Soil

• 1. Introduction

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  Soil holds billions of organisms in a single gram, belonging to thousands of species, mostly microbial and largely still unexplored. A recent study suggested soil is likely home to 59 percent of the species on Earth, give or take 15 percent. Yet most people would call the stuff underfoot dirt. Some scientific definitions reserve that word only for soil that has been displaced. What lies beneath your feet is a complex three-state system of solids, liquids, and gases, the product of climate, terrain, living things, and original minerals interacting across time. How does bare lava rock become living ground? Why are tropical rainforest soils strangely infertile? How did civilizations rise and fall on the quality of what they tilled? The answers begin with a material that scientists treat not as inert matter, but as an ecosystem in its own right.

• 2. A Living System

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  A gram of soil carries a mean prokaryotic density of roughly 10 to the 8th organisms, while the ocean holds no more than 10 to the 7th prokaryotes per milliliter of seawater. Because of this immense range of niches, soil contains a prominent part of the Earth's genetic diversity. The numbers grow stranger by group. Enchytraeidae, the potworms, have 98.6 percent of their species living in soil. Fungi follow at 90 percent, plants at 85.5 percent, and termites at 84.2 percent. About 30 percent of insects and close to half of arachnids live there too. Even among vertebrates, which mostly live above ground, many are fossorial. Moles, pocket gophers, voles, and blind snakes have adapted to subterranean life, an adaptation thought to be inherited from past global ecological crises. A typical soil holds a biomass of 70 percent microorganisms, 22 percent macrofauna, and 8 percent roots. Measured by weight, the living component of an acre of soil may include 900 pounds of earthworms, 2,400 pounds of fungi, 1,500 pounds of bacteria, 133 pounds of protozoa, and 890 pounds of arthropods and algae. These organisms feed on one another in sequence. Bacteria and fungi consume raw organic matter, protozoa consume them, and nematodes, annelids, and arthropods consume the protozoa. This chain has been called the soil food web, through which all organic matter is processed as in a digestive system.

• 3. From Bare Rock

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  Weathering of lava flow bedrock produces a purely mineral parent material, the starting point from which soil texture forms. Development proceeds most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. On basaltic lava, plants establish very quickly even with little organic material. Nitrogen-fixing lichens and cyanobacteria arrive first, then epilithic higher plants take hold. Basaltic minerals weather relatively quickly, following the Goldich dissolution series. Crevasses and pockets in the rock hold fine materials and harbour plant roots. Developing roots pair with mineral-weathering mycorrhizal fungi that help break up the porous lava, and by these means organic matter and finer mineral soil accumulate over time. Such initial stages have been described on volcanoes, inselbergs, and glacial moraines. Five interrelated factors govern the whole process, often referred to by the acronym CLORPT: climate, organisms, topography or relief, parent material, and time. Soil is said to be formed once organic matter has accumulated and colloids wash downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum that produce a distinct layer called the B horizon. Yet more recent definitions embrace soils without any organic matter at all, such as the regoliths that formed on Mars and in the deserts of Earth.

• 4. Reading The Horizons

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  Passing from one soil horizon down to the next, from the top of the profile to the bottom, one goes back in time, with past events registered like sediment layers. Mature profiles typically include three master horizons labeled A, B, and C. The solum normally includes the A and B horizons, and the living component is largely confined there, most prominent in the A horizon. Biological influences such as bioturbation are strongest near the surface, while geochemical influences increase with depth. Sampling pollen, testate amoebae, and plant remains in these layers can reveal environmental changes that occurred during a soil's formation. Horizons can be dated by methods such as radiocarbon, using pieces of charcoal large enough to escape disturbance by earthworm activity. Some soils, called entisols, may have only one horizon or none at all, including incipient soils from mining waste, moraines, volcanic cones, sand dunes, or alluvial terraces. The kind of vegetation above shapes the layers below. In grassland, much of the organic matter comes from deep, fibrous grass root systems, and frequent fires destroy aboveground material while stimulating even greater contributions from roots. Under forests, falling tree leaves are the principal source, and greater acidity inhibits the organisms that would otherwise mix surface litter into mineral soil. Grassland soils therefore develop a thicker A horizon with organic matter spread deeper than in comparable forest soils.

