Soil Components and Formation PDF

# LOS COMPONENTES DEL SUELO Considering the materials that constitute it, the soil is the integration of four components: 1. Mineral matter 2. Organic matter 3. Water 4. Air The volume occupied by each component in a superficial soil, of Extoluenca in ideal conditions for plant development, w...

# LOS COMPONENTES DEL SUELO Considering the materials that constitute it, the soil is the integration of four components: 1. Mineral matter 2. Organic matter 3. Water 4. Air The volume occupied by each component in a superficial soil, of Extoluenca in ideal conditions for plant development, would be approximately as follows: | Component | Percentage | | ------------------- | ---------- | | Mineral matter | 45% | | Organic matter | 5% | | Water | 25% | | Air | 25% | The percentages vary from one soil to another and in the same soil they can vary from one moment to another because the volumes of water and air are in a direct exclusion relationship: the entry of water into the soil excludes the air and vice versa. The subsoil is generally characterized by having less organic matter than the surface soil. Other types of soils called organic (such as humiferous or turbosos) have a greater volume occupied by organic matter than by mineral matter. # FACTORES DE FORMACIÓN DEL SUELO The soil is formed as a result of a natural process and only natural; soil cannot be manufactured. It is the product of the action, over tens or hundreds of years, of several natural factors. Any element that contributes, along with others, to achieving a result, is called a factor. When the parent material is residual, it is said that the soil has a mode of formation *in situ* (in the same place). When the parent material is transported, it is said that they are transported soils. In Mexico, due mainly to the relief and the fluvial activity, most agricultural soils are transported. This feature is highlighted by the presence of rounded stones that give an idea of movement. In other regions of the world, such as Alaska and Siberia, a second type of parent material consisting of deposits of organic matter known mainly as peat, is considered. There is a close relationship between the parent material and the soil formed. In other words, many of the characteristics of a soil are inherited from the material that gave rise to it. # CLIMA Climate is the set of meteorological phenomena that characterize the long-term state of the atmosphere of a given place. The two dominant meteorological phenomena in soil formation are precipitation and temperature (cold and heat). Some direct effects of climate on soil formation are: * In areas of low rainfall, the carbonates of calcium, common in soils, are not dissolved and washed away by the water, so there are accumulations of this mineral. * Erosion of soils located on steep slopes occurs due to rainfall. * Rainfall deposits soil materials in low-lying areas. * In tropical regions, due to rainfall and intense heat, weathering, leaching, and erosion are more intense than in cold regions. * Climate indirectly influences soil formation through vegetation. In the early 20th century, Russian scientists proposed that soil formation was the result of the action of climate and living matter on parent materials (rocks or rock fragments) in a given relief and over a period of time. Based on these ideas, in 1941 Jenny proposed the following expression to denote that soil formation depends on a set of variables, so that if they change, the result (the soil) will change. $S = f (m, cl, o, r, t)$ This expression means: "Soil (S) is the result of the combined action of five factors: * m = parent material (passive) * cl = climate (active) * o = organisms or biosphere (active) * r = relief (passive) * t = time (neutral) # MATERIAL PARENTAL Parent material the (mineral or organic) material from which soils are formed. The parent material of Mexican soils is mineral parent material and is grouped into: * **Residual material**: the disintegrated rock that remains in the original place is called residue. It is this residue that, after a sufficient time, along with other factors, allows for the development of soil. * **Transported materials**: the disintegrated rock that is transported (through the action of water, wind, or gravity) to another place is called sediment. Deltas and dunes are examples of formations with transported materials. # ETAPAS DE FORMACIÓN DEL SUELO Climate, organisms, and relief (considered as factors of soil formation) act on the materials by participating in the processes of weathering. These are the processes caused by those elements of the environment that act on materials or things that are exposed to the open air, not protected. Although there is no single and orderly way of soil formation, theoretically, three stages can be considered, according to the type of weathering. This does not mean that soil formation follows this order strictly. In reality, the processes are simultaneous and it is difficult to distinguish one from another; this distinction is made only for didactic purposes. 1. **Physical weathering**. Disintegration. 2. **Chemical weathering**. Decomposition. 3. **Biochemical weathering**. Invasion of vegetation. 