surface converted the soil shear stress into root tensile stress, thus enhancing the shear strength of the slope soil [27]. [PERSON] derived the mechanical model of root anchorage and summarized the bonding degree of root-soil interface and the pullout force when the root system starts to be pulled out from static friction to sliding friction, namely the ultimate tensile force of the root system, and gave the corresponding calculation formula [28]; [PERSON] and [PERSON] obtained the relationship between root density and shear strength through shear tests of root-soil composites [29; 30]. [PERSON] conducted a pull-out test and concluded that the mechanical features of the root-soil interface are as follows: the smaller the diameter or the greater the loading speed, the more easily the root fractures; the maximum anchorage force is linearly proportional to the root diameter [31]. The majority of the preceding research focuses on the mechanical properties of the root system, such as root-soil complex model, tensile strength, and shear strength, and less on the substrate, particularly the substrate of steep slopes. The substrate serves as a platform and carrier on the rock slope to transform and utilize vegetation water and nutrients [32]. Plants require water and nutrients to grow. Therefore, we plan to investigate the effects of substrate moisture and nutrients on plant root growth and anchorage on rocky slopes. Super absorbent polymer, known as a water retention agent, is a kind of polymer material with high water expansion and strong water absorption capacity, and it is also an excellent cementing agent for soil. It can rapidly absorb and retain hundreds of times its own quality of water, achieving the impact of water storage and moisture retention; at the same time, when the soil is dry and deficient in water, it can rapidly release water for plants to absorb and utilize, so improving crop output. As a result, SAP can improve the soil water and fertility environment as well as the soil structure, preventing deep infiltration and soil nutrient loss while promoting maximum water and fertilizer uptake by the crop root system. Soil moisture is the liquid phase of soil and the main source of water for plant growth and development. Depending on the sort of force applied, soil water can be classified as hygroscopic water, pellicular water, capillary water, and gravitational water. Gravitational water is the water in the soil that plants cannot use on a long-term basis. The SAP can hold it and then gradually release it to the crops, increasing the water utilization rate [33]. In addition, SAP can significantly increase soil saturated water content, field water holding capacity, gravitational water and available water, improve soil \"pore\" water holding capacity, and, especially, significantly increase soil water holding capacity during water loss [34]. According to [PERSON]'s research, the chemical hydrophilic groups and network structure in SAP hydrogel molecule can considerably boost water absorption and utilization of plant development [35]. Soil with SAP can absorb more water, allowing the absorbed water to be released slowly when soil moisture is reduced, and fertilizer nutrients can also be released slowly as moisture is reduced, so nutrient retention in the hydrogel amended substrate can be improved. Therefore, the utilization rate of water and fertilizer is effectively improved, the seed germination rate is increased, and the survival time of plants under water stress is prolonged [36; 37]. However, most studies on SAP have not dealt with plants on slopes. Therefore, the aim of this article is to investigate the effects of water retention agents on soil nutrients and root mechanics of plants growing on rocky slopes. _Amorpha fruitosa_ is a deciduous bushy shrub that thrives on barren slopes, roadsides, and saline-alkali lands due to its wide adaptability and strong resistance to cold, drought, wind, and sand.