Water and soil in the city

We rarely give a thought to the soil beneath our feet. Yet soil is not mere 'dirt' for growing plants but a planetary-scale infrastructure that stores rain, filters it, banks carbon and nourishes countless living things. A single gram of healthy soil hosts billions of microbes, and its crumb structure holds water and air like a sponge. Cities have covered this soil with buildings and pavement, compacted it, and often contaminated it.

Learning about water and soil matters greatly in the context of urban farming. The success or failure of a plot comes down, in the end, to 'how much water the soil can store and hand to the roots.' And across the city as a whole, gardens and green spaces are themselves green infrastructure that soaks up rain and softens floods and heat. Growing a single vegetable and keeping a city's water cycle healthy are connected on the same ground.

On natural ground, much of the rain that falls soaks into the soil, some evaporates and transpires, and only a little flows over the surface. But in heavily paved cities, most of the rain cannot enter the soil and becomes 'surface runoff,' surging into the drains. When a short, heavy downpour exceeds the capacity of the sewers, inland flooding — so-called urban flooding — occurs. As climate change brings more sudden cloudbursts, this problem is worsening in cities worldwide.

From here we go a layer deeper into the soil-and-water science behind this 'water that cannot enter the soil': the structure of soil, the physics of how water moves, how cities break the soil, and the principles of restoring healthy soil. In the second half we then visit the 'cities that drink' around the world that have implemented this science as urban planning.