1. **Physical weathering** that breaks and breaks down rock can be carried out by the action of temperature and gravity. For example, when water enters the cracks of rocks and freezes, its volume increases and a pressure of up to 146 kg/cm² is exerted, which favors the disintegration of rocks. As all bodies are elastic, when heated and cooled the minerals that make up rocks tend to expand and contract differentially, that is, some faster than others, which causes various pressures and favors the fracturing of rocks. 2. **Chemical weathering** corresponds to chemical weathering by means of different reactions that cause changes in the solubility or structure of materials. 3. Finally, the **invasion of vegetation** causes the appearance of a new material, organic matter, which causes biochemical changes in the soil. # ORGANISMOS The organisms that intervene in soil formation are animals, plants, fungi, and bacteria, mainly. Their contribution of organic matter, their disintegrating activity, and their mechanical action on the soil have a marked influence on the development of this substrate. The characteristics of the soil that are most clearly affected by organisms are: * Content and distribution of organic matter. * Soil compaction. * Soil acidity or alkalinity. # RELIEVE Relief or the shape of the Earth's surface influences soil formation through its relationship to water and temperature. Soils on gentle slopes generally have more water than passes through them, have higher organic matter content, more lush vegetation, and are deeper than soils on steep slopes. Similarly, in low-lying areas (depressions) soil conditions are different. For example, if dissolved salts from surrounding areas accumulate, saline areas can develop where only tolerant plants can grow, or they can become toxic areas for all plants. # TIEMPO <start_of_image>といい、 The time it takes for a soil to develop different layers called horizons depends, above all, on the interrelationships of the other factors: climate, nature of the parent material, organisms, and relief. Donahue (1977) points out that under ideal conditions, a soil can be fully formed in about 200 years. To identify horizons, there are rules of nomenclature that include uppercase letters and numbers. Uppercase letters designate two groups of horizons: organic with the letter O and mineral with By C. The difference between organic and mineral horizons is their content of organic matter. The letter R is also used but is to designate bedrock, which is not a horizon. The following figure shows that organic horizons have a higher content of organic matter than mineral horizons, between 20-30%, depending on the clay content. [Image of a figure showing the difference in organic content between organic and mineral horizons] The figure on the following page shows a diagram of an ideal horizon, where all possible horizons are indicated. This situation is difficult to find in reality, as horizons are generally missing. You should review the diagram carefully, but it is not necessary to memorize it, as the study of profiles is the subject of advanced courses on soil, for which much knowledge and experience are required. You can find O horizons in forest areas, jungles, natural grasslands, and other areas of abundant vegetation where the layer [Image of a figure showing a profile of soil horizons] The water of rainfall, upon percolating through the material that forms soil, causes the migration of clay and organic matter, which results in the differentiation of layers, better known as horizons. As a result of its formation, soils develop, naturally, layers of different thicknesses at different depths. For certain field studies, pits of specific shapes and sizes are dug, where these layers are analyzed. The vertical face or section of the soil that shows the arrangement of its layers is called the soil profile. The layers of soil that constitute the profile are called horizons. [Image of a soil profile with horizons labeled] As an exercise to obtain the designation of the textural class, the following figures of the sand, silt, and clay separates are given: | Sand | Silt | Clay | Soil Textural Class | | ---- | ---- | ---- | ------------------ | | 65% | 25% | 10% | Sandy loam | | 20% | 20% | 60% | Clay | | 20% | 70% | 10% | Silty loam | [Image of a triangular diagram used to classify soils by their texture] If there are mineral particles larger than 2 mm in significant quantities, the term "gravelly" or "stony" is added to the texture name, as appropriate. The texture of a soil can be measured accurately. This requires a laboratory. # DETERMINATION IN THE LABORATORY OF SOIL TEXTURE To measure texture in a laboratory, the organic matter of the soil samples is discarded and only the mineral material is considered. The mineral material of the soil is made up of particles of different sizes. There are large particles (visible to the naked eye) and very small particles (visible only under a microscope). According to their size (equal to or less than 2 mm in diameter) they are classified as sand, silt, and clay, as shown in the following table: | Particle Types | Diameter (mm) | | -------------- | --------------- | | Sand | 2.0 - 0.02 | | Silt | 0.02 - 0.002 | | Clay | Less than 0.002 | The proportions of each type of particle are quantified and the textural class is determined using the textural triangle. Therefore, once the measurement is complete, it is customary to say that texture is the percentage of sand, silt, and clay in a soil. The determination of a soil’s texture in a laboratory is done using the textural triangle. The textural triangle is a graphical scale used to designate the textural class to which the soil being studied belongs. The different proportions of the three types of particles yield different types of textures, resulting in 12 textural classes that appear in the aforementioned triangle. The productive capacity of the soil is largely determined by the nature of the subsoil. The practical importance of this fact is observed when we consider that the subsoil is usually subject to few alterations, except when a drainage system is established. [Image of a diagram showing the influence of subsoil on topsoil] Even when roots do not penetrate deeply into the subsoil, its permeability and chemical nature can positively or negatively affect the topsoil where roots develop # CHARACTERISTICS OF THE SOIL Once a soil is formed, it has a mixture that is difficult to define, whose characteristics depend on its composition and the way in which its components are bound together. In this mixture, three portions can be distinguished: 1. Solid portion (mineral and organic material). 2. Liquid portion (water and soil solution). 3. Gaseous portion (air). The solid portion, although very heterogeneous because it is made up of a mixture of different materials, has a specific gravity because it is made up of two layers. This property is used for the characterization of soils because the liquid and gaseous portions are extremely unstable, making their study difficult. The topsoil of the soil is not usually disturbed. In agricultural soils. the A horizon is most commonly found in the surface. The terms topsoil and subsoil are frequently heard. The former is considered synonymous with the arable layer, so its variability depends on the implement used. With an animal-drawn plow. the topsoil would be 15 cm deep and with a tractor 30 cm deep. From these limits, the subsoil begins, and it is common to consider it with a thickness equal to that of the topsoil, that is, 15 to 30 cm. [Image of a soil profile with horizons labeled] If soils contain enough stones to interfere with or prevent cultivation, they are called stony soils. Soil texture is not altered by tillage. # SOIL STRUCTURE Two soils with the same texture can have another physical characteristic that is different because their particles are arranged in a certain way. It is therefore important to study soil structure. Soil is made up of particles. Each particle is made up of mineral matter, organik matter, water, and air. Naturally, the finer particles (clay and humus) tend to bind together, resulting in larger soil units or soil aggregates. Soil structure is the way in which soil particles bind together to form aggregates. The different types of structure are shown in the following table. Note that structure is determined by the general shape of the aggregates or “peds”. | Structure Type | Aggregate Description | Aggregate Shape | Horizon Location | | -------------- | --------------------- | ----------------- | ----------------- | | Granular | Relatively porous, small, spherical, not attached to adjacent aggregates | Rounded | A | | Blocky | Relatively porous, small, angular, not attached to adjacent aggregates | Angular | A | | Subangular Blocky | Relatively porous, small, angular, not attached to adjacent aggregates | Subangular | B | | Prismatic | Relatively porous, columnar with non-rounded tops | Columnar | B | | Columnar | Relatively porous, columnar with rounded tops | Columnar | B | Soil structure affects water infiltration, drainage, aeration, root penetration, and consequently, soil productivity and tillage. Soil structure (a product of a natural process) can be altered by tillage, especially in the most superficial horizons. # SOIL DENSITY Density is a characteristic that indicates how closely or loosely packed together material particles are (of any substance) in a given volume. It is measured. Density is quantified as the ratio between the weight of a material and the volume it occupies (Density = weight/volume). The following table shows the approximate densities of some materials. In general, it can be stated that higher density means higher hardness of the material. | Material | Density (g/cm³) | | -------------- | ----------------- | | Water | 1.0 | | Quartz | 2.6 | | Steel | 7.7 | | Lead | 11.3 | Therefore, soil density is a characteristic that describes how closely or loosely packed together soil particles are in the volume they occupy. There is naturally space between soil particles. If particles are more closely packed together, more particles fit in the same volume, so the density of that volume will be higher. If particles are less closely packed together (more spread out). fewer particles will fit in the same volume, so the density will be lower. In the study of soils, two types of density are distinguished: 1. Apparent soil density. 