The appropriate blend is relevant to the site itself. One can order the blend (recommended) or make your own. Either way the blend must follow the comprehensive guide lines developed by the German Landscape Research, Development and Construction Society know by the initials FLL.

When self made or using commercially available blend soils please consider the followings:

• Make sure that the medium specify is fine-tuned to your requirements

Additional testing is recommended for: density, air-filled porosity, water-holding         capacity, hydraulic conductivity, particle size distribution and organic content (pH, soluble salts, and nutrient content.

• Organic matter should have leaf mold, composed animal waist, worm castings, substrates mushroom cultivation and sewage sludge (sewage sludge needs to be use carefully because it can reduce the blend’s porosity.
  
  
• Regional climatic conditions must be taken into account. FLL guidelines allows a level of fine particles that might not work in areas of high precipitation. This allows the medium to hold too much water increasing weight to the system and creates prime conditions for weed growth.
  
  
• Success on green roofs largely depends on plant selection, though following FLL standards for the right soils can reduce a lot of potential sources of failure.

Typical Issues

• Poor storage: if the blend is not carefully stored the chances of it being contaminated with weeds and other seeds blown onto the area increases.

• Delivery and installation: during this process there are high chances of it getting contaminated, if this is the case you can ask your supplier for a weed germination test.

• Blend not processed enough: compost that has not been broken down consumes lots of nitrogen and oxygen from the plants, causing serious damage to the plants.

• Depth: Three to 4 inches can work well in region with lots of rain, where as in warmer, drier climates a minimum of 6 inches is needed for plant survival. In very hot regions like in southwestern American climates a depth of 8 to 12 inches and sometimes more might be necessary.