Soil chemistry facts you'll dig; Plus a student handout from Access Science
Access to this content is available to Ward's World readers for free from McGraw Hill's AccessScience, an award-winning, digital STEM resource containing exclusive articles written by expert scientists and engineers; biographies of well-known scientific figures; science news, videos, and animations; and much, much more.
Instructors can use AccessScience to guide students on their research project journeys, to help students understand scientific concepts, to support distance learning efforts, in flipped classroom approaches, and in countless other ways.
Ward’s World and AccessScience have partnered to offer educators a no-obligation, free trial subscription to AccessScience. Request your free trial today to discover how valuable AccessScience can be for you and your students! Get your free trial now.
The team at Ward's World is excited to make available this powerful series of science articles from McGraw Hill’s AccessScience. This unique series fits our mission to find ways to bring you tips, ideas, and resources to help inspire students and guide discovery and learning. Look for a new McGraw Hill’s AccessScience article to be posted each month on Ward's World.
Want to prepare lessons on soil with a lot less toil? Scoop up these key concepts on soil chemistry to put the future scientists in your classroom on firm ground.
So, what is soil? Soil is the thin layer of material covering the Earth’s surface and is formed from the weathering of rocks. It’s made up mainly of mineral particles, organic materials, air, water, and living organisms.
Unearthing the science behind soil introduces students to various STEM concepts, including soil formation, classification, and mapping; physical, chemical, biological, and fertility properties of soils; and the relationship these properties have to soil management.
Most people don’t realize the significant role soil plays in our ecosystem. Soil influences the worldwide distribution of plants, animals, and people. It filters our water, provides essential nutrients to our forests and crops, and helps regulate the Earth's temperature and many of the important greenhouse gases. It even supplies most of the antibiotics used to fight diseases.
The nature lovers in your class will enjoy getting their hands dirty when exploring all the ways soil supports life. While they’re at it, make sure they check out the activities in our post Get the dirt on gardening, to inspire their natural curiosity about the world around them. They’ll really dig these activities:
- Soil is Alive, Kids Gardening
In this lesson, students explore the many organisms that call soil home.
- Experiment with Composting
In this experiment, your students will add two of the main ingredients needed for composting.
- Rain Gardens, Kids Gardening
In this lesson, students design their own rain gardens to learn about the water cycle, stormwater drainage, water quality, and the broad environmental impact of urbanization.
McGraw Hill's AccessScience article on soil chemistry has mounds of in-depth, comprehensive facts to support your lesson plans. If you’ve been looking for the perfect soil science resource, you’ve hit pay dirt!
Soil chemistry
Article by: Garrison Sposito, College of Natural Resources, Department of Plant and Soil Biology, University of California, Berkeley, California.
Content
Key Concepts
- Chemical processes, including hydrolysis, ion-exchange, and hydration, change the chemical composition of soil (such as ion and mineral content) over time.
- Soils can be divided into organic soils and mineral soils. The main elements of organic soils are carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur, whereas the main elements of mineral soils are silicon, aluminum, and iron.
- Soils’ cation- and anion-exchange capacities describe the degree to which soils can adsorb and exchange cations or anions, respectively.
- Mineral stress occurs when soil suffers from nutritional deficiencies and toxicities, which limit its plant growth potential.
- On a large scale, the nutrient content of soils impacts both plant and animal production; deficiencies in certain minerals can impact plant growth and development as well as animal health.
The study of the composition and chemical properties of soil. Soil chemistry involves the detailed investigation of the nature of the solid matter from which soil is constituted and of the chemical processes that occur as a result of the action of hydrological, geological, and biological agents on the solid matter. Because of the broad diversity among soil components and the complexity of soil chemical processes, the application of concepts and methods employed in the chemistry of aqueous solutions, of amorphous and crystalline solids, and of solid surfaces is required. In addition, soil chemistry controls the availability of plant nutrients within the soil and thus influences plant growth (Fig. 1), yield, and nutritional value for human or animal consumption.
The elemental composition of soil varies over a wide range, permitting only a few general statements to be made. Those soils that contain less than 12–20% organic carbon are termed mineral. (The exact percentage to consider in a specific case depends on drainage characteristics and the clay content of the soil.) All other soils are termed organic. Carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur are the most important constituents of organic soils and of soil organic matter in general. Carbon, oxygen, and hydrogen are most abundant; the content of nitrogen is often about one-tenth that of carbon, whereas the content of phosphorus or sulfur is usually less than one-fifth that of nitrogen (Table 1). The number of organic compounds into which these elements are incorporated in soil is very large, and the elucidation of the chemistry of soil organic matter remains a challenging problem.
Besides oxygen, the most abundant elements found in mineral soils are silicon, aluminum, and iron. The distribution of chemical elements will vary considerably from soil to soil and, in general, will be different in a specific soil from the distribution of elements in the crustal rocks of the Earth. The most important micro or trace elements in soil are boron, copper, manganese, molybdenum, and zinc because these elements are essential in the nutrition of green plants. Also important are cobalt, which is essential in animal nutrition, and selenium, cadmium, and lead, which may accumulate to toxic levels in soil. The average natural distribution of trace elements in soil is not greatly different from that in crustal rocks (Table 2). This similarity indicates that the total content of a trace element in soil usually reflects the content of that element in the soil parent material and, generally, that the trace element content of soil often is not affected directly by pedochemical processes.