2. Real soil density. Soil density (the ratio of weight to volume) of a soil can be quantified by considering it in its natural condition as it is found in the field, that is, with pore space. When this is done, the apparent density of the soil is obtained, which is the ratio between the weight of the soil particles and the apparent volume they occupy. Apparent soil volume = Volume occupied by solid soil materials + Pore space. Apparent soil density = Soil weight/Apparent soil volume. Apparent density should be calculated for each soil because changes in porosity are reflected in the apparent density values. [Image of a diagram showing a device for measuring apparent soil bulk density] In normal soils, exchangeable cations greatly exceed soluble cations. For example, for every H⁺ ion in the soil solution, there are 50,000 to 100,000 exchangeable H⁺ ions. Dissolved ions in soil solution can be easily removed by leaching because they move with the solution. Exchangeable cations are difficult to remove in this way. In both acidic and calcareous soils, Ca⁺⁺ is usually the predominant exchangeable cation. Acidic soils result from the accumulation of exchangeable H⁺ in the soil. # SOIL REACTION OR SOIL pH Soil reaction or soil pH is the criterion used to determine if a soil is acidic or alkaline. Soil pH can be measured. In acidic soils, pH values range from 3 to just under 7. In alkaline soils, pH values range from just over 7 to 11. When the value is exactly 7, it is said that the pH is neutral. The degree of acidity or alkalinity of a soil expressed in terms of pH is what is commonly called “soil reaction” and the different degrees are shown in the figure below. [Image of a diagram showing the pH scale and the approximate pH values of different soils] This exchange is rapid and reversible; exchangeable cations are in equilibrium with cations in solution. If the cations in the soil solution are absorbed by plants, the above reaction shifts to the right to replenish the supply. In this way, exchangeable cations are an important source of nutrients for plants. # CHARACTERISTICS OF THE SOIL A person with experience can relate soil color to physical, chemical, and biological properties of soils and propose some relationships. Color, for example, is related to the following: * Gray and blue colors are associated with poorly drained soils. * Red color may indicate the presence of free iron oxide. * Black color usually indicates the presence of organic matter. # CHEMICAL CHARACTERISTICS OF THE SOIL The chemical characteristics of any material are those derived from its fundamental structure and its transformations at the molecular level. They can be evaluated using technical procedures and even measured. In the case of soil, the chemical characteristics (which are studied in this course) are as follows: 1. Cation exchange. 2. Soil reaction or soil pH. Both qualities are important because knowing them allows for assessments related to plant nutrition and soil fertility. # CATION EXCHANGE The materials that directly nourish plants are not the soil particles or the organic matter of the soil, as many believe. In reality, plants are nourished by inorganic chemical units as simple as ions and molecules that come from the air and from decomposed soil materials. An ion is an atom or group of atoms with one or more units of electrical charge. If the charge is positive, the ion is called a cation; if it is negative, it is called an anion. In this sense, one of the most common and important processes for plant life is cation exchange, which takes place in the soil. [Image of a diagram showing cation exchange] # LOS ORGANISMOS Fungi are heterotrophic organisms of various sizes. They can be microscopic or macroscopic. They obtain their nourishment from complex organic substances. Some species of fungi develop in association with the roots of higher plants, forming special structures called mycorrhizae that help in obtaining nutrients from the soil. Soil fungi are outnumbered by bacteria, except in acidic soils, soils that have been heavily fertilized, and soils rich in organic matter. Some fungal species are able to break down cellulose, and are considered very active in soil structure formation. It has been calculated that in a fertile soil layer one hectare in area and 20 cm deep, there may be up to 1,500 kg of living weight due to the fact that fungi are larger than bacteria. [Image of a diagram showing different types of microorganisms commonly found in soil] # CHARACTERISTICS BIOLÓGICOS DEL SUELO In the strictest sense, only living organisms can have biological characteristics. Soil is not a living organism. However, considering that soil naturally contains a large amount of organisms and organic matter, and that it is capable of supporting plant life, it is said to have biological characteristics. In the case of soil, the characteristics considered to be biological (and which are studied in this course) are the following: 1. Soil organisms. 