- - AGRICULTURE CORE CURRICULUM - - (CLF6000) Advanced Core Cluster: ORNAMENTAL HORTICULTURE (CLF6400) Unit Title: HORTICULTURAL SOILS & PLANTING MEDIA ____________________________________________________________________________ (CLF6401) Topic: SOIL BASICS Time Year(s) 1 hour 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the student will be able to: Learning Outcome #: (G-1) - Review the four functions of soil. (G-2) - Describe the soil texture considered ideal for gardening. Special Materials and Equipment: Samples of each of the following soil types: sandy, loam, clay-loam, and clay References: Cooper, Elmer L. (1990). AGRISCIENCE: FUNDAMENTALS AND APPLICATIONS. Albany, NY: Delmar Publishers. Hartmann, H. T., Flocker, W. J., & Kofranek, A. M. (1981). PLANT SCIENCE: GROWTH, DEVELOPMENT, AND UTILIZATION OF CULTIVATED PLANTS. Englewood Cliffs, NJ: Prentice-Hall. Hausenbuiller, R. L. (1972). SOIL SCIENCE: PRINCIPLES AND PRACTICES. Dubuque, IA: Wm. C. Brown Company. Editors of Sunset Books and Sunset Magazine. (1988). WESTERN GARDEN BOOK. Menlo Park, CA: Lane Publishing Co. Evaluation: Unit Exam ===================================================================== *** INSTRUCTORS PLEASE NOTE *** The detail of this topic presentation goes beyond the scope necessary to meet the requirements of the Core Cluster in this area. It will take longer to teach than indicated above if covered entirely. It is included for local enrichment as appropriate to the class. ===================================================================== TOPIC PRESENTATION: SOIL BASICS REVIEW: Excellent and in-depth information on soils, soil water, soil microorganisms and organic matter in soils is available in the CLF7000 Advanced Core Cluster on Plant and Soil Science. The unit title is (CLF7400) Soils. These topics are highly recommended to complement the information presented in this Ornamental Horticulture unit. A. Soil and Horticulture: The management of soils for gardening and ornamental horticulture is more intensive than that for most fields of agriculture, and there are significant differences between the two. However, basic soil characteristics and some of the challenges they present are common to all areas of plant culture. The following review summarizes the principles and concepts which form the basis for the management of horticultural soils. B. The Functions of Soil: Soil provides four basic functions for plants: 1. Soil provides important plant nutrients. a. At present, 17 chemical elements are known to be essential for plant growth. Soil is the source of 14 of these. (Note: Nickel has recently been added to the list of essential elements.) b. Thirteen of these 14 originate in the parent rock from which the soils developed. Nitrogen, the exception, comes from organic matter. (Nitrogen actually originates from the atmosphere and is fixed by microorganisms in organic matter.) 2. Water is also provided through the soil. a. Water a plant uses enters the plant through its roots. b. Water dissolves soil minerals and provides the "vehicle" by which nutrients enter the plant. 3. Air is provided to plant roots through the soil. a. Air replaces the space between the particles as water evaporates or is drawn up by the plant and transpired. b. Air supplies the oxygen which is necessary for respiration by plant roots and by other organisms in the soil essential to plant health. 4. Soil also serves as anchorage for plants. a. With the exception of hydroponic operations, all plants produced in ornamental horticulture rely on soil for plant support. b. The roots of plants comprise 20 to 50 percent of a plant's total mass. c. Roots provide anchorage for the plant through attachment to soil particles. C. Composition of Soils: Mineral soils are made up of four major components: mineral materials, organic matter, water, and air. 1. Composition by Volume: The volume composition of an AVERAGE silt loam surface soil when in good condition for plant growth is approximately: 45% mineral matter } 25% water } } solid space } pore space 5% organic matter } 25% air } a. As the soil moisture content varies (such as the time just preceding an irrigation compared with the time just following a rain storm) there will be wide fluctuations in the proportions of air and water in the soil profile. The 25% figures above represent averages, not constants. b. In a typical soil these components exist in a close, mixed condition. c. This closeness encourages interactions among the components and yields a complex of variation in the environment for the growth of plants. d. Many soils which we prepare for HORTICULTURAL use (especially for production of ornamental plants) will have much higher percentages of organic matter and air space. 