The elemental composition of soil varies with depth below the land surface because of percolating water, chemical processes, and biological activity. The principal chemical processes are:
- Hydrolysis: reaction with water to form products containing hydroxide ions
- Complexation: reaction of a metal with a ligand to form a product containing both metal and ligand
- Oxidation-reduction: changes in the oxidation state of an element
- Ion exchange: replacement of one ion by another on a solid surface
- Hydration: reaction with water to form a product containing water molecules
- Flocculation-dispersion: settling-resuspension of solid particles in water; this process affects soil particle removal by erosion or by translocation downward along soil pores.
The principal effect of these processes is the appearance of illuvial horizons in which compounds (for example, aluminum and iron hydrous oxides, aluminosilicates, or calcium carbonate) have been precipitated from solution or deposited from suspension.
The minerals in soils are the products of physical, geochemical, and biologically driven (pedological) weathering (Fig. 2). Soil minerals may be either amorphous or crystalline. They may be classified further, approximately, as primary or secondary minerals, depending on whether they are inherited from parent rock or are produced by chemical weathering, respectively.
The bulk of the primary minerals that occur in soil are found in the silicate minerals, such as the olivines, garnets, pyroxenes, amphiboles, micas, feldspars, and quartz. The feldspars, micas, amphiboles, and pyroxenes commonly are hosts for trace elements that may be released slowly into the soil solution as weathering of these minerals continues. Chemical weathering of the silicate minerals is responsible for producing the most important secondary minerals in soil. These are found in the clay fraction, sometimes in the form of coatings on other minerals, and include aluminum and iron hydrous oxides, carbonates, and aluminosilicates.
A portion of the chemical elements in soil is in the form of cations that are not components of inorganic salts, but that can be replaced reversibly by the cations of leaching salt solutions. These cations are said to be readily exchangeable, and their total quantity, usually expressed in units of centimoles of positive charge [cmol(+)] per kilogram (kg) of dry soil, is termed the cation-exchange capacity (CEC) of the soil. The cation-exchange capacity of a soil usually varies directly with the amounts of clay and organic matter present and with the distribution of clay minerals.
The stoichiometric exchange of the anions in soil for those in a leaching salt solution is a phenomenon limited to chloride and nitrate in the general scheme of anion reactions with soils. Under acid conditions (pH < 5), exposed hydroxyl groups at the edges of the structural sheets or on the surfaces of clay-sized particles become protonated and thereby acquire a positive charge. The degree of protonation is a sensitive function of pH, the ionic strength of the leaching solution, and the nature of the clay-sized particle. The magnitude of the anion-exchange capacity (AEC) usually varies from near 0 at pH 9 for any soil colloid to as much as 50 cmol(−)/kg (centimoles of negative charge per kg) of allophanic clay at about pH 4.
The solution in the pore space of soil acquires its chemical properties through time-varying inputs and outputs of matter and energy that are mediated by several parts of the hydrologic cycle and by processes originating in the biosphere (Fig. 3). Thus, the soil solution is a dynamic and open natural water system whose composition reflects the many reactions that can occur simultaneously between an aqueous solution and an assembly of mineral and organic solid phases. This type of complexity is not matched normally in any chemical laboratory experiment, but nonetheless must be amenable to analysis in terms of chemical principles. An understanding of the soil solution in terms of chemical properties has proven to be essential to progress in the maintenance of soil fertility and the quality of runoff and drainage waters.
The macrosolute composition of a soil solution will vary depending on pH, pE (negative common logarithm of the electron activity), organic matter content, input of chemical elements from the biosphere, and effectiveness of leaching. Under conditions of near-neutral pH, high pE, low organic matter content, no solute input from agriculture, and good (but not excessive) drainage, the expected macrosolutes are Ca2+, K+, Mg2+, Na+, Cl−, biocarbonate ion (HCO3−), silicic acid [Si(OH)4], and sulfate ion (SO42−). If the pH is low, H+ and Al3+ should be added to this list; if it is high, carbonate ion (CO32−) should be added. If the soil has been fertilized, nitrate ion (NO3−) and hydrogen phosphate ion (H2PO4−) become important.
The important microsolutes in soil include trace metals, such as iron, copper, and zinc, and trace element oxyanions, such as those formed by arsenic, boron, molybdenum, and selenium. The tableau of microsolutes in a given soil solution is more dependent on inputs from the lithosphere and the biosphere and less on proton or electron activity and hydrologic factors than is the composition of macrosolutes. The trace metals present, for example, usually are derived from the chemical weathering of specific parent rocks, from the application of fertilizers, pesticides, and urban wastes, and from air pollution.
This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this.
Access the "Test Your Understanding" teacher key here.
For more ways to explore this topic in your classroom, check out these recommended materials:
[StartProductBlock]
Field Guide for Soil & Stratigraphic Analysis Chart
Enhance soil and sediment descriptions. Improve the accuracy of logs and classification of soils in the field using this field guide.
[EndProductBlock]
[StartProductBlock]
APES Lab #1: The Soil Factor: How Does Your Garden Grow?
Students examine the role of soil and soil dynamics in our environment in this comprehensive introduction to soil properties and classification methods. They explore soil texture, porosity, soil nutrients, soil moisture, and holding capacity, and the impact these factors have on growing plants.
[EndProductBlock]
[StartProductBlock]
Ward's® Introductory Mineral Collection
Beginning geology students will easily recognize each of the wide range of physical properties when they examine the basic minerals. High-quality specimens, cut by Ward's geologists.
[EndProductBlock]
