Main research question
The ROOFFOOD project focuses on soil characteristics in the Ghent roof garden of the same name with the intention of being an inspiring example case for other roof agriculture projects. ILVO characterizes the chemical and microbiological characteristics of the substrate of the roof garden and studies how a good root environment for the vegetables must be obtained. Based on the measurements, we determine the criteria for a "roof garden enhancer" (= a good substrate enhancer for the roof garden). A better fit between this improver and the needs of the vegetable garden contributes to a better closing of the cycle.
The project has now ended. The chemical properties of the substrate indicate that the pH is high, the carbon content is on the low side and the release of mineral N is very low. Of the total amount of N in the rooftop vegetable substrate, only 1% is initially present under mineral form and the substrates supply rather little mineral N. Sufficient supply of carbon with a sufficiently low C/N ratio to the rooftop vegetable garden is important: the higher the carbon content in the substrate, the more mineral N this organic matter can supply for the vegetable garden. The microbiology of the rooftop vegetable garden substrate was characterized based on microbial activity, microbial biomass, and microbial diversity. The microbial activity in the rooftop vegetable garden substrates was found to be low, indicating stable organic matter in the substrate. The Roof Food samples are more bacterially diverse than the 3 groups of reference samples (compost, woody material, and potting soil). The fungal population in the Roof Food samples is more diverse than potting soil, but not more diverse than compost and woody material. To manage the rooftop vegetable garden in a circular manner, researchers searched for a vegetable garden-improving agent that would fit the needs of the rooftop garden. Different soil enhancing agents (worm compost, GFT composts, green composts) were compared in order to maintain the fertility of the rooftop vegetable garden substrate.
Clarity has been brought to the challenges for the future of roof gardens in Ghent. One of them is to maintain sufficient C (and thus organic matter) in the top layer of the rooftop vegetable garden, to keep the pH from rising further and preferably even slightly decreasing, and to strive to add organic matter that can supply sufficient mineral N. In order to induce the desired combination of carbon supply, supply of rapidly mineralizing organic matter, and acidification in the topsoil, there does not appear to be one ready-made solution in the form of one product. Acidification and supply of C can be obtained through a woody material, while supply of a source of mineral N and possibly other nutrients can be through a compost or through an organic fertilizer.