- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7400) Unit Title: SOILS ____________________________________________________________________________ (CLF7402) Topic: SOIL FORMING Time Taught in Year(s) FACTORS 2 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: None (G-1) - Name the five major factors in the formation of soils. Special Materials and Equipment: None References: Donahue, Roy L. SOILS. Pages 36 to 43. Hartman and Hudson. PLANT SCIENCE. Pages 168-173. Evaluation: Quiz by instructor TOPIC PRESENTATION: SOIL FORMING FACTORS A. Man is dependent on soils. 1. Soils are the natural bodies on which plants grow. 2. Humans use the soil sorce to raise plants for their beauty and to supply food and fiber for themselves and their livestock. 3. People's standard of living is effected directly by the quality of soil and the quality and types of plants and animals grown on them. B. There are five major factors usually considered in the formation of soils: climate, parent rock, living organisms, topography, and time. We will look at them individually and explain how each of them contributes to the formation of soil. C. Parent rock from which a soil is formed has a significant effect on its qualities. 1. Parent materials influence the formation of soils by their rates of weathering, the nutrients that they supply, and the particle sizes that they contain. 2. The less developed a soil is, the more influence that the parent material has on its characteristics. 3. Mineral particle size has a great effect on the properties of soil in the field. D. Climate is a dominant factor in the formation of soils. Climate determines soil. 1. The major components of climate that affect soil are precipitation and temperature. a. In areas of low rainfall an accumulation of lime may occur. There is insufficient water to leach the lime, so these soils are usually alkaline. b. In areas of high rainfall there is intense weathering and leaching resulting in acid soils. c. Erosion of sloping lands removes developing layers of soil and deposits them down slope. d. Erosion, leaching and weathering are more intense and take place over a longer period each year in warm and humid areas. 2. Climate also has an indirect affect on soils by its action on vegetation. 3. Changes in temperature strongly affect the rate of physical chemical weathering. Different rates of expansion and contraction bring about cracking and peeling beginning the breakdown of parent rock. Rates of chemical reactions increase as temperature increases, if sufficient water is present. D. Parent rock from which a soil is formed has a great affect on its qualities. 1. Parent materials influence the formation of soils by their rates of weathering, the nutrients that they hold, and the particle sizes that they contain. 2. The less developed a soil is, the more influence that the parent material has on its characteristics. 3. Mineral particle size has a great affect on the properties of soil in the field. E. The activity of living plants and animals (macro and microorganisms) has major significance on the development of soil. 1. The kind and amount of organisms present are influenced by climate (i.e., the presence of trees will have a different effect on soil forming processes than would the presence of grasses). 2. Microorganisms help develop soils by decomposing organic matter and forming weak acids that dissolve minerals faster than would pure water. 3. Fibrous root systems of grasses have a distinctly different effect on soils than do the coarser roots of trees. 4. Lichens, which are a combination of algae and fungi, are often the first plants that grow on weathering rocks. F. Topography (the lay of the land) influences drainage and runoff. 1. Steeper hill sides typically have thinner A and B horizons than more level areas, given the same parent materials and climate. 2. Soils on gentle slopes will have more water passing through them vertically than soils on steeper slopes. a. The profile on gentle slopes will be generally deeper, sustain more luxuriant vegetation, and contain more organic matter than soil profiles on steeper slopes. b. Soils developed in flat areas with high rainfall are often severely leached. 3. In our hemisphere soils on west and south facing slopes receive more direct rays from the sun. They are, therefore, warmer and drier than east and north facing slopes. 4. Soils lying in a lower land locked area receive runoff waters from the surrounding higher area. If salts are present they will be dissolved and accumulate in the "low lands" causing a salt marsh or toxic salt conditions in which no plants can grow. G. It requires time, up to about a million years, to form soils. 1. Rocks like granite are extremely hard to decompose. 2. Softer rocks such as limestone take less time. 3. As soils age they differentiate into defined profiles consisting of three different layers (A horizon, B horizon and C horizon). a. Horizons tend to develop faster under humid, warm, and forested conditions. b. A recognizable soil profile may develop in as few as 200 years or, under less favorable conditions, take several thousand years to develop. H. The five major factors that we have mentioned affecting soil formation interact in a variety of ways to produce a variety of soils. 1. Parent material along with other factors effect the capacity of a soil to support plant life. 2. This, then influences the type of vegetation that will grow. 3. As we have discussed, temperature and topography also influence plant growth. 4. Topography, temperature, and time influence the conversion of rock into soil. 5. Each factor therefore is affected by each of the others. 6. It would be difficult to separate these factors, or say that any one is more important than the others. __________________________________________________________ ACTIVITY: 1. Take the class on a field trip to view road cuts, excavations or pits dug that show the soil profiles. a. Relate the soil layers, colors and textures to the soil forming factors studied in this unit. b. Note the darker the color of the surface few inches caused by the presence of organic matter. __________________________________________________________ 7/24/90 sg #%&C