Improved soil fertility from compost amendment increases root growth and reinforcement of surface soil on slopes

• On a purpose-built experimental slope, use of quality green compost could increase vegetation cover of grass by up to 20 times.
• Surface applications of quality green compost produced a greater effect than incorporation to 10 cm depth.
• Compost alone did not affect cohesion or angle of friction of the amended soil.
• Increased soil cohesion from root reinforcement at 5 cm depth quadrupled on slopes with 300 t ha−1 added compost.

Vegetation such as grasses and shrubs can improve slope stability, reducing the risk of shallow failures once roots have permeated soil to enhance cohesion. However, establishment of vegetation is hindered by poor soil fertility, frequently a characteristic of disturbed soils used in engineering projects. We evaluated whether compost could improve the rate of vegetation establishment and hence soil mechanical reinforcement by plant roots and therefore protect against shallow failures. Over 1200 t of material was formed into a slope 40 m long × 15 m wide, with an experimental soil slope angle of 20°. Washings from recycled mineral fill were used for the surface soil. Five amendment treatments were replicated three times in strips of 1.5 m by 8 m in a randomised block design on this slope; treatments were a no compost control, standard compost addition at a rate of 35 t ha−1 and a high level compost amendment at 300 t ha−1, applied either to the surface or incorporated into the topsoil to 10 cm.Thirteen weeks after planting an amenity grass mix, vegetation cover increased up to 6 times compared to the control for 35 t ha−1 surface applied compost and similarly 20 times for 300 t ha−1 compost that had been surface applied. Root length density was about 3 km m−3 with no added compost and about 30 km m−3 for 300 t ha−1 added compost. At 35 t ha−1 compost, peak shear stress of the vegetated soil at 5 cm depth was not affected, but it almost doubled with 300 t ha−1 compost compared to no amendment. Cohesion from plant roots was 8.1 kPa for 300 t ha−1, in comparison to 2.1 kPa for no amendment and 2.3 kPa for 35 t ha−1 compost. Whereas surface application resulted in better vegetation cover, there were no differences in peak shear stress between plots with surface application or incorporation of compost. This study provided experimental evidence in the field that compost improvement to soil fertility has a positive impact on soil stabilisation by plant roots.