ASTOUNDING as it may seem, a shovelful of soil

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Bonnie Hines
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1 The Nature of Soil ASTOUNDING as it may seem, a shovelful of soil holds more living things than all the humans that have ever been born. Living things are just one component of soil, a material undervalued by many people. The composition of soil makes it extremely important to our existence. Objective: Identify the components and functions of soil. Key Terms: amend clay humus loam mineral matter organic matter sand silt soil symbiosis tilth Soil Soil is the outer portion of the earth s crust that supports plant growth. SOIL COMPONENTS Mineral soils consist of mineral matter, organic matter, living organisms, air, and water. Soils may vary from one area to another, but most will contain these basic components. Page 1

Mineral Matter Mineral matter is the inorganic, or non-living, substances that come from weathered rock to form soil.

Climate is also a factor in soil formation. Soils are often distinguished by the amount of mineral matter found in them. The three types of mineral matter found in soil are sand, silt, and clay.

Water flows through the large pore spaces quickly. Sand particles are, therefore, considered to be well-drained. Sand soils lack the ability to hold nutrients and are not fertile.

2 Mineral Matter Mineral matter is the inorganic, or non-living, substances that come from weathered rock to form soil. The mineral matter in soil varies from one location to another based on the type of rock or parent material. As parent material is weathered or worn away, small bits accumulate to form soils. Climate is also a factor in soil formation. Soils are often distinguished by the amount of mineral matter found in them. The three types of mineral matter found in soil are sand, silt, and clay. Sand is the largest size particle. You can physically separate and see individual grains of sand. Sand particles create large pore spaces that improve aeration. Water flows through the large pore spaces quickly. Sand particles are, therefore, considered to be well-drained. Sand soils lack the ability to hold nutrients and are not fertile. Also, soils that contain sand particles feel gritty to the touch. Silt is the mid-size soil particle produced by the action of water breaking down minerals. It has good water-holding and nutrient-holding abilities. Silt feels like flour when dry and smooth like velvet when moist. Clay is the smallest size soil particle. Clay has the ability to hold both nutrients and water that can be used by plants. It creates very small pore spaces, resulting in poor aeration and poor water drainage. Clay forms hard clumps when dry and is sticky when wet. FIGURE 1. Soil can consist of several different components, such as air, water, mineral matter, and organic material. FIGURE 2. Sand, silt, and clay differ greatly in size. Sand, silt, and clay, the three minerals found in soil, make up about 45 percent of soil composition. When there are equal amounts of sand, silt, and clay present in soil, it is called a loam. Page 2

Organic Matter An average soil is composed of 5 percent organic matter. Organic matter is the accumulation of decayed or partially decayed plants and animals, roots, and living organisms.

This is because the organic matter contributes to soil fertility. Additional advantages to organic mat- FIGURE 3. This soil contains a high amount of organic matter.

3 Organic Matter An average soil is composed of 5 percent organic matter. Organic matter is the accumulation of decayed or partially decayed plants and animals, roots, and living organisms. Soils with high organic matter content tend to be dark in color. Organic soils are more productive than soils with low organic matter content. This is because the organic matter contributes to soil fertility. Additional advantages to organic mat- FIGURE 3. This soil contains a high amount of organic matter. ter are improved aeration and water-holding capacity. Most organic matter in soil is in the form of humus. Humus is organic matter that has reached a point of stability, where it will break down no further. It is made up of plant leaves, roots, and stems. A common practice is to amend soils with organic matter. Amend means to change for the better. Peat moss is the most common soil amendment used to improve soil. Living Organisms Abundant life can be found in soil in the form of plant roots, earthworms, insects, bacteria, fungi, and other organisms. Roots of living plants contribute to the soil mass. Earthworms, ants, crawfish, moles, and other organisms improve the soil tilth (the ease at which soil can be worked). These organisms create openings in the soil as they tunnel. This enhances drainage and improves air exchange. Bacteria and fungi have an important role in the soil. They break down organic matter and release nutrients. Mycorrhizae fungi have symbiotic relationships with plants. Symbiosis means the living together of unlike organisms. The mycorrhizae fungi get glucose and sucrose from the plant roots, and, in return, the plant is assisted in the absorption of water and minerals. FIGURE 4. Earthworms are some of the many living organisms that improve soil tilth. Page 3

