Soil Science

Summary

Soil science is the multidisciplinary study of soils, which are composed of mineral, water, air, and organic matter. Soil supports the growth of terrestrial plants, controls the percolation of water, recycles elements and organic molecules, provides habitats for biota, alters dust and gaseous components in the atmosphere, and stabilizes the foundations of buildings and constructions.

Definition and Basic Principles

Soil science is the study of the physical and chemical properties of soils. Soil is the end product of the weathering of rocks and minerals and the decomposition of living organisms. Soil solids include minerals (inorganic particles) and humus (organic matter). As soil solids cling together, pore spaces, or voids, are formed between inorganic and organic materials to accommodate air and water. A typical loam soil contains 45 percent minerals, 5 percent humus, 25 percent water, and 25 percent air. Among the three inorganic particles (sand, silt, and clay), pure sands have the largest particle size, the lowest surface area per unit volume, and the lowest percent of pores. Sands have the largest pores, which allow faster water percolation and drainage. Clay particles are the smallest and have the highest surface area per unit volume and the highest percent of pore spaces. Therefore, clays have the greatest water-holding and element-adsorbing capacity among inorganic particles. Organic humus is the lightest and has the greatest percent of pore spaces among the soil solids. Adding humus increases the retention of water and elemental nutrients in sandy soils and improves water drainage in clay soils.

Soils have a net negative charge because the surfaces of soil particles have more negative sites than positive sites. Negative sites attract positively charged ions (cations) of nutrient elements. The attracted cations are loosely adsorbed and can be easily replaced by other cations. For example, the application of ammonium fertilizers in the soils releases ammonium cations (NH₄⁺), which can replace other adsorbed cations, such as the cations of potassium (K⁺), hydrogen (H⁺), magnesium (Mg²⁺), calcium (Ca²⁺), or sodium (Na⁺). This chemical property of the soils plays an important role in crop production and retaining chemical wastes and other pollutants.

Background and History

Edaphologists who studied soil as a medium for plant growth could be considered the pioneers of soil science. The next group of scientists, the pedologists, studied soil as a geologic entity, examining its origin, formation, morphology, and classification.

Edaphology research began in the early 1600s when Dutch scientist Jan van Helmont conducted an experiment involving soil. He planted a five-pound willow tree in 200 pounds of soil and let it grow for five years. During those years, the plant received only rainwater, no nutrients or fertilizers. At the end of the experiment, the weight of the soil had decreased while that of the plant increased. Helmont concluded that the elements from the soil had contributed to the weight increase of the willow tree. In the 1800s, German chemist Justus von Liebig postulated that plant growth is limited by sixteen essential nutrient elements, a number that scientists later increased to seventeen.

In the early 1870s, Russian geomorphologist Vasily Vasilyevich Dokuchayev, the father of pedology, conceived of soils as natural bodies, each with its own specific characteristics resulting from a unique process of development. In the 1890s, Dokuchayev created a classification system for Russian soils. In the 1920s, the Russian technique was adapted by soil scientist C. F. Marbut, who applied it to the soils of the United States (US). In 1941, Swiss-born scientist Hans Jenny published Factors of Soil Formation: A System of Quantitative Pedology. His work summarized and illustrated investigations into soil science and became a standard work in the field. In 1965, an official soil classification system was completed for the US National Cooperative Soil Survey. This was later published in 1975 as Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys.

How It Works

Soil Formation. It takes hundreds of years for the soils to develop from parent materials. The first step in soil formation is the weathering of rocks and minerals by physical, chemical, or biological processes. Over time, the consolidated rocks become unconsolidated soil materials lying on top of the existing bedrock. Some unconsolidated materials can be carried by wind, water, gravity, or glaciers and then deposited as sediment in different locations. Bedrock and transported sediments become the parent materials of inorganic (mineral) soils. Some land areas, which are cold or inundated with stagnant waters, are subject to faster accumulations of plant and animal residues. The decomposed residues serve as the parent materials of organic soils.

