- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7400) Unit Title: SOILS ____________________________________________________________________________ (CLF7410) Topic: SOIL WATER Time Taught in Year(s) 3 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (G-11) - Describe the effect of soil texture on depth of water penetration and water and nutrient holding capacity. (G-12) - Describe the difference between infiltration and percolation. (G-13) - Identify two ways of modifying soil to improve moisture relationships. Special Materials and Equipment: Soil mixture samples for percolation tests. References: Interstate Printers and Publishers. WESTERN FERTILIZER HANDBOOK. Pages 225-240. Hartman, and Hudson. PLANT SCIENCE. Page 180. Evaluation: Quiz by instructor TOPIC PRESENTATION: SOIL WATER A. There are several reasons why we are interested in soil water relationships. 1. Large amounts of water must be supplied to growing plants as water is lost through transpiration and evaporation. 2. The water must be available as plants need it and most of it must come from the soil. 3. Water is the solvent that holds the dissolved nutrients making up the soil solution. 4. Soil moisture also affects or is related to soil air and soil temperature. B. We are also concerned about the role and type of precipitation as it will effect soil erosion. *************************************************************************** Note to Instructor: See Management Practices Necessary for Irrigation and Drainage (CLF7550). *************************************************************************** C. Water penetration is affected by texture. 1. The rate of water penetration into sandy soils is much greater than the movement into finer-textured soils (often considered a "heavier"soil.) 2. Also the movement of water into a sandy soil will be in a downward direction, whereas in finer-textured soils water will move more laterally. 3. The speed of flow, mostly because of pore size, varies with texture. D. The water holding capacity is also directly related to the soil texture. 1. Fine textured soils will have a higher water holding capacity than sandier soils. 2. In the fine textured soils there are more total void (pore) spaces, so they have a greater capacity to hold water. 3. Even though the individual pore spaces are larger in a sandier soil, the total porosity is greater in fine textured soils. E. Soils vary in the amounts of the different nutrients they can provide to growing plants. 1. An important property of fine textured clay soils is their ability to attract and hold ions of essential plant nutrients (cation exchange capacity). 2. This ability to hold the essential elements depends somewhat on the soil pH. 3. Other things being equal, the nutrient holding capacity is lower in acid soils and higher in alkaline soils. 4. Also, a soil high in clay, because of it higher cation exchange capacity, will lose less of the essential ions by leaching than the sandier soils. F. There is often confusion on the differences between infiltration and percolation. 1. Water infiltration is water movement into the soil when rain or irrigation water is on the soil surface. This is when the water enters the soil through the surface. 2. Percolation is the flow of more water through wetted soil. It is this percolating water moving through the soil which carries away nutrients and other salts dissolved from the soil. G. Soils can sometimes be modified to improve moisture relationships. 1. The availability and types of soil amendments used to improve the physical properties of soil continually change. a. Many of these material are byproducts of other manufacturing processes. b. Organic wastes such as bark, sawdust, sludge, and mushroom composts are examples. c. Inorganic soil amendments do not contain humus nor do they contribute to the production of it. Their function is to act as wedges in physically separating soil particles. (e.g., pumice, vermiculite, perlite, and sand). 2. Sand is a low cost and long lasting amendment but must be used at very high rates (up to 80% of mix) to be effective. a. It is low in both water and nutrient holding capacity. b. It causes finer silts or clays to compact. c. It is especially useful in nursery production and on golf green to improve drainage in clay soils. 3. There are several soil amendments that will increase the water holding capacity in sandy soils. Examples of organic and inorganic amendments are: a. Organics include peat moss, wood shavings, rice hulls, and sludges. b. Inorganics include vermiculite and perlite. __________________________________________________________ ACTIVITY: 1. Prepare several different soil mixtures, place all of them in separate (same size) containers with drainage holes. a. Observe the rates of water penetration. b. Measure the water holding capacities of each of the mixtures. c. Evaluate the two above characteristics in relation to the use of these mixes for several different purposes. __________________________________________________________ 7/25/90 sg #%&C