Mineral matter and organic matter together make up about 50 percent of soil. What two substances make up the other 50 percent?

Explanation:

Key Points

The chemical composition of the soil, the topography, and the presence of living organisms determines the quality of soil.

In general, soil contains 40-45% inorganic matter, 5% organic matter, 25% water, and 25% air.

In order to sustain plant life, the proper mix of air, water, minerals, and organic material is required.

Humus, the organic material in soil, is composed of microorganisms (dead and alive) and decaying plants.

The inorganic material of soil is composed of rock, which is broken down into small particles of sand (0.1 to 2 mm), silt (0.002 to 0.1 mm), and clay (less than 0.002 mm).

Loam is a soil that is a mix sand, silt, and humus.

Key Terms

loam: soil with no dominant particle size that contains a mixture of sand, silt, and humus

humus: a large group of natural organic compounds found in the soil composed of decaying plants and dead and living microorganisms

Soil Composition

Plants obtain inorganic elements from the soil, which serves as a natural medium for land plants. Soil is the outer, loose layer that covers the surface of Earth. Soil quality, a major determinant, along with climate, of plant distribution and growth, depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms.

Soil consists of these major components:

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Components of soil: The four major components of soil are shown: inorganic minerals, organic matter, water, and air.

inorganic mineral matter, about 40 to 45 percent of the soil volume

organic matter, about 5 percent of the soil volume

water, about 25 percent of the soil volume

air, about 25 percent of the soil volume

The amount of each of the four major components of soil depends on the quantity of vegetation, soil compaction, and water present in the soil. A good, healthy soil has sufficient air, water, minerals, and organic material to promote and sustain plant life.

The organic material of soil, called humus, is made up of microorganisms (dead and alive), and dead animals and plants in varying stages of decay. Humus improves soil structure, providing plants with water and minerals. The inorganic material of soil is composed of rock, slowly broken down into smaller particles that vary in size. Soil particles that are 0.1 to 2 mm in diameter are sand. Soil particles between 0.002 and 0.1 mm are called silt, and even smaller particles, less than 0.002 mm in diameter, are called clay. Some soils have no dominant particle size, containing a mixture of sand, silt, and humus; these soils are called loams.

Soil Formation

Soil formation is the result of a combination of five factors: parent material, climate, topography, biological factors, and time.

LEARNING OBJECTIVES

Describe the five factors that account for soil formation

KEY TAKEAWAYS

Key Points

Parent material is the organic and inorganic material from which soil is formed.

Climate factors, such as temperature and wind, affect soil formation and its characteristics; the presence of moisture and nutrients is also needed to form a quality soil.

Topography, or regional surface features, affects water runoff, which strips away parent material and affects plant growth (the steeper the soil, the more erosion takes place).

The presence of microorganisms in soil creates pores and crevices; plants promote the presence of microorganisms and contribute to soil formation.

Soil formation takes place over long periods of time.

Key Terms

rhizosphere: the soil region subject to the influence of plant roots and their associated microorganisms

bedrock: the solid rock that exists at some depth below the ground surface

horizon: a soil layer with distinct physical and chemical properties that differ from those of other layers

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands