Answers Underground
Scientists are discovering that the soil beneath our feet is one of the least understood and most important ecosystems on Earth
A A single teaspoon of healthy agricultural soil contains more microorganisms than there are people on Earth. This figure, which has become something of a scientific touchstone, captures the almost incomprehensible biological richness of the underground world. For most of human history, soil has been treated primarily as a physical medium -- a substrate in which plants anchor their roots and from which they extract water and a limited range of mineral nutrients. The emerging science of soil ecology is revealing that this picture is radically incomplete. Far from being an inert backdrop to plant growth, healthy soil is a dynamic, interconnected community of organisms whose interactions underpin the productivity of every terrestrial ecosystem on the planet.
B The most numerically abundant and ecologically significant inhabitants of soil are bacteria and fungi. Bacteria decompose organic matter, cycling nutrients back into forms that plants can absorb. Some species fix atmospheric nitrogen, converting the gas into compounds accessible to plant roots. Fungi form intricate networks of fine filaments known as hyphae that penetrate the soil and, in the case of mycorrhizal species, form intimate associations with plant roots, extending their effective reach by orders of magnitude. In exchange for carbohydrates supplied by the plant, mycorrhizal fungi provide mineral nutrients -- particularly phosphorus -- that the plant cannot access on its own. This exchange is not a curiosity of botanical biology: it is estimated that more than ninety percent of land plant species engage in some form of mycorrhizal relationship.
C Larger soil organisms -- including nematode worms, mites, springtails, and earthworms -- play equally important roles. Earthworms, which Charles Darwin devoted much of his later career to studying, ingest organic matter and mineral particles, processing them through their digestive systems and depositing casts that are richer in nutrients and bacteria than the surrounding soil. Their burrowing activity improves soil structure, creating channels that allow air and water to penetrate. In compacted or heavily tilled agricultural soils, earthworm populations are frequently depleted, contributing to the drainage problems and reduced fertility that characterise degraded agricultural land.
D The network of fungal hyphae and the chemical signals exchanged between plants through the soil have attracted popular attention under the name 'the wood wide web' -- a term that, though catchy, risks oversimplifying a phenomenon whose ecology is still being worked out. Research has demonstrated that established trees in forest ecosystems can transfer carbon and nutrients to seedlings through fungal networks, and that stressed plants can send chemical signals through the soil that prime neighbouring plants to activate defence responses against pests. The precise mechanisms, the selectivity of resource transfer, and the extent to which these interactions constitute genuinely cooperative behaviour remain active areas of investigation and debate.
E Agricultural practices have profoundly altered soil biology, in most cases with damaging consequences. Intensive tillage disrupts fungal networks and exposes organic matter to rapid oxidation, reducing soil carbon. The widespread application of synthetic fertilisers, while boosting short-term crop yields, has in many cases reduced plant investment in mycorrhizal partnerships -- because plants supplied with abundant nutrients from outside have less incentive to maintain the energetically costly exchange with fungi. Pesticides, applied to control crop pests, frequently affect non-target soil organisms, reducing the diversity and abundance of the microbial communities that maintain soil health.
F The recognition that soil biology is central to agricultural sustainability has generated growing interest in farming approaches that restore rather than deplete underground communities. Reducing or eliminating tillage, maintaining continuous plant cover through cover cropping, and reducing reliance on synthetic inputs are all associated with improvements in soil biological activity. Carbon farming -- managing land to maximise the sequestration of carbon in soil organic matter -- is increasingly discussed as both an agricultural and a climate strategy, though the quantities of carbon that can realistically be stored, and the permanence of that storage, remain subjects of scientific debate.