Fleming

Physical Characteristics of Soil

Based on a Lab Outline distributed by Carolina Biological Supply Company

Discussion:

Soil serves many essential functions including decomposing dead organic matter for recycling into plants and animals, filtering water for purification, and providing essential nutrients for plant growth (and indirectly animal growth). For humans, soil also serves as an engineering material for constructing buildings and roadways, an agricultural medium, and a waste receptacle. Soil differs from area to area, and different soils are not equally suited to all of the above activities.

In general, soil is a mixture of weathered rocks and minerals and decomposed organic matter. The sizes and chemical identity of the rock fragments, the relative amount of organic matter, and the environmental conditions under which decay occurred all affect the nature of the soil. Soil is constantly developing and changing, though the time scale for the process is so slow that humans do not normally observe it.

Soil Organization – Soil is organized in layers, a product of how it is formed. Soil formation typically starts with the weathering of bedrock to produce very fine particles. In time, plants may begin to grow in this material. As these plants die, their remains add organic material to the weathered rock, which brings bacteria, fungi, and microscopic animals to feed on the organic material. Their physical activities and decayed remains further alter the soil; in time, a reasonably thick, dark-colored soil layer is formed. Rain washes dissolved minerals and very fine particles through this layer, often forming a clay layer below it. And over time, major geologic factors such as glaciers or floods may introduce new layers of soil.

The bottom soil layer rests on bedrock; the point at which the soil is saturated with water is called the water table, and its depth varies per location and season.

Characteristics of Soil – The ability of a specific soil to support different activities depends on both the chemical and physical aspects of its composition. Chemical features of soil include pH, ion content, and ion-holding capacity and are determined by factors such as the chemical nature of soil particles and bedrock. These characteristics affect how well a particular soil can supply essential minerals to plants and filter ions out of wastewater.

Plant roots require air and water. Just as a plant can die from lack of water, it can die in waterlogged soil from lack of air. Soil must retain water, allow for easy plant root penetration, and provide physical support for the plant.

Physical features of soil such as particle size and arrangement, nature of the soil layers, soil texture, and land slope determine how well a soil holds water, how freely water passes through it, how easily it permits root growth, and how readily oxygen permeates it. These physical characteristics not only profoundly influence the ability of a soil to support plant life, but also determine its suitability for such things as supporting materials for buildings and roads, and hosting landfills and septic tank systems.

SOIL TEXTURE, STRUCTURE, AND CONSISTENCE

Soil texture is determined by the ratio of sand, silt, and clay in the sample. Sand, silt, and clay are all mineral components of soil, and are defined by their particle size. Particles with a diameter greater than 0.05 mm are considered sand; between 0.002 mm and 0.05 mm, silt; and less than 0.002 mm, clay. (By definition, organic matter does not contribute to soil texture.) Soil scientists group soil into three broad classes based on texture: the sands, the clays, and the loams (a mixture of sand, silt, and clay).

A common field-test method to determine texture is the ribbon test. In this test, a small amount of soil is moistened, formed into a ball, then squeezed and pinched to form a ribbon. The behavior of the sample during the test (for example, whether it forms a ball or a ribbon – and, if so, how long a ribbon) determines its classification.

Soil Structure – Primary soil particles (sand, silt, and clay) are arranged into secondary units called peds. The shape of the peds and the way in which they aggregate in a soil is referred to as soil structure. Soil structure affects how easily air, water, and plant roots move through soil. Human activity such as repeated trampling or plowing when wet can alter it.

Soils that separate easily into rounded peds are called granular. Granular soils have high permeability and therefore do not pack tightly. They are usually found near the soil surface where organic matter is abundant. Granular soils are particularly suitable for plant growth, because their structure permits air, water, and plant roots to easily penetrate the soil. Clay and loamy soils often have blocky peds, which are angular and somewhat irregular in shape. Their irregularity ensures that soils composed of blocky peds contain pores that permit passage of air and water. Soils with plate-shaped peds, which can resemble stacked sheets of ice, are tightly packed and difficult for air and water to penetrate. Platy soils usually have a high clay content and tend to be found in frequently flooded areas. These soils are often called “clay-pan.” On the other hand, sand itself is a structureless soil; the primary particles do not aggregate but instead fall apart.

Soil Consistence – The degree to which soil resists pressure is referred to as its consistence. Farm and construction machinery and even a herd of cattle can put a great deal of pressure on the soil, so consistence is important when considering how land should be managed. The terms sticky, plastic, loose, friable, soft, firm, very firm, and hard are used to describe the consistence of the soil and how well the soil resists effects of wind, water, and machinery.

