Text 2 composition of soils
TEXT 1 SOIL
Soil is an important natural resource that covers much of the earth’s land surface. Most life on earth depends upon the soil as a direct or indirect source of food. Plants are rooted in the soil and obtain nutrients (nourishing substances) from it. Animals get nutrients from plants or from animals that eat plants. Certain microbes in the soil cause dead organisms to decay, which helps return nutrients to the soil. In addition, many kinds of animals find shelter in the soil.
Soil contains mineral and organic particles, other plant and animal matter, and air and water. The contents of soil change constantly. There are many kinds of soils, and each has certain characteristics, including color and composition. The kind of soil in an area helps determine how well crops grow there. Soil forms slowly and is destroyed easily, and it must be conserved so it can continue to support life.
Soil scientists, called pedologists, use the term polypedons for the bodies of individual kinds of soil in a geographic area. Polypedons can be indefinitely large, but some have a surface area of only about 10.8 square feet (1 square meter).Some polypedons measure less than 5 inches (13centimeters) deep. Others are more than 4 feet (1.2meters) deep.
TEXT 2 COMPOSITION OF SOILS
The mineral and organic particles in soil are called soil particles. Water and air occupy the spaces between the particles. Plants and animals live in these pore spaces. Plant roots also grow through the pore spaces.
Minerals supply nutrients to green plants. Particles called sands, silts, and clays make up most of the mineral content of soil.
Sands and silts are particles of such mineral as quartz and feldspars. Clays consist of illite, kaolin, micas, vermiculite, and other minerals. Trace amounts of many minerals add nutrients, including calcium, phosphorus, and potassium, to the soil. Most soils are called mineral soils because more than 80 per cent of their soil particles are minerals.
Plants and animal matter consists of organic material in various stages of decay. Many organisms also live in the soil. These soil organisms include plant roots, microbes, and such animals as worms, insects, and small mammals. Bacteria, fungi, and other microbes decompose (break down) dead plants and animals. Many soil organisms help mineral and organic particles aggregate (come together) and form clumps of soil. Roots, burrowing animals, and natural weathering break apart large clumps of soil.
Decaying organic material release nutrients into the soil. In addition, some organic material combines with mineral particles. Other decaying material forms organic soil particles called humus. Most humus is black or dark brown, and it holds large amounts of water. Only 6 to 12 per cent of the volume of particles in most mineral soils is organic. However, these small quantities greatly increase a soil’s ability to support plant life. In some soils, called organic soils, more than 20 per cent of the soil particles are organic.
Water that enters the soil dissolves mineral and nutrients and forms a soil solution. Much of the solution drains away, but some remains in the pore spaces. Green plants obtain water and some nutrients by absorbing soil solution through their roots.
Air replaces the water that drains from the larger pore spaces. Soil organisms live best in soils that contain almost equal amounts of air and water.
TEXT 3 HOW SOIL IS FORMED
Soil begins to form when environmental forces break rocks and similar materials that lie on or near the earth’s surface. Pedologists call the resulting matter parent material. As soil develops through the centuries, organic material collects, and the soil resembles the parent material less and less. Glaciers, rivers, wind, and other environmental forces may move parent material and soil from one area to another.
Soils are constantly being formed and destroyed. Some processes, such as wind and water erosion, may quickly destroy soils that took thousands of years to form.
Soil formation differs according to the effects of various environmental factors. These factors include kinds of parent material, climate, land surface features, plants and animals, and time.
Kinds of parent material. The type of parent material helps determine the kinds of mineral particles in a soil. A process called weathering breaks down parent material into mineral particles. There are two kinds of weathering, physical disintegration and chemical decomposition. Physical disintegration is caused by ice, rain and other forces. They wear down rocks into smaller particles that have the same composition as the parent material. Sand and silt result from physical disintegration.
Chemical decomposition mainly affects rocks that are easily weathered. In this kind of weathering, the rock’s chemical structure breaks down, as when water dissolves certain minerals in a rock. Chemical decomposition results in elements and in chemical compounds and elements that differ from the parent material. Some of these substances dissolve in the soil solution and become available as plant nutrients. Others recombine and form clay particles or other new minerals.
The mineral content of parent material also affects the kinds of plants that grow in a soil. For example, some plants, including azaleas and rhododendrons, grow best in acid soils that contain large amounts of iron.