Effects of steel slag mixed substrate on rooting of Hydrangea macrophylla cuttings

To investigate the effect of steel slag used as substrate on the rooting of Hydrangea macrophylla cuttings, and to develop a new mixed substrate that can partially replace conventional cutting substrates and realize the high-efficient utilization of solid waste, we examined the physical and chemical properties of different mixed substrates containing 10% (T₁), 20% (T₂), 30% (T₃), and 40% (T₄) volume fractions of steel slag, and investigated the rooting of H. macrophylla 'Red Beauty' cuttings growing on these substrates, with conventional cutting substrates (peat and perlite) as the control (CK). The results showed that pH value, electrical conductivity, and bulk density of the mixed substrates were significantly higher than those of CK. The aeration porosity of T₂ was higher than other treatments, while the total porosity and water holding porosity differed little from others. Both fresh weight and dry weight of all the four treatments were higher than those of CK, with stem diameter being higher than that of CK (except T₄), plant height showing no significant difference compared to CK (except T₄), and leaf chlorophyll content being significantly lower than CK. Root length ranked as T₂>CK>T₁>T₃>T₄, the root surface area and root volume both ranked as T₂>T₁>CK>T₄>T₃, the root tip ranked as T₂>CK>T₁>T₄>T₃. Both average root diameter and root activity were significantly higher than that of CK, with the highest value being observed in T₂. Soluble sugar content in the leaves of T₂ was the highest, followed by T₄, T₃, CK, and T₁. The weight ranking of root growth indices was root activity > average root diameter > root volume > root surface area > root tip number > root length. Redundancy analysis indicated that pH value, electrical conductivity, aeration porosity, and water holding porosity of substrates were key factors influencing root growth and development of cuttings. Our results suggested that substrates mixed with 10% to 40% steel slag could be used for H. macrophylla cutting propagation, and 20% (T₂) being the best one because it could significantly improve the survival rate, growth status, and root development of cuttings. Steel slag would be a novel substrate to partially replace conventional unrenewable substrates such as peat and perlite for flower seedling propagation, which could reduce agricultural production cost and provide a high-value utilization way of industry solid waste.