Chemical Characteristics of Soil

Plants absorb two essential things from soil: water and nutrients. The availability of water in soil is related to physical properties of the soil such as percolation and retention rates. The availability of nutrients is a function of the chemical characteristics of the soil, in which water also plays a key role.

Soil is made up of decomposed organic matter (humus) and weathered minerals (sand, silt, and clay). Minerals are compounds with a definite chemical composition, crystal structure, and an orderly internal organization of atoms. In all, about 90 elements are found in the earth’s crust, and these combine in different ways to produce about 2000 different minerals. These minerals are generally found in mixtures we call rocks. Approximately 98% of the earth’s crust by weight is made up of only 8 elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. In fact, 75% of the weight of the earth’s crust consists of just 2 elements: oxygen and silicon. These elements comprise the majority of the material found in the earth’s minerals and in the mineral fraction of soil.

The total mineral composition of a particular soil is a function of the parent rocks and the minerals from which the soil particles were formed. However, only a relatively small fraction of the total mineral content of soil is typically available and important for plant nutrition. Plants absorb essential minerals in the form of ions dissolved in the water fraction of soil. Most of these ions originate from the mineral fraction of the soil itself, but they can also arise from the decomposition of organic matter. The mineral component of soil slowly releases ions in a process called weathering.

Chemical Weathering – Weathering refers in general to natural processes that cause physical and chemical changes in rocks and minerals. Some weathering processes are physical; for example, water seeps into small cracks in a rock, freezes, and enlarges the crack; wind blowing sand gradually erodes the outside surface of a rock. Other weathering processes are chemical, and these are very important in both soil formation and plant nutrition.

Some of the most important chemical weathering processes occur at exposed edges of mineral surfaces where they interact with water. Some minerals, such as halite (sodium chloride), are gradually dissolved by the water and washed away in a process called leaching. Insoluble minerals take up water molecules on their surfaces (an interaction called adsorption), where the water molecules can be a source of hydrogen ions. These small positively charged ions can very gradually replace other cations found in the mineral, such as Mg2+, Ca2+, K+, and Na+. Gradually, the composition of the mineral is altered.

Once the ions are dissolved, they are in the proper chemical form to be absorbed by plant roots. Whether plant roots will have time to absorb the ions depends in large part on how well the soil holds them. Bulk retention of dissolved ions depends in part upon how fast water percolates through soil. Soil also holds dissolved ions through electrostatic interactions at the surfaces of soil particles. Large particles such as sand have relatively little surface area and are therefore relatively unimportant for ion binding. In addition, highly sandy soil has little capacity to retain water, so most of the dissolved ions entering sandy soil will be leached away. Leaching is similar to rinsing; percolation of water through the soil eventually rinses out ions that are not bound by soil particles.

Clay and humus particles have much greater surface area and are the centers of ion binding in soil. Their ion binding properties depend on their chemistry, and also the environment within the soil.

Cations in Soil – Humus is composed of humic acid, a highly complex material with many carboxylic acid groups. At relatively neutral pH values, the carboxylic acid groups are negatively charged and can bind cations. Most clays are composed of systems of silicates containing varying amounts of aluminum, magnesium, iron, calcium, potassium, and trace minerals. Silicate clays are negatively charged, and also bind positive ions.

The adsorption of cations by negatively charged binding sites is, like other chemical associations, governed by a chemical equilibrium. This means that a given cation spends some time associated with a binding site, and some time dissociated with it. If the soil water contains other dissolved cations, one of these can replace the original ion on the binding site. This process is called cation exchange.

Cation exchange is the means by which plants absorb essential positive ions such as potassium. Plant roots secrete hydrogen ions (H+) which eventually affect an exchange with adsorbed cations such as potassium. In this manner, plants slowly deplete soil of available nutrient cations, unless these are replaced by mineral weathering or by application of fertilizer. The cation-binding capacity of soil also enables soil to purify water of positively charged contaminants.

Anions in Soil – Although silicate clays and humus have an overall negative charge, particles of these materials do contain a few sites where anions can bind. However, the capacity of these soils to bind anions is far less than their capacity to bind cations. A few soils are positively charged. In these rare soils, the interactions with cations and anions are reversed: anions are adsorbed and cations are leached.

Plant Nutrition and Soil Minerals – The elements required for plant growth can be grouped roughly according to the relative amount the plant needs: macronutrients, which are necessary at concentrations of at least 500 parts per million in plant tissue; and micronutrients, which are necessary only in extremely small amounts, usually less than 50 parts per million. Like all living things, plants require carbon, which they obtain from carbon dioxide in the atmosphere. The macronutrients that plants get from soil are nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.