4 Air and Water The 50 percent of soil not made of mineral matter or organic matter is composed of air and water. The air and water occupy pore spaces between the soil particles. A typical soil might have 25 percent air and 25 percent water. However, these percentages fluctuate as soil dries out or is soaked by rain. What s Your Profile? Interested in the information presented in this E-unit? You might make a great Soil Scientist! Check out: Looking for something a little different? Explore your options! Visit: SOIL FUNCTIONS There are four basic functions of soil related to plant growth. Soil serves as a reservoir of water to be absorbed by plants roots. Soil provides 13 of the 16 essential nutrients for plant growth and development. Soil permits the exchange of gases, especially oxygen and carbon dioxide, to and from the plant roots. Soil also provides support for the plant by giving roots firm anchorage. AGRICULTURAL USES OF SOIL Agriculture depends on soil to grow food, fiber, and ornamental plants. Various uses include cropland, grazing land, forest land, water structures, and natural resources. Cropland is land on which soil is worked and crops are planted, cared for, and harvested. Most cropland is devoted to annual crops, such as corn, soybeans, cotton, vegetables, and so on. EXPLORING OUR WORLD SCIENCE CONNECTION: Soil Microorganisms In a handful of soil, there is believed to be billions, to hundreds of billions, of soil microorganisms. Of those organisms, there are thousands of different species of bacteria and hundreds of different species of fungi and protozoa. There is also likely to be dozens of nematode species and an assortment of mites. Nearly all of these soil organisms are beneficial. Soil microorganisms play an active role in soil fertility as a result of their involvement in the cycle of nutrients like carbon and nitrogen, which are required for plant growth. Soil microorganisms are responsible for the decomposition of the organic matter entering the soil. They recycle nutrients in soil. Certain soil microorganisms, such as mycorrhizal fungi, increase the availability of mineral nutrients to plants. Other soil microorganisms, such as nitrogen-fixing bacteria, transform nitrogen gas into soluble nitrogen compounds that plants use for growth. Page 4

Grazing land is used for grazing cattle and sheep. It is often planted with perennial forage. Forest land is used for growing trees that are later harvested for building materials, paper, and so on.

5 Grazing land is used for grazing cattle and sheep. It is often planted with perennial forage. Forest land is used for growing trees that are later harvested for building materials, paper, and so on. Water structures include ponds and other reservoirs constructed with soil. Natural resources include plant and animal wildlife that depend on soil. NONAGRICULTURAL USES OF SOIL People require soil for many other uses besides growing plants. Such uses include recreation, foundations of buildings, waste disposal, and building materials. Recreation is activity that requires playgrounds, sports fields, jogging paths, golf courses, parks, campgrounds, and other facilities. Foundations of buildings depend on a solid soil base to remain structurally sound. Waste disposal involves the use of soil for the treatment of human sanitary wastes. Soil filters some of the material, while microorganisms break down organic portions into less dangerous compounds. Building materials for homes and other structures sometimes involve soil. Occasionally, structures are built underground, into hillsides, or even with soil piled over them. Earth-sheltered buildings have lower heating and cooling costs. FIGURE 5. Soil is required for the construction of a playground. Summary: Soil is the outer portion of the earth s crust that supports plant growth. Mineral soils consist of mineral matter, organic matter, living organisms, air, and water. Mineral matter is composed of non-living substances that come from weathered rock. Three types of mineral matter found in soil are sand, silt, and clay. Organic matter is accumulated decayed or partially decayed plants and animals, roots, and living organisms. Abundant life can be found in soil in the form of plant roots, earthworms, insects, bacteria, fungi, and other organisms. Air and water occupy pore spaces between the soil particles. Four basic functions of soil related to plant growth are to serve as a reservoir of water, to provide nutrients, to permit the exchange of gases, and to provide support for the plant. Page 5

6 Soil is essential for cropland, grazing land, forest land, water structures, and natural resources. People use soil for recreation, building foundations, waste disposal, and building materials. Checking Your Knowledge: 1. What makes up a mineral soil? 2. What are the types of mineral matter found in a soil? 3. What are the four basic functions of soil? 4. What are some agricultural uses for soil? 5. What are some nonagricultural uses for soil? Expanding Your Knowledge: Collect soil samples from a forest and from a field. Examine the textures and components of the soil. Inspect them with a magnifying lens or microscope. Pay special attention to living organisms. Make notes as to how you would describe the samples. How do they compare? Web Links: Soil What on Earth is Soil? What is Soil? The Dirt on Soil Agricultural Career Profiles Page 6

Recommended Resources: The following resources may be useful in teaching this lesson:

Recommended Resources: The following resources may be useful in teaching this lesson: Unit A: Principles of Soil Lesson 1: Importance of Soil to Afghanistan Student Learning Objectives: Instruction in this lesson should result in students achieving the following objectives: 1. Explain how