• 5. The Chemistry Of Holding

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  A colloid is a small, insoluble particle ranging from 1 nanometer to 1 micrometer, small enough to stay suspended by Brownian motion without settling. Most soils contain organic colloids called humus and inorganic colloids of clay. Their very high specific surface area and net electrical charges give soil its ability to hold and release ions. Negatively charged sites attract and release cations in what is called cation exchange. The strength of that grip follows a hierarchy: in equal amounts, aluminium replaces hydrogen, which replaces calcium, then magnesium, then potassium and sodium. Add one cation in large amounts and it can displace the others by sheer force of numbers, a principle called the law of mass action, which is largely what happens when potash or lime is applied. Cation-exchange capacity, or CEC, measures how many exchangeable cations a soil can hold, expressed in milliequivalents per 100 grams. The values reveal stark differences. A Charlotte fine sand from Florida sits at 1.0, while a Davie mucky fine sand, also from Florida, reaches 100.8. By material, sands range from 1 to 5, while humus ranges from 100 to 300. Most CEC occurs on clay and humus colloids, and their absence in hot, humid climates, due to fast leaching and decomposition, explains the apparent lack of fertility of tropical soils.

• 6. Acidity And Air

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  Soil pH practically ranges from 3.5 to 9.5, because values beyond those extremes are toxic to life forms. A pH of 3.5 holds a million times more hydronium ions per litre than a pH of 9.5. Soils with high acidity tend to carry toxic amounts of aluminium and manganese, so most agricultural crops do best in mineral soils at pH 6.5 and organic soils at pH 5.5. In high rainfall areas, basic cations are forced off the colloids and washed out, leaving soils inhabited only by organisms efficient at uptake in very acid conditions, as in tropical rainforests. In low rainfall areas, unleached calcium pushes pH to 8.5, and with added exchangeable sodium, soils may reach pH 10, beyond which plant growth is reduced. Sodium can be lowered by adding gypsum, since calcium clings to clay more tightly and pushes sodium into solution to be washed away. The soil's own atmosphere differs sharply from the air above. Atmospheric carbon dioxide sits at 0.04 percent, but in soil pore space it may run 10 to 100 times that level, potentially inhibiting root respiration. When oxygen runs short, anaerobic bacteria strip oxygen from nitrate to form nitrogen gases lost to the atmosphere, a process called denitrification. People meet this hidden atmosphere through the after-the-rain scent, when infiltrating rainwater flushes out the whole soil atmosphere, attributed to compounds such as petrichor or geosmin.

• 7. What Soil Does For Us

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  Soil produces 98.8 percent of the food consumed by humans, serving as the anchor and primary nutrient base for plants. It absorbs rainwater and releases it later, making flood regulation one of its major services, and it cleans water as that water percolates through it. Pollutants are filtered out along the way: persistent organic pollutants, oils, heavy metals such as lead, zinc, and cadmium, and excess nutrients like nitrates and phosphates. Soil organisms metabolise or immobilise these in their living and dead biomass, locking them into stable humus. The material reaches well beyond farms. Soil serves as a foundation for most construction, and moving massive volumes of it figures in surface mining, road building, and dam construction. Earth sheltering uses soil as thermal mass against building walls, and many building materials are themselves soil based. Soil is also a carbon sink, since about 57 percent of its biotic content is carbon. Even in deserts, cyanobacteria, lichens, and mosses form biological soil crusts that capture carbon through photosynthesis. Intensive farming and grazing have degraded soils and released much of this stored carbon, yet restoring the world's soils could offset rising greenhouse gas emissions while improving crop yields and cutting water needs.

• 8. Lessons From The Land

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  Civilizations have prospered or declined as a function of the availability and productivity of their soils. The Greek historian Xenophon, who lived from 450 to 355 BCE, was the first to praise green-manuring crops, writing that weeds turned into earth enrich the soil as much as dung. Around 60 CE, Columella's Of husbandry advocated lime and the turning under of clover and alfalfa, advice followed for 15 generations across 450 years until the Roman Empire's collapse. Experiments later chased a single essential element. Around 1635, the Flemish chemist Jan Baptist van Helmont grew a willow tree for five years on rainwater alone and concluded water was that element, since the soil's weight barely fell. Antoine Lavoisier showed around 1778 that plants and animals must combust oxygen to live, and deduced that most of the willow's 165-pound weight came from air. The hard lessons continued into the modern era. Wind erosion known as the dust bowl ruined American and Canadian prairies during the 1930s, after immigrant farmers, encouraged by both federal governments, converted shortgrass prairie to crops and ranching. The remedy can be as simple as the right clay. In field trials in northeast Thailand by the International Water Management Institute, a single application of 200 kilograms per rai of bentonite raised yields by an average of 73 percent, a result that drew an estimated 20,000 more farmers to the technique.