- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7400) Unit Title: SOILS ____________________________________________________________________________ (CLF7403) Topic: SOIL HORIZONS, Time Taught in Year(s) TEXTURE AND STRUCTURE 4 hours 3 AND 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (G-3) - Compare and contrast the three horizons in a soil profile. (G-6) - Define soil texture and soil structure and identify soil texture by feel. Special Materials and Equipment: Soil samples (sand, loam, clay) and water bottles. Evaluation: Quiz by instructor TOPIC PRESENTATION: SOIL HORIZONS, TEXTURE AND STRUCTURE A. The soils on our earth's surface are undergoing continual change. 1. The impelling evolutionary forces that form soils are an integrated effect of climate and living matter acting on parent material, affected by slope, acting over time. 2. Soil development cannot be described in distinct stages. 3. As they develop, soils attain better defined profiles. 4. Not all soil properties develop at the same rate. B. A soil profile is a vertical section of the soil through all of its layers (horizons) extending from the surface into the parent material. C. These soil layers (horizons) are zones of soil, that are approximately parallel to the soil surface, showing distinct characteristics that have been produced by the soil forming processes. D. The morphology of these horizons makes it possible to classify soil into types, in order that its structure and fertility may be predicted. E. We will discuss in some detail each of these three soil horizons. 1. The surface layer (A horizon) is often referred to as the zone of leaching. Note: Unless strongly leached, the A horizon has all of the important nutrients and is the most fertile horizon because the nutrients are recycled to the surface. a. It varies in depth and contains most of the plant roots. b. If this zone is heavily leached, and in areas of high rainfall lacks some of the important mineral nutrients, it can be referred to as the E horizon. c. Ordinarily, however,this horizon (A) does contain the largest amount of organic matter. d. This organic matter makes the A horizon permeable and dark colored. 2. The B horizon is below the A horizon. a. It is known as the "zone of accumulation" or "zone of alteration" since plant nutrients, silts, clays and other material from the A horizon are leached into and accumulate in this zone. It is also the zone affected by soil forming processes, other than accumulation, but that is not delivered by organic matter. b. As less organic matter is present in this layer, the color is usually lighter than that in the A horizon. It is often red and yellow colored relative to the parent material. c. The A and B horizons together make up the solum, or the zone affected by soil forming processes. 3. The C horizon is the unconsolidated material underlying the root zone. a. It may or may not be the same as the parent material from which the A and B horizons were formed. b. It is outside the zones of major biological activities. c. It is little affected by soil forming processes. F. The relative size of soil particles is expressed by the term "texture". 1. This refers to the fineness or coarseness of the soil. 2. Specifically soil texture is defined as the percentage of sand, silt and clay particles in a soil. (sample < 2mm) 3. Soil texture is measured by separating the soil particles into their respective sizes, then the percentage of each category is calculated. 4. Texture is an important soil characteristic, because it will partly determine: a. water absorption rates, b. soil water storage capacity, c. the ease of tillage, d. the amount of aeration, and e. soil fertility. G. Soil "structure" is defined as the physical arrangement of soil particles. (How the particles clump and hold together). 1. These clumps (aggregates) act as individual larger particles with their own characteristics. 2. The kind of soil structure is determined by the relative amounts of each primary particle and the way they are arranged into aggregates. 3. The structure of soil then is determined by the size and shape of the aggregates. 4. Descriptive words that are based upon the size and shape of the aggregates are used to classify soil structure (e.g., prismatic, blocky, columnar, platy, crumb, granular). 5. Structure can be altered by tillage. a. Tilling when a soil's moisture level is higher than field capacity can ruin good structure. b. Soil crusts and other compacted layers can have their structure improved by tillage. 6. Adding organic matter usually improves soil structure. __________________________________________________________ ACTIVITY: 1. Have students texture soil by the feel method. You will need a sample of soil from each of the textural groups: a. sand; b. loam; c. clay loam and d. clay. Follow these directions: a. Have each student place 2 teaspoons of each of these soils in the palm of his/her hand. Do one sample at a time. b. Add drop by drop of water to moisten the soil; kneading the soil while adding the water. c. Make soil like moist putty. d. Place ball of soil between the thumb and forefinger. e. Gently push the soil so it is squeezed upward by the thumb against the forefinger. f. Here is an explanation of soil casts and ribbons formed in moist soil: 1) A cast will form when the moist soil is squeezed in the hand. The cast cannot be handled without breaking. No ribbon can be formed. It feels very gritty. This is sand, a coarse textured soil. 2) A short ribbon can be formed with moist soil. The ribbon will split readily and will break away when it becomes 1/2 inch long. A moist cast will bear some handling. This is a loam, a medium textured soil. 3) A ribbon can be formed easily in moist soil. This ribbon is moderately strong, but will break away when it is 3/4 of an inch long. A moist cast will bear moderate handling. This is a clay loam, a medium textured soil. 4) A strong ribbon can be formed in moist soil. The ribbon often will be more than an inch long. A moist cast will bear considerable handling. This is a clay, a fine textured soil. 2. Have the class view actual soil profiles in the field. a. Show the individual soil horizons and point out their characteristics and differences. b. Collect samples to be used in the determination of soil texture by the feel method above. c. Be sure the horizons are distinct. Subtle differences in poorly developed soils are hard for students to see. __________________________________________________________ 7/24/90 sg #%&C