Abstract. A field experiment was undertaken to assess the effectiveness of a combined treatment, involving addition of Aspergillus niger ‐treated sugar beet (SB) residue in the presence of rock phosphate and mycorrhizal inoculation of seedlings with Pisolithus tinctorius . The aim was to improve the physical, chemical, biochemical and biological properties of a degraded semiarid Mediterranean soil. Short‐term effects of such improvements on the establishment of Cistus albidus L. seedlings were evaluated. Eight months after planting, macronutrients (NPK), total carbohydrates, water‐soluble C, water‐soluble carbohydrates, microbial biomass C and enzyme activities (dehydrogenase, urease, protease, acid phosphatase and β‐glucosidase) measured in the rhizosphere soil of C. albidus were increased greatly by addition of fermented SB residue. Soil structural stability improved only with the fermented SB addition (about 79% higher in the amended soils than in the non‐amended soils). The mycorrhizal inoculation was the most effective treatment in improving the growth of C. albidus plants, but only slightly improved soil quality. Growth of inoculated plants was about 33% greater than plants grown in the amended soil and about 131% greater than control plants. The combined benefit of mycorrhizal inoculation of seedlings and addition of fermented SB residue to soil on plant growth was similar to that of the treatments applied individually.