Nitrogen – Nitrogen presents a special case because it is not a component of any mineral, and higher plants cannot use the elemental nitrogen found in the atmosphere. Plants require nitrogen in the form of ammonium (NH4+) or nitrate (NO3-) ions. Atmospheric nitrogen is converted into these forms by soil microbes in reactions that form part of the global nitrogen cycle. Plants absorb and use some of the converted nitrogen, then eventually die and decompose, releasing the nitrogen back to the soil. Alternatively, plants are ingested by animals, which excrete nitrogen-containing wastes into the soil and eventually die and decompose. Other soil microbes convert part of this organic nitrogen back into nitrogen gas, keeping the concentration of nitrogen in the atmosphere constant.

The amount of nitrogen introduced into the soil via natural processes is not sufficient to sustain the intensive agriculture upon which our economy depends. Therefore farmers add nitrogen to the soil in the form of nitrate salts. Nitrate is an anion and is not bound by soil to any significant extent, so excess nitrate leaches through the soil and can end up in surface water or groundwater. In addition, since nitrate is not retained by the soil, farmers must frequently reapply nitrate fertilizer.

Phosphorus – In theory, there is enough phosphorus present in the earth’s crust to supply the needs of plants. However, the distribution of phosphorus around the world is not uniform, and even when phosphorus is present, it is not necessarily available to plants. Most phosphorus is present as phosphate salts, which generally have quite low solubility. The low solubility means that the concentration of available phosphorus in soil water is low at any given time. Although phosphate is an anion (PO43-), it does not usually leach from the soil into groundwater, due to its low water solubility. Phosphate ions usually bind to the surface of clay particles through exchange with OH groups. From there they maintain a low concentration in soil water, governed by their solubility constants. Phosphorus availability commonly limits plant growth. For this reason, the introduction of phosphates (for example, from detergents) into the environment can result in rapid plant growth. This stimulation of growth by phosphorus often results in algal blooms, which create problems for aquatic ecosystems.

Other Nutrients – The other macronutrients are absorbed from soil as cations, except for sulfur, which is absorbed as sulfate (SO42-). All of these elements are introduced into the soil via mineral weathering. The cations Mg2+, Ca2+, and K+ are all bound by negatively charged clay and humus particles, where they can be exchanged into the soil water and absorbed by plants.

Soil pH – Soil pH is measured by suspending a sample of water in soil, allowing it to equilibrate, and measuring the pH of the water. Therefore, soil pH is determined by the amount of hydrogen ions dissolved in soil water. Recall from the above discussion of weathering that mineral cations in soil are gradually replaced by H+ over time. This means that, over time, the concentration of H+ ions in soil water will also increase (because they are in equilibrium with the bound H+), and the pH of the soil will decline. The rate of replacement of nutrient cations by H+ is dependent on the moisture in the soil and the buffering capacity of the soil. Soils in the humid tropics weather the most quickly. Many tropical soils are ultimately weathered; that is, their nutrient cations have been depleted and replaced by H+, so that the only nutrient cycling in the ecosystem occurs in the living organisms.

Soil pH is important for several different reasons, one being that it affects the solubility of soil nutrients. For example, phosphorus (in the form of phosphates) is most soluble between pH 6 and 8. Soil pH also affects the ability of certain microorganisms to fix nitrogen. For example, azotobacteria, a group of free-living nitrogen fixers, can survive at pH values below 6 but can no longer fix nitrogen. Another consequence of soil pH is its effect on the solubility of the potentially toxic aluminum ions.

Aluminum Toxicity – As the cations Mg2+, Ca2+, and K+ are removed from soil by weathering and absorption by plants and are replaced by H+, the soil pH drops, making the aluminum ion (Al3+) become more soluble. As soil pH drops, aluminum ions begin to dissolve in the soil water and occupy cation binding sites in greater and greater concentration. This has two effects. One effect is on soil pH. Aluminum ions increase the acidity of a solution, because they interact with water molecules to liberate H+. The other effect is plant toxicity. Many plants show toxic effects when the concentration of Al3+ in soil water exceeds one part per million.

Soil Chemistry and Acid Deposition – Acid rain and other forms of acid deposition can, in essence, put chemical weathering processes on fast forward. If a soil lacks buffering capacity (provided by carbonate material), acid rain introduces relatively high concentrations of H+, which speeds replacement and leaching of nutrient cations and increases aluminum solubility.