- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7550) Unit Title: MANAGEMENT PRACTICES NECESSARY FOR IRRIGATION AND DRAINAGE ____________________________________________________________________________ (CLF7554) Topic: SOIL WATER Time Taught in Year(s) 4 hours 3 AND 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (J-11) - Discuss how water in soil can move upward against gravitational pull. (J-12) - Demonstrate how to test for soil moisture content by touch. (J-13) - Calculate soil moisture content. Special Materials and Equipment: Oven for drying soil samples, containers for soil samples, variety of soils with various textures (sand, silts and clays). References: Hartman. PLANT SCIENCE. Pages 183 to 188. Curriculum Development Project. LESSON PLANS FOR THE BASIC CORE CURRICULUM Donahue. SOILS. Pages 176 to 184. Evaluation: Quiz by instructor TOPIC PRESENTATION: SOIL WATER A. Water content in soil has an effect on soil formation and erosion, but the growers primary concern is the availability of water for plant growth. Three forces are responsible for water movement within the soil. They are gravity, adhesion, and cohesion. B. Gravity causes water to move downward under no tension, and is the principle force when the soil is saturated. C. Adhesion is the force of attraction between unlike molecules, water and the soil particles. D. Cohesion is the attractive force between like molecules, water and water. E. Adhesive and cohesive forces can cause water to move by capillarity in any direction, laterally, upward or downward. They are the two main forces that move water in an unsaturated soil. F. The upward movement of water in soil is called capillary rise. 1. It is responsible for water lost from the soil surface by evaporation. 2. Water can move further by capillary action in soils with small pores (clay) than soils with large pores (sand). 3. Capillary water moves usually from a wetter to a drier soil. 4. Adhesive forces and cohesive forces, then can actually move water upward against the forces of gravity. G. There are several methods used for measuring soil water content. 1. The gravimetric method, however, is the classical procedure used as a check for all of the other methods. 2. This is the way it works. a. A soil is sampled, b. the soil is placed in a container, c. the soil and container are weighed (in the moist condition as collected), d. the sample is oven-dried (at 221 to 230 degrees F), and e. the container and soil is weighed again after drying. 3. The formula for determining percent soil water is: a. = (wet soil weight) - (oven dry soil weight) Percent water _________________________________________ (100) oven dry soil weight b. Sample problem: A soil sample was collected from a field, (Note: Soil scientists have agreed that for percent water calculation, the reference point - the number used in the denominator is ALWAYS THE DRY SOIL WEIGHT.) placed in a container, weighed, dried in an oven at 221 degree F and re-weighed. c. The measurements were: Moist soil plus container is 168 grams. Oven dried soil plus container is 145 grams. Empty container weight is 49 grams. d. Calculate the moisture content of this soil when it was collected. e. All calculations must include only soil and water weights, so the weight of the container must be subtracted. f. Moist soil only = 168 grams - 49 grams = 119 grams g. Dried soil only = 145 grams - 49 grams = 96 grams h. moist soil - oven dry soil Moisture percentage = __________________________ (100) oven dry soil = 119 grams - 96 grams ____________________ (100) 96 grams = 24 percent moisture. **************************************************************************** Note: Instructor see "Knowing when to Irrigate" (CLF353) of the Basic Core Lesson Plans for details on how to determine soil moisture content by touch. Included there is a feel chart for determining moisture in medium to fine textured soil. **************************************************************************** __________________________________________________________ ACTIVITY: 1. Demonstrate to students capillary rise of water by using various sizes of glass tubing. Place tubes in a vessel of water. Note: The water ascends highest in the smallest tube. Explanation: Doubling the diameter of the tube doubles the area for water molecules to adhere to. However it also qradruples the weight of the water to be pulled up. Therefore, water does not rise as high in the tubes with layer diameters. 2. Have each students bring a soil sample from home and determine the soils moisture percent. Use the procedure and formula shown in the text of this topic. 3. Have students determine moisture content of soils by using touch (or feel) method outlined in (CLF353) "Knowing when to Irrigate." Note: Soils containing low to high degree of moisture may have to be prepared several days before use to give them time to dry to the degree desired. __________________________________________________________ 7/25/90 sg #%&C