Components and properties of soil
- 1. COMPONENTS AND PROPERTIES OF SOIL Prepared By: Ar. Neelesh Kant Saxena
- 2. ORIGIN OF SOIL  Agronomists customarily refer to the origin of parent material* of soils when referring to the components of soil. Thus, it is desirable to have an understanding of the origin of parent material when considering the origin of soils.  Soil parent material may be referred to as a geological formation (a layer of material varying from a few feet to many feet in thickness). Wherever a geological formation is exposed at the earth’s surface, either soil formation is taking place or soil has already developed from the exposed material.  Productive soils develop from parent materials that supply the essential elements needed to support plant life. Parent material from which these productive soils are formed may be classified as residual, transported,or cumulose.
- 4. Residual Materials  Residual materials develop from underlying bedrock materials. Most rock formations at the earth’s surface are sedimentary. Examples of sedimentary formations are limestone, sandstone, shale, and slate. Other rock formations are classified as either igneous or metamorphic. Metamorphic rocks include marble and slate, while granite and basalt are considered igneous rocks. Transported Materials  Parent material that was transported from its place of origin by water is classified as alluvial, lacustrine, or marine materials. Alluvial materials are sediments that were transported by flowing water of streams and rivers. Lacustrine materials are sediments that were deposited into freshwater lakes due to erosion of surrounding uplands during the glacial period. The greatest lacustrine deposits in the United States border the Great Lakes. Marine materials are sediments that were washed out to sea by rivers and then later lifted above sea level. The principal marine sediments in the United States occur along the Gulf Coast and theAtlantic Coast.
- 5. Cumulose Materials (Organic) - Peat and Muck ď‚´ Cumulose materials are plant residues that were developed in shallow lakes and are referred to as peat and muck. Peat is the term used for cumulose materials when the plants are recognizable. Muck describes the cumulose materials when plants are decayed beyond recognition. Weathering ď‚´ Parent material is constantly being transformed into soil as rocks and minerals are weathered by both physical and chemical processes. Physical weathering processes include constant temperature changes, climatic conditions, rainfall, erosion, and action of animals, plants, and humans. Chemical processes include such factors as oxidation, reduction, solution, hydrolysis, hydration, and carbonation.
- 6. SOIL COMPOSITION  It should be emphasized at this point that soils develop from parent material by processes of soil formation that differ from processes of rock weathering, which produce the parent material. As a soil develops from parent material, certain changes occur that produce the various layers of soil that are characteristic of the soil’s profile.  Soil consists of living and nonliving materials in three forms: solid, liquid, and gas. Organic matter and mineral matter in soil are the solids. Air and water, which move through soil pores, are the gas and liquid. Approximately 45% of a soil’s volume is mineral matter, approximately 5% is organic matter, and approximately 50% is water and air.
- 8. SOIL COMPOSITION  It should be emphasized at this point that soils develop from parent material by processes of soil formation that differ from processes of rock weathering, which produce the parent material. As a soil develops from parent material, certain changes occur that produce the various layers of soil that are characteristic of the soil’s profile.  Soil consists of living and nonliving materials in three forms: solid, liquid, and gas. Organic matter and mineral matter in soil are the solids. Air and water, which move through soil pores, are the gas and liquid. Approximately 45% of a soil’s volume is mineral matter, approximately 5% is organic matter, and approximately 50% is water and air.
- 9.  Increases the soil’s porosity  Supplies nitrogen and other nutrients to growing plants  Holds water in the soil to protect growing plants against droughts  Aids in soil moisture content management  Furnishes food for soil organisms  Serves as a store house for mineral nutrients  Minimizes leaching of nutrients  Serves as a source of nitrogen and growth promoting substances  Improves the soil’s structure The physical features, called tilth, of the soil are affected by the amount of organic matter in the soil. A high organic matter content provides the following benefits:
- 10. PROPERTIES OF SOIL ď‚´ Although soils in general contain the same components, they differ in their properties or characteristics. These differences affect or determine the management of soils. Soils differ in color, texture, structure, consistence, and fertility and productivity. Many years of proper soil management are required to change soil properties or characteristics. However, agricultural producers can make considerable improvement in the structure, consistence, and fertility and productivity of their soils. Soil Color ď‚´ Color is one of the most easily recognized soil characteristics and is very apparent in the horizons of a soil profile. Topsoil colors are classified as dark, moderately dark, light, or very light. Subsoil colors are classified as red, yellow, brown, gray, or mottled. Color usually is a good indicator of organic matter content, moisture availability, and texture of a soil. The dark color of topsoil usually indicates presence of organic matter. Light-colored soils generally are low in organic matter content, high in sand particles, or contain reduced iron or manganese oxide.
