You’ve been learning a lot about traditional soil classification and management. In this module, you’ll learn about western methods of evaluating and classifying soils. You’ll explore the chemical and physical properties of soil as well as soil formation and development.
To give you a more exploratory and visual approach, start by watching this foundational video about soil and the importance of soil in global ecosystems.
Film 1: Soil Stories – The Whole Story
Soil Formation & Development
Soil consists of minerals in many different sizes, in which spaces are filled with air or water. The water, known as the soil solution, contains colloids and dissolved chemicals, plus many living organisms.
The minerals found in the soil are a product of the weathering over time of parent material. Parent material, also known as bedrock, comes in a large variety of forms, and cycles over long geological time spans. Through chemical, physical and biological forces parent material is broken down into smaller mineral particles, eventually becoming a soil.
Parent material interacts with climate, biota, topography and time, and through chemical and physical forces, it gradually weathers into smaller and looser materials. After the parent material has been weathered by physical and chemical forces to a consistency suitable for plant growth, plants begin to establish themselves on the weathered material. Plants send roots into the mineral matter, and draw nutrients from them. Deep roots break down the regolith. When these plants die, they decompose on top of the regolith and add these nutrients back to the top in an organic form. These plant residues serve as an energy source for bacteria, fungi, earthworms other soil organisms to establish themselves in that regolith. Soil development is accelerated by these organisms, as they regulate and carry out the biological, chemical and physical processes to establish and maintain soil fertility. This is a gradual process- to put it in perspective, it takes more than 100 years for 1 inch of topsoil to form.
Soil Physical Properties
Over time, the local chemical, physical and biological processes in regolith leads to the development of observable layers in the soil, known as horizons. These horizons vary, and together, the horizons in a particular location are known as a soil profile.
Generally, the soil profile is made up of four major horizons. The O horizon lies at the soil surface and is generally made up of organic deposits and non-decomposed plant materials. The A horizon, just below the O horizon, generally contains the most organic matter, and the macro- and micro- organisms responsible for decomposition are most active in the A horizon. Due to chemical weathering, this layer is usually depleted of iron, clays, aluminum and soil colloids. In agro-ecosystems, the A horizon is eroded and depleted by tillage. Below the A horizon is the subsoil, or B horizon. Depending on the parent material and age of the soil, the B horizons vary greatly. Soils can develop more horizons than just these master horizons. There are many subdivisions for each master horizon, denoted by another letter. Beneath the master B horizon is the parent material or bedrock, known as the C or R horizons respectively.
If you’re interested in learning more about soil horizon naming, please visit this page.
Soil particle size determine soil texture. Solid material in soil, usually minerals, are classified according to size. The International Society of Soil Science classifies solid soil material as follows: gravel (>2.0mm), sand (between 0.02 and 2.0 mm), silt (between 0.002 and 0.02 mm) and clay (<0.002 mm).
The largest particles found in soil are known as sand. These particles are irregularly shaped, and because of their large size, when they are packed together they leave a lot of air space in between them. As a result, water flows quickly through soil with a lot of sand in it, and they dry out quickly. Sand particles are often made of quartz (SiO2), although feldspars and micas can occur also.
Silt particles are in between the size of sand and clay. They are irregularly shaped, and usually quartz. Frequently, silt particles are coated in a layer of clay, making them sticky and as a result of the clay, able to absorb water.
Clay particles are the smallest sized particles in soil. Clay particles are any mineral particles smaller than 0.002mm in size.
Soil colloids are particles smaller than 0.001 in size. Not all clay particles are strictly colloidal, but those that are are held in colloidal suspension in the soil solution. Their surface area per unit mass is enormous-you can fit about 1,000,000 clay particles in the area occupied by a single sand particle, and each of those clay particles has about 100 times the surface to volume ratio as the sand particle. Because many features of soil physics and chemistry are related to consequences of surface area phenomena, the colloidal clays largely determine the physical and chemical properties of a soil.
Inorganic colloids usually make up the bulk of soil colloids. They include clay minerals and hydrous oxides. The organic colloids include highly decomposed organic matter generally called humus. Organic colloids are more reactive chemically and generally have a greater influence on soil properties per unit weight than the inorganic colloids. Clay minerals are usually crystalline (although some are amorphous) and usually have a characteristic chemical and physical configuration. Both inorganic and organic colloids are intimately mixed with other soil solids. Thus, the bulk of the soil solids are essentially inert and the majority of the soil’s physical and chemical character is a result of the colloids present.
