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Soil: the story on HearLore | HearLore
Soil
A single gram of soil contains more living organisms than there are people on the entire planet. This invisible world is a bustling metropolis of bacteria, fungi, protozoa, and tiny animals that function as the planet's primary digestive system. While humans often view dirt as a lifeless substrate for construction or a nuisance to be cleared, soil is actually a complex three-state system of solids, liquids, and gases that supports the life of plants and soil organisms. It is a mixture of organic matter, minerals, gases, water, and organisms that together create a dynamic ecosystem. The solid collection of minerals and organic matter forms the soil matrix, while the porous phase holds gases in the soil atmosphere and a liquid phase that holds water and dissolved substances in ionic or molecular form. This complexity makes soil a critical provider of ecosystem services, acting as a medium for plant growth, a means of water storage and purification, a modifier of Earth's atmosphere, and a habitat for soil organisms. The pedosphere, the collective body of Earth's soil, interfaces with the lithosphere, hydrosphere, atmosphere, and biosphere, creating a fragile yet resilient boundary that has sustained life for millions of years.
The Five Forces of Creation
Soil formation is governed by five interrelated factors that drive the development and evolution of the ground beneath our feet. These factors, often remembered by the acronym CROPT, are climate, relief, organisms, parent material, and time. When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT, highlighting the dynamic interplay between environmental conditions and geological history. Soil development begins with the weathering of parent material, such as lava flow bedrock, which produces the purely mineral-based parent material from which the soil texture forms. In warm climates with heavy and frequent rainfall, soil development proceeds most rapidly from bare rock of recent flows. Plants, in a first stage nitrogen-fixing lichens and cyanobacteria then epilithic higher plants, become established very quickly on basaltic lava, even though there is very little organic material. Basaltic minerals commonly weather relatively quickly, according to the Goldich dissolution series. The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries minerals dissolved from the rocks. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral-weathering mycorrhizal fungi that assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes, inselbergs, and glacial moraines. The alteration and movement of materials within a soil causes the formation of distinctive soil horizons, which differ in one or more properties such as in their texture, structure, density, porosity, consistency, temperature, color, and reactivity.
Common questions
What is the composition of a single gram of soil?
A single gram of soil contains more living organisms than there are people on the entire planet. This composition includes bacteria, fungi, protozoa, and tiny animals that function as the planet's primary digestive system. The soil is a complex three-state system of solids, liquids, and gases that supports the life of plants and soil organisms.
What are the five factors of soil formation known as CROPT?
Soil formation is governed by five interrelated factors often remembered by the acronym CROPT. These factors are climate, relief, organisms, parent material, and time. When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT, highlighting the dynamic interplay between environmental conditions and geological history.
What are the three basic master horizons of a mature soil profile?
Mature soil profiles typically include three basic master horizons labeled A, B, and C. The solum normally includes the A and B horizons, while the living component of the soil is largely confined to the solum. These horizons differ in properties such as texture, structure, density, porosity, consistency, temperature, color, and reactivity.
How does cation exchange capacity affect soil fertility?
Cation exchange capacity is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100 grams of soil. Negatively charged sites on colloids attract and release cations, which resist being washed downward by water and are at first out of reach of plant roots. This process preserves the soil fertility in areas of moderate rainfall and low temperatures.
What is the biomass composition of a typical soil?
A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900 pounds of earthworms, 2400 pounds of fungi, 1500 pounds of bacteria, 133 pounds of protozoa, and 890 pounds of arthropods and algae. This biological community processes all organic matter as in a digestive system.
What historical event caused large-scale soil erosion known as the dust bowl?
Historically, one of the best examples of large-scale soil erosion due to unsuitable land-use practices is wind erosion, the so-called dust bowl. This event ruined American and Canadian prairies during the 1930s. Immigrant farmers, encouraged by the federal government of both countries, settled and converted the original shortgrass prairie to agricultural crops and cattle ranching.