As the soil develops on top of the parent materials, a soil profile is formed with distinct layers of soil horizons. Soil taxonomists use the unique properties of horizons in a profile as the basis for soil classification. Soils are classified into twelve orders.

Physical Properties. The physical properties of soil (such as texture, density, and structure) affect their role in agriculture, the environment, and engineering. Soil texture represents the percentages of sand, silt, and clay particles. The greater the proportion of clay, the more the soil responds to compaction, which increases bulk density, decreases porosity, and reduces aeration. As a result, compacted clays prohibit root penetration but provide a better foundation for engineering projects.

Chemical Properties. The chemical properties of soil—cation exchange capacity (CEC), pH (acidity-alkalinity), and nutrient availability—depend solely on microscopic soil colloids, clays, and humus. Each colloid possesses more negatively charged surfaces than positively charged surfaces. The total amount of cations the soil colloids can adsorb is called the cation exchange capacity. Exchangeable cations held by soil colloids are not easily removed by percolating water but can be easily removed by root absorption and replaced by other cations. Other adsorption reactions bind the ions more snugly, preventing them from leaching into the environment.

The cation exchange capacity influences the development of acidic or alkaline soils. In rainy locations, H⁺ ions from rain replace most basic cations (K⁺, Ca²⁺, Mg²⁺, Na⁺) on the negative sites. As rainwater percolates through the soil, the basic cations leach with water to the deeper horizons, leaving the topsoil acidic with a pH lower than seven because of high H⁺ accumulation in the exchange sites. In arid regions, alkaline soils with pH greater than seven are found because basic cations are retained in the exchange sites.

Soil Biota. Based on their feeding habits, soil-dwelling organisms (plants, animals, fungi, protists, bacteria, and archaea) play a role in the formation of soil, contribute to the accumulation of organic matter, and recycle nutrients in the soil. Large animals (badgers, mice, and prairie dogs) dig underground tunnels and aerate the soil. Earthworms strengthen the soil structure. As they burrow, they ingest plant and animal residues, including clays, and excrete them as granular aggregate. Most soil fungi (molds and mushrooms) are decomposers. They use the nutrients and organic molecules from the tissues of dead and living plants and animals. Protists include plant-like (algae), animal-like (protozoa), and fungus-like (slime molds) organisms. Algae, like plants and cyanobacteria, are not decomposers but rather primary producers because they are photosynthetic organisms. Protozoa (such as amoeba) are unicellular organisms that ingest small living organisms. Most bacteria and archaea, which decompose the tissues of all dead organisms, are found in the lowest part of the food chain.

Applications and Products

Medium for Plant Growth. Soil anchors a plant's roots (or shoot) and holds its above-ground parts in place. Pore spaces between soil solids supply oxygen to established plants' roots or germinating seeds during respiration. Soil pores also store water, which is supplied to the roots for absorption or to the seeds for imbibition. Moisture in the pores minimizes soil temperature fluctuations in the rhizosphere. As a result, soil serves as an insulator that protects plant roots from extremely hot and cold conditions.

All higher plants require seventeen essential elements to complete their life cycle. Three elements—carbon (C), hydrogen (H), and oxygen (O)—mostly come from air and water. The remaining fourteen elements—nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), boron (B), copper (Cu), chlorine (Cl), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn)—are supplied by the soil. Macronutrients (N, P, K, Ca, Mg, and S) are the elements plants need in large amounts. Therefore, incorporation of these nutrients in the soil (as fertilizers) is often necessary. Plants require micronutrients (B, Cu, Cl, Fe, Mn, Mo, Ni, and Zn) in small amounts, so applying fertilizers containing micronutrients is usually unnecessary.

