Roof Garden Soils

From the laboratory analysis, the physical compositions of the Roof Garden mixes were very similar, being soil texture class SAND. They were comprised of mainly fine to coarse sand sized particles and predominantly medium sand. The chemical testing also found the Roof Garden samples to be similar, being non-saline, alkaline in reaction, and, with the exception of nitrogen, contained high levels of organic matter and major plant nutrients which will not need improvement before use. In conclusion these soils would be considered suitable for their respective purposes provided the nitrogen deficiencies are remedied by routine application of fertiliser at planting The two Roof Garden Mixes contain significant quantities of soluble potassium ions, which elevated the water extract pH levels. To give an indication of the soil pH values without the influence of excess potassium, additional pH analysis was carried out using a potassium chloride extract. This confirmed moderately alkaline pH values of 7.6 (turf) and 7.5 (shrubs) which are considered acceptable for a wide range of plant species, including most commercially grown trees and shrubs, and grass cultivars.

The bulk densities and water-storage capacities of the roof garden mixes were measured to provide supporting information to the designers and engineers to calculate the potential loadings required for these soils. The bulk densities are comparatively low to those of standard soil and rootzone mixes, demonstrating the light weight nature of the products.

The permeability and porosity information was determined to confirm that the mixes will provide sufficient drainage and aeration once placed and firmed. The tests were based on a Turf Rootzone of 150mm depth and a Shrub Rootzone of 400mm depth. The results indicate that both mixes will provide satisfactory levels of drainage and aeration for the normal range of roof garden planting environments. The permeability rates indicate slow draining soils that will retain good quantities of water for plant uptake, whilst removing surplus water to prevent water ponding and waterlogging. This drainage rate reduces the risk of drought stress and the reliance on excessive amounts of irrigation water.

Conclusions
The results of the mixing trials and tests confirm that the two Roof Garden Rootzones have the necessary physical and chemical properties to support plant growth in the proposed landscape environments. At the mixing ratios used, there will be no need to apply additional composts or mineral nutrients for at least the first growing season, and all that should be applied is a small application of nitrogen fertiliser.

Certificate of Analysis