A typical soil is about 50% solids and 50% voids, with the voids being half occupied by water and half by gas. This structure allows for the infiltration and movement of air and water, both of which are critical for life existing in soil. Given sufficient time, an undifferentiated soil will evolve a soil profile that consists of two or more layers, referred to as soil horizons. These horizons differ in one or more properties such as in their texture, structure, density, porosity, consistency, temperature, color, and reactivity. The biological influences on soil properties are strongest near the surface, while the geochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. The solum normally includes the A and B horizons. The living component of the soil is largely confined to the solum, and is generally more prominent in the A horizon. It has been suggested that the pedon, a column of soil extending vertically from the surface to the underlying parent material and large enough to show the characteristics of all its horizons, could be subdivided in the humipedon, the copedon, and the lithopedon. The soil texture is determined by the relative proportions of the individual particles of sand, silt, and clay that make up the soil. The interaction of the individual mineral particles with organic matter, water, gases via biotic and abiotic processes causes those particles to flocculate to form aggregates or peds. Where these aggregates can be identified, a soil can be said to be developed, and can be described further in terms of color, porosity, consistency, reaction, and other physical properties.
The Chemistry of Life and Death
The chemistry of a soil determines its ability to supply available plant nutrients and affects its physical properties and the health of its living population. A colloid is a small, insoluble particle ranging in size from 1 nanometer to 1 micrometer, thus small enough to remain suspended by Brownian motion in a fluid medium without settling. Most soils contain organic colloidal particles called humus as well as the inorganic colloidal particles of clays. The very high specific surface area of colloids and their net electrical charges give soil its ability to hold and release ions. Negatively charged sites on colloids attract and release cations in what is referred to as cation exchange. Cation-exchange capacity is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100 grams of soil. The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources: isomorphous substitution, edge-of-clay oxygen atoms, hydroxylation, and ionization of hydroxyl groups. Cations held to the negatively charged colloids resist being washed downward by water and are at first out of reach of plant roots, thereby preserving the soil fertility in areas of moderate rainfall and low temperatures. There is a hierarchy in the process of cation exchange on colloids, as cations differ in the strength of adsorption by the colloid and hence their ability to replace one another. If present in equal amounts in the soil water solution, aluminum replaces hydrogen, which replaces calcium, which replaces magnesium, which replaces potassium, which replaces sodium. If one cation is added in large amounts, it may replace the others by the sheer force of its numbers, a process called the law of mass action.
The Underground Food Web
A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900 pounds of earthworms, 2400 pounds of fungi, 1500 pounds of bacteria, 133 pounds of protozoa, and 890 pounds of arthropods and algae. Bacteria and fungi feed on raw organic matter, which are fed upon by protozoa, which in turn are fed upon by nematodes, annelids, and arthropods, themselves able to consume and transform raw or humified organic matter. This has been called the soil food web, through which all organic matter is processed as in a digestive system. Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water. Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as peat, are infertile. In its earliest stage of decomposition, the original organic material is often called raw or fresh organic matter. The final stage of decomposition is called humus. In grassland, much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest. Another difference is the frequent occurrence in the grasslands of fires that destroy large amounts of aboveground material but stimulate even greater contributions from roots. Also, the much greater acidity under any forests inhibits the action of certain soil organisms that otherwise would mix much of the surface litter into the mineral soil. As a result, the soils under grasslands generally develop a thicker A horizon with a deeper distribution of organic matter than in comparable soils under forests, which characteristically store most of their organic matter in the forest floor and thin A horizon.
The History of Human Cultivation
The history of the study of soil is intimately tied to humans' urgent need to provide food for themselves and forage for their animals. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils. The Greek historian Xenophon, who lived between 450 and 355 BCE, was the first to expound upon the merits of green-manuring crops. Columella's Of husbandry advocated the use of lime and that clover and alfalfa should be turned under, and was used by 15 generations under the Roman Empire until its collapse. From the fall of Rome to the French Revolution, knowledge of soil and agriculture was passed on from parent to child and as a result, crop yields were low. During the European Middle Ages, Yahya Ibn al-'Awwam's handbook, with its emphasis on irrigation, guided the people of North Africa, Spain, and the Middle East. Olivier de Serres, considered the father of French agronomy, was the first to suggest the abandonment of fallowing and its replacement by hay meadows within crop rotations. He also highlighted the importance of soil in the management of vineyards. His famous book contributed to the rise of modern, sustainable agriculture and to the collapse of old agricultural practices such as soil amendment for crops by the lifting of forest litter and assarting, which ruined the soils of western Europe during the Middle Ages and even later on according to regions. Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element, but the role of nitrogen was overlooked, which is not left on the ground after combustion, a belief which prevailed until the 19th century. In about 1635, the Flemish chemist Jan Baptist van Helmont thought he had proved water to be the essential element from his famous five years' experiment with a willow tree grown with only the addition of rainwater. John Woodward experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded that earthy matter was the essential element.
The Dust Bowl and Modern Crisis
Historically, one of the best examples of large-scale soil erosion due to unsuitable land-use practices is wind erosion, the so-called dust bowl, which ruined American and Canadian prairies during the 1930s. Immigrant farmers, encouraged by the federal government of both countries, settled and converted the original shortgrass prairie to agricultural crops and cattle ranching. A serious and long-running water erosion problem occurs in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion in the Loess Plateau region of northwest China. Soil piping is a particular form of soil erosion that occurs below the soil surface. It causes levee and dam failure, as well as sink hole formation. Turbulent flow removes soil starting at the mouth of the seep flow and the subsoil erosion advances up-gradient. The term sand boil is used to describe the appearance of the discharging end of an active soil pipe. Soil salination is the accumulation of free salts to such an extent that it leads to degradation of the agricultural value of soils and vegetation. Consequences include corrosion damage, reduced plant growth, erosion due to loss of plant cover and soil structure, and water quality problems due to sedimentation. Salination occurs due to a combination of natural and human-caused processes. Arid conditions favour salt accumulation. This is especially apparent when soil parent material is saline. Irrigation of arid lands is especially problematic. All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlying water table. Rapid salination occurs when the land surface is within the capillary fringe of saline groundwater.
The Future of the Pedosphere
Soils filter and purify water and affect its chemistry. Rain water and pooled water from ponds, lakes and rivers percolate through the soil horizons and the upper rock strata, thus becoming groundwater. Pests and pollutants, such as persistent organic pollutants, oils, heavy metals, and excess nutrients are filtered out by the soil. Soil organisms metabolise or immobilise pollutants in their biomass and necromass, thereby incorporating them into stable humus. The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes. Degradation is a human-induced or natural process which impairs the capacity of land to function. Soil degradation involves acidification, contamination, desertification, erosion, or salination. Intensive farming and grazing methods have degraded soils and released much of this sequestered carbon to the atmosphere. Restoring the world's soils could offset the effect of increases in greenhouse gas emissions and global warming, while improving crop yields and reducing water needs. Waste management often has a soil component. Septic drain fields treat septic tank effluent using aerobic soil processes. Land application of waste water relies on soil biology to aerobically treat BOD. Alternatively, landfills use soil for daily cover, isolating waste deposits from the atmosphere and preventing unpleasant smells. Composting is now widely used to treat aerobically solid domestic waste and dried effluents of settling basins. Although compost is not soil, biological processes taking place during composting are similar to those occurring during decomposition and humification of soil organic matter. Organic soils, especially peat, serve as a significant fuel and horticultural resource. Peat soils are also commonly used for the sake of agriculture in Nordic countries, because peatland sites, when drained, provide fertile soils for food production. However, wide areas of peat production, such as rain-fed sphagnum bogs, are now threatened and protected because of their high patrimonial interest. As an example, Flow Country, covering 4,000 square kilometres of rolling expanse of blanket bogs in Scotland, is now recognized as a UNESCO World Heritage Site. Under present-day global warming peat soils are thought to be involved in a self-reinforcing process of increased emission of greenhouse gases and increased temperature, a contention which is still under debate when replaced at field scale and including stimulated plant growth.