Nutrient elements from the soil are absorbed by plant roots as negatively charged ions (anions) or positively charged ions (cations). The forms of ions easily absorbed by plant roots are anions—nitrate (NO₃⁻), hydrogen phosphate (H₂PO₄⁻), sulfate (SO₄²⁻), hydrogen borate (H₄BO₄⁻), Cl⁻, and molybdate (MoO₄²⁻)—on the positive sites and cations—NH₄⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Mn²⁺, Ni²⁺, and Zn²⁺—on the negative sites.

Soil amendments increase cation exchange capacity, improve the water-holding capacity and drainage of the soil, and supply more nutrients to the plants. Commercial potting mix for ground and container gardening contains a combination of fertilizer, perlite, vermiculite, and compost. Synthetic soils, which are made of potting mix plus additional amendments, such as sludges, hydrogels, and organic mulches, are commercially manufactured in large quantities and used for reclamation or remediation of abandoned mine sites, barren soils, or unproductive wetlands.

Water Storage and Filtration System. Water is stored in the soil, but some water travels through soil pores and moves to streams, rivers, lakes, and aquifers. Contaminated waters that are spilled onto soils undergo purification and clarification processes that remove toxic ions and impurities before the cleansed water enters waterways. This filtration mechanism relies on the pores and exchange sites of the soil. As water moves through the soil, large molecules of pollutants are trapped within the microscopic pores, allowing only small molecules of water to pass through. Soil further removes ions of toxic wastes and heavy metals by electrostatic reaction with the ion exchange sites of the soil colloids.

Not all soils can store and filter water effectively. Sandy soils make ineffective water storage and filtration systems because of their large pores. Compacted clays have tiny pores that become impervious to water. Torrential rains that pour into denuded mountains with impermeable clays result in flash floods of muddy water.

Nutrient and Waste Recycling. Soils are the major repositories of wastes created by humans. The capacity of soil to recycle nutrients from wastes depends on the living organisms that inhabit the soils. Organic wastes are decomposed into valuable humus by fungi and microbes. Mineral nutrients from humus are used by plants and animals. Plants acquire carbon as carbon dioxide from the atmosphere and release oxygen into the atmosphere through photosynthesis. Animals release carbon dioxide into the atmosphere through respiration. When plants and animals decompose, mineral nutrients are deposited in the soil.

Atmosphere Modification. Exposed soils that are dry and poorly structured are susceptible to erosion by wind, which can carry and disperse the soil particles as dust in the atmosphere. Suspended dust decreases the clarity of the air, increases the chance of developing respiratory problems, and cools the atmosphere and the ground. Soils that are covered with vegetation are less affected by erosion.

Soil alters the amounts of ammonia (NH₃) and nitrogen gas (N₂) in the atmosphere. The soil's decomposition of organic matter by bacteria and fungi generates NH₄⁺ through ammonification. Plants absorb some NH4+ and volatilize into the atmosphere as ammonia (NH₃). Soil bacteria convert NH₄⁺ into nitrate (NO₃⁻) by nitrification. Plants assimilate some nitrate, and soil bacteria denitrify some into nitrogen gas (N₂). Some N₂ in the soil pores is fixed by bacteria that live in the roots of the plants. Some atmospheric N₂ is converted into NO₃⁻ by lightning. Whatever form of nitrogen is in the soil, nitrogen is subject to rapid loss. For this reason, soil nitrogen is the element most needed by plants. Nitrogen is often replaced by adding nitrogen-containing fertilizers or compost to soil.

Medium for Engineering Projects.Soil mechanics, an examination of the architecture and physical properties of the soil, determines which soil types are best suited for construction. Soil, as compacted solid ground, serves as a stable and firm foundation on which to build roads, bridges, houses, towers, and other infrastructure. Most structures are erected on the soil surface, and many engineering projects (such as underground tunnels) involve digging deeper into the soil. Soil stability is based on its compressibility. Sands are difficult to compress, but compressed clays form a very stable compacted material. However, clays that shrink and form deep cracks when dry and expand when wet (such as montmorillonite) are not suitable sites for buildings. Highly weathered clays, such as kaolinite, do not shrink and swell and are appropriate types of clays for engineering projects.

