Historically, some experts have said it takes three hundred to a thousand years to build inches of topsoil. But we’ve learned that’s actually not the case. Soil is not built primarily by decaying leaf matter and so forth. Living, healthy topsoil is created by plant root exudates – the carbohydrates, vitamins, organic acids, and other nutrients released into the soil by the root systems of plants. Of the sugars that plants create through photosynthesis, 30–40 percent transfers to the soil through the roots in exchange for nutrients. In this way, plants feed soil biology: fungi, bacteria, microorganisms, and mycorrhizae, the symbiotic associations between plants and fungi in the root zone. Those take the sugar and convert it to humus, which is topsoil.
So topsoil can actually be built quite quickly. But it won’t happen without diverse plant life. This diversity is key, and it has everything to do with the way we farm.
It’s a cutting-edge area of scientific research. We’re learning that as plant diversity increases, there’s a certain trigger point – called quorum sensing – where topsoil begins to build rapidly. How many species of plants do you need for a quorum? The more the merrier, microbiologists are saying. Different plants produce different root exudates, allowing access to specific soil nutrients. There have been positive results from as few as twelve species, and more rapid success with forty.
Our best native pastures at Danthonia contain fifteen to twenty species. We’re a long way from the hundreds of species these landscapes once enjoyed – all forming a richly diverse pasture sward that allowed topsoil to build and be maintained, enabling it to hold water and release it during drought.