Chapter 6: Section 2

The Nature of SoilChapter 7: Section 2

I. Formation of SoilWeathering gradually breaks rock into smaller and smaller fragments. Does not become high quality soil until plants and animals live on them. Plants and animals add organic matter to the rock fragment. Remains of plants and animals including leaves, twigs, roots and dead worms and insects. Soil is a mixture of weathered rock , decayed organic matter, mineral fragments, water, and air.

II. Five Factors of Soil Formation

III. Factors of Soil FormationComparing the FactorsDiscussion Points

Tropical vs. PolarSteep vs. FlatLimestone vs. CoalTrees vs. DesertNew Soil vs. Old

C. Visualization of Soil FormationIt may take thousands of years to form but soil is constantly evolving from solid rock.

IV. Visualization of Soil FormationSoil enters the cracks providing some nutrients for plants, weeds, etc. to grow.

IV. Visualization of Soil FormationThese organisms provide nutrients, mechanical, and chemical weathering.

IV Visualization of Soil FormationOver time the soil becomes more fertile and allows more growth; which reduces the erosion rate.

V. Composition of SoilMost organic matter in soil comes from plants. Leaves, stems, and roots all contribute organic matter to soil.Animals and microorganisms provide additional organic matter when they die. After plant and animal material gets into soil, fungi and bacteria cause it to decay. The decayed organic matter turns into a dark-colored material called HUMUS. Animals and insects mix the humus with the fragments of rock. Good quality surface soil has approximately equal amounts of humus and weathered rock material.

V. Composition of SoilWater FiltrationSoil has many small spaces between individual soil particles that are filled with water or air. When soil is moist, spaces hold water needed for plants. During a drought, the spaces are almost entirely filled with air. When water soaks into the ground, it infiltrates the pores.Infiltration rate is determined by calculating the time it takes for water sitting on soil to drop a fixed distance.

VI. Soil Profile

HorizonsDifferent layers of soil are called horizons. All the horizons of a soil form a SOIL PROFILE. Most soils have three horizons-labeled A, B, and C. Most plant roots grow in the top layer of soil.

VI. Soil Profile

The A HorizonTop layer of soil; in a forest may be covered with litter. LITTER consists of leaves, twigs, and other organic matter that can be changed to humus by decomposing organisms. Litter helps prevent erosion and evaporation. A Horizon is known as topsoil. Generally dark and fertile and has more humus than the other horizons.Dark color is caused by the humus, which provides nutrients for plant growth.

VI. Soil Profile

The A HorizonSince dark color absorbs solar energy more readily, soil color can greatly affect soil temperature. Darker color may also indicate a higher content of soil moisture, which is important in determining seed germination for plants and the vitality of decomposing organisms.

VI. Soil Profile

The B HorizonAlso known as subsoil. Less organic matter is added so it is lighter in color than the A Horizon. Less fertile than A Horizon. Contains material moved down from the A Horizon by the process of LEACHING.

VI. Soil Profile

The B HorizonLeaching is the removal of minerals that have been dissolved in water. Resembles making coffee in a drip coffeemaker. Water seeps through the A Horizon and reacts with humus and carbon dioxide to form acid. The acid dissolves some of the of the minerals in the A Horizon and carries the material into the B Horizon.

VI. Soil Profile

The C Horizon Bottom horizon; consists of partially weathered rock. Does not contain much organic matter and is not strongly affected by leaching. Composed of coarser sediment than the above soil horizons.Bottom of C Horizon is the rock that gave rise to the soil horizons above it. Called the parent material of soil. C Horizon is the soil layer that is most like the parent material.

VII. Soil Structure

Four Classes of Soil Structure Affect pore space and will affect a plants ability to penetrate roots. Natural clumps of soil are called peds. GranularGranular structures are common in surface soils with high organic content that glues minerals together. Earthworms, frost, and rodents mix the soil, keeping the peds small, which provides good porosity and movement of air and water.

VII. Soil Structure

Platy StructuresOften found in subsurface soils that have been leached or compacted by animals or machinery. Blocky StructuresCommon in sub-soils or surface soils with high clay content which shrinks and swells, producing cracks.

VII. Soil Structure

Prismatic StructuresFound in B HorizonsDense and difficult for plant roots to penetrate.Vertical cracks result from freezing and thawing, wetting and drying, and downward movement of water and roots. Soil ConsistencyAbility of peds and soil particles to stick together and hold their shapes.

Section 2 Mini Lab (15 minutes)

Section 2 Mini Lab (15 minutes)Use the information on the previous slide to answer the questions below.

Calculate the percentage of sand in the sample. Convert the percentage sand to degrees. Calculate the percentage of silt in the sample. Convert the percentage of silt to degrees. Convert the percentage of clay to degrees.

VIII. Soil TypesSoil types vary in color, depth, texture, fertility, pH, temperature, and moisture content. Soil Types and ClimateSoil temperature and moisture content affect the quality of soils. Desert soil contains little organic material and are also thinner than soils in wetter climates. Prairie soils have thick, dark A Horizons because the grasses that grow there contribute lots of organic matter.Temperate forest soils have less organic matter and thinner A Horizons than prairie soils. Other regions such as tundra and tropical areas also have distinct soil.

VIII. Soil Types of the United States

IX. Other Factors for SoilParent RockAffects vegetation that grows. Affects how soil is weathered. Soil pHControls many chemical and biological activities that take place in soil. Activities of organisms, acid rain, or land management practices could affect soil quality. TimeIf weathering has only been occurring for a short time the parent rock dominates the landscape. SlopeOn steep slopes, soils are often poorly developed. In bottomlands, sediment and water are plentiful; and are often dark, thick, and full of organic material.

IX. Other Factors for SoilThe slope of the land affects soil development. Thin, poorly developed soils form on steep slopes, but valleys often have thick, well developed soils.

Why does this happen?