2. Soil organic matter. # SOIL ORGANISMS Soil is the medium where a wide variety of organisms of various sizes develop. These range from those that can only be seen under a microscope, such as fungi and bacteria, to those that can be seen with the naked eye, such as earthworms. These organisms are important because they contribute to improving soil productivity through various activities. Almost all the mineral elements related to plant nutrition are subject, in one way or another, to the action of soil organisms. In this sense, one of the main activities of soil microorganisms is the mineralization of organic matter. This consists of the decomposition of organic materials into simpler compounds, as a result of the nutrition and other activities of various organisms, including bacteria, fungi, and actinomycetes. # GROUPS OF ORGANISMS THAT LIVE IN THE SOIL The idea that bacteria predominate among soil organisms has been refined with years of research, revealing that fungi, actinomycetes, protozoa, algae, and many small animals also play an important role. The following is a look at some of these. # BACTERIA Bacteria are unicellular organisms. The largest individuals rarely exceed 0.005 mm in diameter. In cultivated soils, they are the most abundant organisms because they exceed all others in number and species. They multiply very quickly, as a new individual can be formed in less than 20 minutes. In soil, like other microorganisms, bacteria exist in millions. It is estimated that in a fertile soil layer one hectare in area and 20 cm deep can have more than 500 kg of living weight. The abundance of the different types of bacteria depends on the supply of nutrients, moisture levels, temperature, and other environmental conditions present in the soil. [Image of a diagram showing the abundance of bacteria in soils of different depths] # THE ORGANIC MATTER OF THE SOIL Soil organic matter is the decomposing material that occurs in soil and comes from the remains and waste products of plants, animals, and other living or dead organisms. Soil organic matter is directly or indirectly affected by the digestive process or indirectly by influencing the activities of bacteria, fungi, and other soil microorganisms. [Image of a diagram showing various soil organisms] Soil organic matter is considered to be a soil fertility factor, which is why its effect on crop yields is usually emphasized. # CHARACTERISTICS BIOLOGICALS OF THE SOIL In the strictest sense, only living organisms can have biological characteristics. Soil is not a living organism. However, considering that soil naturally contains a large amount of organisms and organic matter, and that it is capable of supporting plant life, it is said to have biological characteristics. In the case of soil, the characteristics considered to be biological (and which are studied in this course) are the following: 1. Soil organisms. 2. Soil organic matter. # SOIL ORGANISMS Soil is the medium where a wide variety of organisms of various sizes develop. These range from those that can only be seen under a microscope, such as fungi and bacteria, to those that can be seen with the naked eye, such as earthworms. These organisms are important because they contribute to improving soil productivity through various activities. Almost all the mineral elements related to plant nutrition are subject, in one way or another, to the action of soil organisms. In this sense, one of the main activities of soil microorganisms is the mineralization of organic matter. This consists of the decomposition of organic materials into simpler compounds, as a result of the nutrition and other activities of various organisms, including bacteria, fungi, and actinomycetes. # GROUPS OF ORGANISMS THAT LIVE IN THE SOIL The idea that bacteria predominate among soil organisms has been refined with years of research, revealing that fungi, actinomycetes, protozoa, algae, and many small animals also play an important role. The following is a look at some of these. # ACTINOMYCETES Actinomycetes strongly resemble bacteria because they are also unicellular organisms and abundant. Many of them produce spores and are similar to fungi. In terms of actual living weight per hectare. they may exceed bacteria, but not fungi. Under optimal conditions, their weight can be 700 kg/ha, or more, considering a depth of 20 cm. # EARTHWORMS Perhaps the most important group of macroorganisms that inhabit soil are earthworms. There are many species of earthworms. These animals prefer a humid environment with an abundance of organic matter and readily available calcium. It is estimated that, in some soils, these organisms can move several tons of soil annually through their bodies, thereby improving the availability of nutrients for plants. The number of earthworms in the arable layer per hectare can range from a few hundred to over two million. It is estimated that there are between 200 and 1,000 kg of earthworms per hectare. # OTHER LARGER ANIMALS Various species of rodents, ants, snails, arachnids, mites, myriapods, worms, etc., also inhabit soil and contribute to decomposing organic matter and improving soil fertility. Some of these organisms spend their entire lives in soil, while others spend only part of their lives there. As a result of their vital processes, they cause chemical changes in soil, either directly (through their digestive process) or indirectly by influencing the activities of bacteria, fungi, and other soil microorganisms. The soil is the medium where a wide variety of organisms of various sizes develop. These range from those that can only be seen under a microscope, such as fungi and bacteria, to those that can be seen with the naked eye, such as earthworms. # SOIL ORGANIC MATTER Soil organic matter is the decomposing material that occurs in soil and comes from the remains and waste products of plants, animals, and other living or dead organisms. Soil organic matter is directly or indirectly affected by the digestive process or indirectly by influencing the activities of bacteria, fungi, and other soil microorganisms. [Image of a diagram showing various soil organisms] Soil organic matter is considered to be a soil fertility factor, which is why its effect on crop yields is usually emphasized. It is a complex mixture of compounds that includes: 1. **Fresh organic matter**: This includes plant residues, animal remains, and other materials that have not yet been completely decomposed. 2. **Humus**: This is the most stable form of soil organic matter. It is a dark, amorphous substance formed by the partial decomposition of fresh organic matter. Humus is made up of a complex mixture of organic molecules, including polysaccharides, proteins, and lipids. The amount of organic matter in the soil is variable, ranging from very small amounts in mineral soils to up to 20% in some organic soils. Humus is generally concentrated in the topsoil, as it is the location where most of the organic matter is added through plant residues, animal waste, and other sources. Factors that influence the content of soil organic matter: * **Climate**: Warm, humid climates, which are conducive to microbial activity, generally have higher levels of soil organic matter than cold, dry climates. * **Vegetation**: Soils under dense vegetation or those that support a diverse mix of plant species typically have higher levels of soil organic matter than those under sparse vegetation or with a limited diversity of plant species. * **Tillage**: Tillage can lead to the loss of soil organic matter through oxidation and decomposition. No-till farming practices can help maintain or increase soil organic matter levels. * **Soil type**: Soils with a high clay content tend to hold more organic matter than those with a high sand content. Benefits of soil organic matter: * **Improves soil structure**: Organic matter acts as a binder, helping to create aggregates that improve soil aeration, drainage, and water infiltration. * **Increases water holding capacity**: Organic matter acts like a sponge and helps to retain moisture in the soil. * **Provides nutrients**: Organic matter is a source of essential nutrients for plants, such as nitrogen, phosphorus, and sulfur. * **Reduces soil acidity**: Organic matter can neutralize soil acidity. * **Stimulates microbial activity**: Organic matter provides food and habitat for beneficial soil microorganisms, which are essential for nutrient cycling and other soil processes. * **Reduces soil erosion**: Organic matter cover on the soil surface protects the soil from wind and water erosion. * **Suppresses soil-borne diseases**: Organic matter can suppress the growth of plant diseases, such as root rot and damping off. * **Enhances soil fertility**: Organic matter helps to create a more fertile soil system, ultimately contributing to higher crop yields. # SOIL COLOR Soil color is the most visible characteristic and the one that is easiest to observe. It can be inherited from the parent mineral material from which the soil originated or it can be the result of significant changes in the profile. Soil color is related to climate and the presence of organic matter. In the same climatic region, soils derived from different parent materials can have the same color and, conversely, soils originating from the same parent mineral material can differ in color if they have developed in different climates. The color of soil horizons can be uniform or non-uniform (mottled, stained, streaked). * Mottle is usually due to poor drainage. * Stains are usually due to the accumulation of lime, organic matter, and the oxidation state of iron. * Streaks are due to the infiltration of organic colloids and iron oxides from the upper layers. Soil colors are determined using the Munsell Soil Color Chart. # DETERMINING SOIL pH To accurately measure soil pH, a meter is needed. A less accurate, but practical, method is the colorimetric method, which involves immersing strips of special paper in a soil/water suspension and determining the pH level based on the color that the wet strips acquire. Below is a table of optimal pH ranges for different crops. While it is not necessary to memorize this table, it is helpful to analyze it and ask yourself questions, such as: * What is the optimal pH range for the majority of crops? * What climate are the crops with a more acidic pH range associated with? * For which crops does the optimal pH range begin after 7.0? | Crop | Optimal pH Range | | -------------- | ------------------ | | Alfalfa | 6.5 - 8.0 | | Pineapple | 5.0 - 6.0 | | Rice | 5.0 - 6.5 | | Oats | 5.0 - 7.0 | | Banana | 6.0 - 7.5 | | Cacao | 5.0 - 7.0 | | Coffee | 4.5 - 7.0 | | Sugarcane | 6.0 - 8.0 | | Coconut | 6.0 - 7.5 | | Peach | 6.5 - 8.0 | | Sunflower | 5.5 - 7.5 | | Corn | 6.0- 7.0 | | Apple | 6.0 - 8.0 | | Potato | 5.0 - 7.0 | | Pear | 6.0 - 8.0 | | Sorghum | 5.0 - 6.5 | | Tobacco | 5.5 - 7.5 | | Tomato | 5.5 - 7.0 | | Wheat | 6.0 - 8.0 | # SOIL DENSITY Soil density (the ratio of weight to volume) of a soil can be quantified by considering it in its natural condition as it is found in the field, that is, with pore space. When this is done, the apparent density of the soil is obtained, which is the ratio between the weight of the soil particles and the apparent volume they occupy. Apparent soil volume = Volume occupied by solid soil materials + Pore space. Apparent soil density = Soil weight/Apparent soil volume. Apparent density should be calculated for each soil because changes in porosity are reflected in the apparent density values. [Image of a diagram showing a device for measuring apparent soil bulk density] # REAL SOIL DENSITY Real soil density is a value derived from a theoretical situation where a soil is considered with 0% pore space. It is determined by the following equation: Real soil density = Soil weight/Real soil volume In practice, real soil density is complex to measure. For classification purposes, a value of 2.65 g/cm³ is often adopted as the average real density of all soils. This value represents an approximation of the average density of the dominant minerals: quartz, feldspars, micas, and clay minerals. # USES OF APPARENT SOIL DENSITY The apparent soil density data can be used in several cases: * Identifying compacted soil layers. These layers hinder root growth and cause problems for agricultural crops. Typically, compacted layers have a higher apparent density of greater than 2.0 g/cm³. * Calculating the weight of a soil layer. * Calculating soil porosity. # SOIL CONSISTENCE As soil particles bind together, soil develops a certain degree of firmness. This characteristic is called soil consistence. Soil consistence is the degree of cohesion or the resistance of the soil mass to deformation or rupture. Soil consistence is influenced by factors such as: * **Moisture content**: The amount of water in the soil plays a significant role in determining consistence. As the moisture content increases, soil tends to become more cohesive. * **Organic matter content**: Organic matter acts as a binder, helping to create aggregates that increase cohesion and contribute to soil consistence. * **Clay content**: Soils with a high clay content are generally more cohesive than those with a high sand content. * **Mineral composition**: Some minerals have a higher tendency to bind together than others. ## FIELD DETERMINATION OF SOIL TEXTURE Soil texture can be determined directly in the field by a trained person who uses their experience and the texture results are considered estimations. The procedure consists of taking a small amount of soil between one’s fingers, moistening it, and rubbing or molding it. The feel is then used to classify soil texture. The traditional criteria used are: * **Sandy texture**: Feels gritty, like sandpaper. * **Silty texture**: Feels smooth and soapy. * **Clayey texture**: Feels sticky and plastic. It can be rolled into a thin ribbon that will hold its shape. Another method to distinguish clay soils from loam soils is to roll the soil between one’s fingers; clay soils form smooth balls that can then be rolled into a thin ribbon. Loam soils can also form balls that are rolled into thin ribbons, but the ribbons are much smaller and eventually break. Generally, soils are classified as fine-textured or coarse-textured based on the dominant particle size. Fine-textured soils are dominated by clay, while coarse-textured soils are dominated by sand. # INFLUENCE OF SOIL TEXTURE ON SOIL PROPERTIES Soil texture is a characteristic that affects the physical and chemical properties of soil. Fine-textured soils have a larger total surface area than coarse-textured soils; they tend to have a higher capacity to retain nutrients, making them more fertile. Clay soils have a greater capacity to retain water, due to their larger surface area and greater total pore space than sandy soils. They also tend to hold more water but their rate of infiltration is slower because of their smaller pores. Sandy soils retain less water because they have larger pores, which allow water to infiltrate quickly. <start_of_image> Soil texture also affects soil aeration and drainage; fine-textured soils have a lower aeration and drainage rate than coarse-textured soils. # SUMMARY This document provided an overview of the components, factors of formation, and properties of soils. Specifically, it covered topics such as: * **Soil Components**: Mineral matter, organic matter, water, and air. * **Factors of Soil Formation**: Parent material, climate, organisms, relief, and time. * **Soil Properties**: Texture, structure, density, consistence, and pH. * **Soil Organisms**: Bacteria, fungi, actinomycetes, and other macroorganisms. * **Soil Organic Matter**: Its importance, composition, and role in soil fertility. Understanding soil properties is crucial for developing sustainable agricultural practices and improving crop yields.