- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7400) Unit Title: SOILS ____________________________________________________________________________ (CLF7409) Topic: SOIL MOISTURE Time Taught in Year(s) BEHAVIOR 3 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (G-10) - With the use of samples determine field capacity, saturation, wilting point, permanent wilting point and available water. Special Materials and Equipment: A soil survey from a local (area) county. References: Hartman, and Hudson. PLANT SCIENCE. Pages 183 to 188. Evaluation: Quiz by instructor TOPIC PRESENTATION: SOIL MOISTURE BEHAVIOR A. All water in soil is not available to plants. 1. Water moving within the soil or held by the soil supplies plants with mineral nutrients and oxygen as well as water. 2. It moves through the soil in both the liquid and gaseous state. a. The pore spaces of a soil are always filled with water (actually soil solution) or air or a mixture of both. b. The forces of adhesion, cohesion and gravity are responsible for water movement within the soil. B. So that we may better understand the demonstration on soil moisture behavior, a glossary of terms that we will use follows: 1. Adhesion is the force of attraction between unlike molecules, (soil particles and water). Water held by adhesion is not available to the plant. 2. Cohesion is the force of attraction between like molecules (water and water). Water held by cohesion is available to the plant. 3. Gravitational water is water that moves into, through, or out of the soil under the influence of gravity. It causes water to move downward under no tension and is the principle force when the soil is saturated. 4. Saturation is when all pore spaces are filled with water. This is an unhealthy state for plants when prolonged because the oxygen dissolved in the water that is needed for respiration is quickly utilized. 5. Field Capacity (F.C.) is the percentage of water remaining in a soil after the free water (gravitational water) has been allowed to drain away (one to three days) after the root zone had been previously saturated. The water has drained from the macropores, but the micropores will contain water. 6. Permanent wilting percentage (P.W.P.) is the moisture content of soil at which plants wilt and fail to recover even when placed in a humid atmosphere. The water is held so strongly that plants are not able to absorb it fast enough for their needs. Wilting plants do not recover except when additional water is added to the soil. 7. Available water (A.W.) is the portion of water in a soil that can be absorbed by plant roots. It is the moisture held in the soil between field capacity and permanent wilting percentage. (Available water = F.C. - P.W.P) 8. Capillary water is water held in the soils "capillary" pores by its own surface tension and adhesive properties. It is often the most important component of soil moisture available to plants. 9. Hygroscopic water is a thin layer of water held so tightly by the soil particles that it cannot be removed by plants. C. An instructor can use this procedure as a class demonstration or individual students can make observations on soil moisture behavior as they follow through this experiment. __________________________________________________________ ACTIVITY: 1. Into a glass of water place a paper towel (or sponge) folded lengthwise. Observe the rise of water up the towel. This is capillary movement or capillarity. 2. Place the towel into the glass of water and let it become completely saturated. Lift the wet towel out of the glass. a. The towel represents a soil that has been completely wetted. All of the pore spaces are completely filled with water. b. It is saturated. 3. Hold the paper towel up and observe the water dripping from it. a. This is gravitational water. b. In a soil this water would continue movement downward until the cohesive forces overcame the gravitational force. 4. When the towel stops dripping it is at field capacity. a. It is now holding the maximum amount of water that it can against the pull of gravity. b. Like a soil, the water has drained away from the macropores, but the micropores still contain water. 5. Take your hands and squeeze or wring out all possible water from the towel. a. This is available water. b. The squeezing out of the water can be compared to the water that the plant uses. 6. When you have squeezed out all possible water the towel is like a soil that has reached its wilting percentage. a. The towel will still feel damp, but no more water can be squeezed out. b. The water is being held by adhesion and is called hygroscopic water. c. It is in the soil, but it is unavailable to plants. d. Permanent wilting percentage is reached when a plant is at the wilting percentage for such a period of time that it will not revive even when placed in a saturated atmosphere. __________________________________________________________ 7/25/90 sg #%&C