- 11. Soil Texture ď‚´ Soil texture refers to the sizes of individual soil particles. It is often thought of as the fineness or coarseness of soil particles determined by the percentages of sand, silt, and clay in the surface layer of soil. Soil particle size is important because it affects the water-holding capacity and workability of soil. Sand, the larger particles in a soil, can be seen with the naked eye. Soils with a high percentage of sand are infertile because they do not provide nutrients to growing plants. Sandy soils dry out quickly and are droughty. ď‚´ Silt particles are between sand and clay particles in size. Until they weather to fine-sized particles, they provide few nutrients for growing plants. After rainfall, topsoil with a high percentage of silt tends to run together and forms a surface crust. Silt type soils erode very easily.
- 12. ď‚´ For convenience in determining the textural name of a soil from the mechanical analysis, an equilateral triangle has been adopted as shown in figure. The textural triangle gives percentages of sand, silt, and clay in each of the textural classes.
- 13. Soil Structure ď‚´ Soil structure, the arrangement of soil particles into various sizes and shapes, is important because it affects soil tilth. Agricultural producers can control soil structure through methods of cultivation, times of cultivation, and applications of organic matter and lime. ď‚´ Soils with good structure contain a large number of crumbs or aggregates, which indicates good tilth. Good soil structure permits deep root penetration and a large particle area from which plants can secure nutrients. ď‚´ Two different soil structures can be compared by securing a soil sample from an area with a dense, heavy growth of grass and securing a sample from an area in continuous cultivation. The sample from the grassy area will be grainy and contain a large amount of aggregates. The sample from the cultivated field will be firm, hard, and lack aggregates, or crumbs. Soils scientists classify structural aggregates of soils into three main categories based on their shapes, sizes, and strengths.
- 14. Grades of Soil Structure  Soil structure grade is determined by strength of aggregates. Soil aggregate strength is determined by measuring a soil’s resistance to crushing. Soil has no structure if coarse soil particles fail to cling together (single grain structure) or if fine soil particles break into large cement-like clods (massive structure). Other descriptive grades of soil structure are weak, moderate, and strong.  In order to describe soil structure, scientists named each structural shape that soil particles could arrange themselves. Six common types of soil structure are single grain, blocky, platy, massive, prismatic, and granular.
- 15. Soil Consistence  Consistence refers to cohesiveness of soil (or its resistance to rupture). Soil consistence and structure are interrelated. Structure deals with shape, size, and distinctness of natural aggregates while consistence refers to the force required to rupture soil material.  Consistence is described under wet, moist, or dry soil moisture conditions. When the soil is wet, consistence is described by degrees of stickiness and plasticity. A soil’s friability and firmness, its tendency to break into smaller masses, characterized consistence under moist conditions, and its ability to cohere again when pressed together. Consistence of soil under dry conditions is characterized by degrees of hardness, maximum resistance to pressure, and inability of crushed material to cohere again when pressed together.
- 16. Soil Fertility And Productivity  Soil fertility is the ability of soil to provide nutrients for plant growth. Regardless of soil fertility levels, plant food must be returned to soil if it is to remain fertile. As soil nutrient levels decline, plant growth decreases.  Soil productivity is the present capability of soil to produce a specified plant or sequence of plants under a defined set of management practices. Productivity is measured in terms of outputs or harvests in relation to production inputs (fertilizer, lime, etc.) for a specific kind of soil.  The capability of a soil under a particular management system is dependent upon factors such as climatic conditions and the soil’s natural fertility, organic matter content, texture, and structure.