Soil and the threats it faces nowadays are often underestimated by citizens. This is rather due to a lack of awareness than to a lack of interest. In France, the surface covered by private gardens is not negligible, as it represents 2% of the territory, which is four times larger than the area of natural reserves in the country. Therefore, ensuring the protection and preservation of healthy soils in these areas is of great importance. Citizen science is a good way to raise awareness about scientific issues. Using a scientific and educational approach, the citizen science project “La clé du sol,” funded by the Fondation de France coordinated by the evertéa Foundation, aimed to raise home gardeners’ awareness of soil complexity and how to maintain healthy soil, as well as help them realize the possible consequences of their practices. For 2 years, fourteen gardeners participated in the project which involved theoretical and practical workshops where cultivation practices were listed, garden soils were sampled, and their physico-chemical and biological parameters as well as contaminants were analyzed. The strength of this project was the close interaction between the gardeners, a local citizen association, agronomy students, and researchers to decipher and interpret the results and learn how to translate them into practical advice. The progress of the project is presented in this article. Moreover, the scientific, social, and educational contributions of “La clé du sol” as well as the challenges inherent in citizen science projects involving long-term citizen engagement will be discussed.