Soils are extensively used in building houses. Walls of African huts are made of mud, and walls of modern Western houses use brick (baked clay), hollow cement blocks (a mixture of sand and cement), silica glass (made from sand), and other soil-cement mixtures. Green roofs are partly or totally covered with soil and vegetation. They insulate the structures they cover and help lower the air temperature within cities.

Careers and Course Work

Soil science has adopted tools and techniques from a wide variety of basic and applied sciences to study soil. Courses include introductory soil science, soil fertility, soil chemistry, soil physics, soil microbiology, soil genesis, soil taxonomy, pedology, soil conservation, and soil management. The minimum requirement for a career in soil science is a Bachelor of Science degree in soil science or agronomy, chemistry, physics, or environmental science with selected courses in soil science. For advanced positions, a Master of Science or Doctorate in soil science is preferred.

Soil scientists may work in an office, in the field, or both. Soil scientists employed in agronomy and crop production provide recommendations to farmers regarding the correct amount of fertilizer for specific plants in large plots of cultivated land. Scientists at the US Department of Agriculture (USDA)'s Natural Resources Conservation Service map and describe the soil using digital and satellite imagery. They take soil samples and chemically analyze them, classify soils, and evaluate the soil, identifying any problems. The USDA's Agricultural Research Service employs many soil scientists in numerous programs throughout the US, including those of the Natural Resources and Sustainable Agricultural Systems.

Social Context and Future Prospects

Soils are highly used and abused natural resources. For example, farmers who employ conventional crop production methods cultivate the soils each year. If they routinely apply more than the recommended amounts of nutrient fertilizers, this could lead to overfertilization. In rainy areas, the excess nutrients leach down to the aquifer and run off horizontally, polluting the surface water and groundwater. In arid regions, the excess nutrients facilitate the development of saline soils. Annual soil cultivation removes organic matter and exposes the soil to wind and water erosion. Many modern farmers use windbreaks and no-till farming to reduce soil erosion.

The US government has implemented many programs to combat soil problems. For example, the Natural Resources Conservation Service planted grasses on barren soils in Texas to counteract wind erosion. As a result, dust particles in the air were reduced drastically. Bioremediation projects operated by the Environmental Protection Agency have been able to identify plants that can extract toxic heavy metals from the soil and water. Although technologies are being continually developed to increase crop production and improve soil and water quality, it is ultimately up to humans to ensure that environmental quality is maintained for future generations. In the twenty-first century, the demands for food, water, and energy will only increase with the global population. Soil science will continue to play a pivotal role in addressing environmental challenges and securing sustainable goals. Global climate change, access to food and water, urban development, and ecosystem biodiversity are all rooted in soil science, and its study will greatly impact the future. 

Blibliography

Evans, Daniel L., et al. "Sustainable Futures over the Next Decade Are Rooted in Soil Science." European Journal of Soil Science, vol. 73, no. 1, 2022, p. e13145, doi.org/10.1111/ejss.13145. Accessed 4 Jun. 2024.

Lal, Rattan, and Manoj K. Shukla. Principles of Soil Physics. New York: Marcel Dekker, 2004.

Liu, Cheng, and Jack Evett. Soil Properties: Testing, Measurement, and Evaluation. 6th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2009.

Popkin, Gabriel. “A Soil-Science Revolution Upends Plans to Fight Climate Change.” Quanta Magazine, 27 July 2021, www.quantamagazine.org/a-soil-science-revolution-upends-plans-to-fight-climate-change-20210727. Accessed 4 June 2024.

Sposito, Garrison. The Chemistry of Soils. 2d ed. New York: Oxford University Press, 2008.

Sylvia, David M., Jeffrey J. Fuhrmann, Peter G. Hartel, and David A. Zuberer, eds. Principles and Applications of Soil Microbiology. 2d ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2005.

“Trends in Soil Science.” Soil Science Society of America, www.soils.org/about-society/trends. Accessed 4 June 2024.