Growing on the roof : But in what? Examing substrates/growing media for rooftop farming

Abstract: Urban Rooftop Agriculture (URTA) is a novel field within urban vegetable production, which can help create access to local food in a rapidly urbanizing world. For these systems to succeed, it is vital to implement a proper choice of substrate/growing medium (SGM). This study uses a literary synthesis, to analyze URTA SGM by investigating three of its key features, namely components, depth, and organic matter (OM) type and amount, with the intention of facilitating thoughts for future research and providing URTA-stakeholders with guidelines for choice of SGM. Results indicate that leafy vegetables and tomatoes can be cultivated in URTA-systems, potentially delivering yields comparable to conventional in-ground farming, using a wide range of SGM-substances, depths, and OM sources. It is not advisable to advocate for one specific SGM-substance. Instead URTA-farmers should strive to integrate locally produced light-weight material with numerous internal pores into their SGM. Shallow depths, corresponding to those found in extensive green roof systems (<15 cm) suffice for satisfactory URTA-yields. Continuous OM-/compost addition is a necessity for URTA-systems and consequently the proportion of OM/compost exceeds guidelines for more conventional green roof systems. More research is needed to understand the behavior of specific compost components in relation to density and URTA-crop production. An advanced knowledge of soil science is vital for engineering an URTA SGM that is light-weight simultaneously providing for sufficient cation exchange capacity (CEC), aeration, plant available water (PAW) and permeability. Therefore professionals, such as landscape engineers play a central role in URTA-SGM implementation.