Let’s start by looking at the theory of mole draining with ex-Simba engineer Philip Wright, whose Wright Resolutions consultancy specialises in soil structure assessment and cultivator development. He explains that a clay-based soil needs a mix of 50% mineral matter — this includes the vital but small component of organic matter — 25% air and 25% water. It’s a balance that allows air in, roots to respire and CO2 out. The bad guy, ‘excessive water at depth’, not only hinders root development, but causes surface water to run away, taking minerals and topsoil with it.