Soil Chemical Properties
Film 2: Soil Formation | Science Learning Hub
Professor Louis Schipper from Waikato University briefly explains the five factors involved in soil formation.
How do colloids affect the physical and chemical properties of the soil?
Soil colloids absorb, hold and release ions. Generally, soil colloids have a net negative charge- that means that they attract positively charged ions, and water. The amount of water affects the amount of attraction between positively and negatively charged ions.
Dissolved ions in soil are what causes soils to be acidic or basic. In humid regions, the cations associated with the colloids are dominated by Ca+2, H+, and often A1+3, resulting in acidic soils. As the soil becomes more acid, H+ and Al+3 become more predominant. The cations Mg+2, K+, and Na+are usually found in lesser amounts, while NH4+ may be present in considerable quantities if the soil has been recently fertilized with ammonium fertilizers. In semiarid and arid regions, Ca2+ usually dominated the cations, but Mg2+ and Na+ are often found in large quantities. H+ and A13+ are usually present only in small concentrations.
These cations are used by plants, which we will learn more about in the Human Nutrition and Soil Fertility Course. It is important to note that some important mineral nutrients, such as potassium and calcium are positively charged ions, whereas others such as nitrate and phosphate are in the form of negatively charged ions. Ions not being taken up through plant roots or fungi run the risk of being leached through soil.
Since clay and humus particles have negatively charged surfaces that hold positively charged ions in the soil. The number of sites available to hold positively charged ions determines what is called the soil cation exchange capacity, measured as milliequivalents of cations per 100g of soil. The higher the CEC, the better the soil’s ability to hold and exchange cations, prevent nutrient leaching, and provide plants with adequate nutrition. The cation exchange capacity varies from soil to soil, depending on the structure of clays and the amount of humus in the soil. Organic matter in the form of humus (organic soil colloids) is more effective than clay at increasing CEC since it has a more extensive surface area-to-volume ratio (more sites for absorption). As a result, farming practices that reduce the Soil Organic Matter (SOM) content can reduce soil fertility.
Acidity and pH
This video goes over acidity, alkalinity and their importance for crops.
Film 3: Soil pH – Volunteer Gardner
Soil acidity influences the electrical charge of soil colloids, and controls whether other ions are displaced, it greatly affects the retention of ions in the soil and the short-term availability of nutrients, both of which are key to soil fertility. It also changes the soil ecosystem, and changes the ability of different organisms to survive in the soil.
Soils can range in pH from very acidic (a pH of 3) to strongly alkaline (a pH of 8). Any soil over a pH of 7 (neutral) is considered basic, and those less than pH 6.6 are considered acidic. Few plants, especially agricultural crops, grow well outside the pH range of 5 to 8. Legumes are particularly sensitive to low pH. Legumes form a symbiotic relationship with nitrogen-fixing bacteria in their roots, and these bacteria cannot survive in acidic conditions. At low pH, it is more difficult for plants to absorb nutrients, so acidic soils are seen as less fertile. Therefore, it is important to maintain soil pH in the optimal range.
Soils can increase in acidity through natural processes. Soil acidification happens as a result of the loss of basic compounds by leaching of water downward through the soil profile, the uptake of nutrient ions by plants, their removal through harvest or grazing, and the production of organic acids by plant roots and microorganisms. Soils will go out of a pH balance more quickly if they are not buffered to be resilient to these input or removal processes.
Salinity and Alkalinity
In arid or semiarid regions of the world, it is common for soils to accumulate salts. Salts are released through the weathering of parent material, and added by limited rainfall. In areas of low rainfall and high evaporation rates, dissolved salts such as Na⁺ and Cl⁻ are common, and combine with others such as Ca²⁺, Mg²⁺, K⁺, HCO₃⁻ and NO₃⁻. Irrigation can add even more salts to the soil, especially in places with high evaporation potential, as evaporation draws salts to the surface of the soil. Inorganic fertilizers increase salinity as they are in the form of water-soluble salts.
Soils with high concentrations of neutral salts, (eg. NaCl or NaSO₄) are called saline. When these combine with weak anions, alkaline soils develop, which generally have a pH greater than 8.5. When soils have high levels of neutral salts, it is difficult for plants to maintain the water in their cells (also known as osmotic imbalances), and to properly absorb nutrients. Soil water management and proper irrigation management are key for dealing with saline, alkaline or acidic conditions.