One of the most striking things about soil is that it harbours a remarkable diversity of life. A handful of soil from any well-kept garden, forest, or agricultural field, can contain literally billions of individual organisms and thousands of species. In some cases, as much as 10 per cent of the soil’s total weight could be alive, although in most cases it will be 1–5 per cent. The bulk of these organisms are microorganisms, which aren’t visible to the naked eye: the bacteria, fungi, and algae. But the soil is also home to many animals, including microscopic nematodes and protozoa, and large faunas such as springtails, earthworms, spiders, and even moles. The diversity of all these organisms is vast, with some scientists estimating that soils probably contain as much as one-quarter of the living diversity on Earth. The importance of soil organisms for soil fertility has long been known. The philosopher Aristotle (384–322 BC) referred to earthworms as ‘the intestines of the earth’, and Cleopatra (69–30 BC), the last pharaoh of Egypt, declared them to be sacred because of their contribution to Egyptian agriculture. Darwin detailed the importance of earthworms for soil fertility in his last book, published in 1881. He commented: . . . It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures. . . . Also, the benefits of leguminous plants for soil fertility and crop growth have been known since Roman times. But it wasn’t until the late nineteenth century that it was discovered that nitrogen fixation is down to microscopic bacteria (Rhizobium) that live in small modules in roots. Around the same time, it was also discovered that bacteria that live freely in soil, outside plant roots, also fix nitrogen from the atmosphere and boost nitrogen supply to soil.