- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7400) Unit Title: SOILS ____________________________________________________________________________ (CLF7407) Topic: SOIL ENVIRONMENT Time Taught in Year(s) PROBLEMS 4 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (G-16) - Determine soil pH using a soil test kit. (G-17) - Distinguish between alkali and alkaline (sodic) and saline soil conditions and describe their effects on plant growth. (G-18) - List soil environmental factors affecting plant nutrient availability. Special Materials and Equipment: Soil test kit. References: Donahue, Roy L. SOILS. Pages 135 to 136, 193, 211. Brady, Nyle C. THE NATURE AND PROPERTIES OF SOILS. pages 220 to 222. WESTERN FERTILIZER HANDBOOK. (7th edition). Interstate Printers and Publishers. Danville. Illinois. Pages 8, 125 to 135. Evaluation: Quiz by instructor TOPIC PRESENTATION: SOIL ENVIRONMENT PROBLEMS A. Soil pH provides an important clue in diagnosis of many soil problems. 1. Both saline and sodic soils present specific problems. 2. Plant nutrient availability can also present the grower with a multitude of challenges. B. The pH of soils is usually determined by one of two methods: the electrometric method or one of various colorimetric (dye) methods. 1. In the electrometric method the hydrogen concentration of the soil solution is measured with an electrode specific for hydrogen ions. This instrument gives very consistant results but requires a skilled technician. 2. The second method requires the use of colorimetric indicators. This method is widely used in the field, because it is simple and accurate enough for most purposes. a. Many dyes change color with an increase or decrease of pH, making it possible to estimate the approximate hydrogen ion concentration of a solution. b. By using several dyes a pH range of 3 to 8 is easily covered. c. To do this test, the soil sample is saturated with a dye. d. After standing in contact with the soil for a few minutes, a drop of liquid is run out and its color compared to a color chart to determine the pH. e. Commercial kits come with directions for using the color chart, reagents and plates, so field testing is simple, fast, and relatively accurate. f. Most agricultural soils will range in pH from 5.5 to 8.0. g. To distinguish between the relative degrees of acidity and alkalinity of a soil, a pH scale from 0 to 14 is used. 1) At the middle of the scale (pH 7.0) the reaction is neutral. 2) Below 7.0 the reaction is acidic, so the soil would be said to be "acid." 3) Above 7.0 the reaction is akaline (basic), so the soil would be called "alkaline". 3. In the west, acid soils are usually found in areas of heavy rainfall, on light textured soils, where high rates of acid-forming fertilizers have been used, and where organic soil deposits occur. a. Soils become acid because the cations of the soil colloids are replaced by hydrogen ions. b. Strongly acidic soils are not productive for most crops. (Exceptions are blueberries, cranberries, watermelons, white potatoes, and pineapples). c. Strongly acid soils may be less productive because of toxicities of aluminum and manganese and deficiencies of some major and minor plant nutrients. C. Saline and sodic soils present specific problems. 1. A saline soil is one that contains enough soluble salts to impair productivity for plants but does not contain an excess of exchangeable sodium. a. The soluble salts that occur come indirectly from the weathering of minerals and from waters which carry salts from other locations. b. The presence of these soluble salts (including sodium chloride and sodium sulfate) interferes with the absorption of needed nutrients and water from the soil. 2. A sodic soil is one that contains sufficient exchangeable sodium to interfere with the growth of most plants, (either with or without appreciable quantities of soluble salts.) For those of you who like to think about details: generally, if soluable salts are present, the soil can be called "saline-sodic"; if soluable salts are not present, but exchangeable sodium is, the soil can be called "sodic". a. Sodic soils contain excessive amounts of sodium. Sodium becomes dominant in the soil solution and replaces calcium and magnesium on the clay, consequently alkali soils are formed. b. Adsorbed sodium causes disintegration of the soil aggregates, disperses the soil particles, and reduces the pore spaces. These all make leaching difficult since, under these conditions, the soil becomes almost impervious to water. c. Under these conditions, root penetration may be restricted, and aeration is reduced, setting up anaerobic conditions. D. Soil environmental factors affect plant nutrient availability, so they can become soil problems. 1. Soil physical conditions, soil chemical make-up, soil microorganisms, and crop management can all affect the nutrient supplying capacity of soils. 2. A soil must be moist, warm, and well aerated for active nutrient absorption. 3. Soils will vary in the amounts of the various nutrients they can supply to growing plants. 4. Many of the plant nutrients in soils do not remain available to plants. 5. Losses occur by leaching, volatization, denitrification, and nutrients immobilized temporarily by soil microbes. __________________________________________________________ ACTIVITY: 1. Have students determine the pH of a soil using a soil test kit. a. Have each student bring a soil sample from home. b. The determination of soil pH is very simple using most commercially available test kits. Instruction, color charts, plates, and chemical regents are all included. 2. Take a field trip to areas of saline or sodic soils and explain their differences in appearance, uses, and reclamation methods. __________________________________________________________ 7/25/90 sg #%&C