2. Mineral Composition: The mineral (inorganic) portion is variable in size and composition. a. The rock fragments are remnants of the parent rocks from which the soil was formed. b. Other components such as the colloidal clay particles are so small that an electron microscope must be used to see them. c. In our study of soils we normally break down the sizes of inorganic particles as: 1) gravel = very coarse 2) sands = coarse 3) silt = fine 4) clay = very fine d. This particle size definitely affects soil properties, especially relating to water and nutrient holding capacity. e. These properties vary mainly because of the relative surface area of the different particles. A volume of clay would have a far greater collective surface area than an equal volume of sand because of the great difference in particle size. f. To give a rough idea of the relative size of these particles, a grain of sand could be compared to a basketball, a silt particle to a baseball, and a clay particle a "bb." 3. Air: Soil air is located in the soil pores separated by soil particles. These pores are the passageways for the movement of oxygen needed for plant roots and other organisms living in the soil. a. Soil air can become limiting to plant growth when the soil pores are filled with water or when the soil is compacted allowing smaller or fewer pores. b. Generally, soil air contains less oxygen and more carbon dioxide than the above ground atmosphere. c. Flooding or water logging will reduce the oxygen content in the soil. d. Many plants suffer when the oxygen percentage in the soil drops below 10 to 12 percent (normal atmosphere contains 20.0 percent oxygen) or the carbon dioxide percentage goes up to 5.0 percent (normal atmosphere contains 0.03 percent carbon dioxide). 4. Soil Water: Water is held in the air spaces between soil particles. a. Soil water, with its dissolved salts, makes up the "soil solution" which is the medium supplying nutrients to growing plants. b. Not all of the water in the soil is available to the growing plants. As the amount of water in the soil decreases, the remaining water is held more tightly by small pores and as a film around soil particles. c. Only about half of the water is available to plants because of the mechanics of these water-holding forces in the soil. d. Nutrients are exchanged between the soil solids and the soil solution and then between the soil solution and the plants. 5. Organic Material: Soil organic matter represents an accumulation of plant and animal residues. Living organisms in the soil can also be considerd part of the organic component in soil. a. This material is constantly being decomposed, and is continually being renewed by decomposing plant or animal residues or both. b. The organic matter content of soils is small compared to the other components, about 2 to 6% by weight of a typical loam soil. c. The influence of the organic matter is, however, of great significance. d. Organic matter is largely responsible for the loose, easily managed characteristics of productive soil. e. It is the essential source of soil nitrogen and the major source of sulfur and phosphorous. f. It increases the water holding capacity of soils as well as increasing water available to plants. g. The decomposed organic material called humus improves the ability of the soil to hold and release plant nutrients (cation exchange). h. Organic matter is the main source of energy for soil organisms. i. Soil microorganisms and larger creatures such as earthworms are beneficial to plants, and necessary in the process of making nutrients available. Refer to (CLF7405) Benefits of Soil Organisms for a complete discussion on this topic. D. Physical Characteristics 1. Texture: The relative size of soil particles is expressed by the term, "texture." a. This refers to the fineness or coarseness of the soil. b. Specifically, soil texture is defined as the percentage of sand, silt, and clay particles making up the mineral portion of the soil. c. Soil texture is measured by separating the soil particles into their respective sizes, then the percentage of each category is calculated. For instance, soil which is: 1) 40% SAND, 40% SILT and 20% CLAY is called loam 2) 50% " , 10% " " 40% " " " sandy clay 3) 10% " , 45% " " 45% " " " silty clay 4) 20% " , 20% " " 60% " " " silt loam 5) 60% " , 30% " " 10% " " " sandy loam 6) 40% " , 30% " " 30% " " " clay loam d. Texture is an important soil characteristic, because it will determine to a large degree: 1) Water absorption rates--sandy soils allow fast infiltration. 2) Soil water storage capacity--clay soils hold more water. 3) Ease of tillage--sandy ("light") soils are very workable. 4) Amount of aeration--sandy soils allow air to enter readily. 5) Soil fertility--silty and clay soils hold more nutrients. e. Adding soil amendments can improve some of the above soil characteristics, but it will not change the basic soil texture. Soil texture and soil quality, therefore, are often equated. f. The loamy soils, especially true loam and the sandy loams, are considered texturally ideal for gardening. They combine the best of the above-mentioned qualities. __________________________________________________________ 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 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 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 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. Structure: Soil structure is defined as the physical arrangement of soil particles (how the particles clump and hold together). a. These clumps (aggregates) act as individual larger particles with their own characteristics. b. The kind of soil structure is determined by the relative amounts of each primary particle and by the way they are arranged into aggregates. c. Descriptive words that are based on the size and shape of the aggregates are used to classify soil structure (e.g., prismatic, blocky, columnar, platy, crumb, granular). d. Structure can be altered by tillage. 1) Tilling when a soil's moisture level is higher than field capacity can ruin good structure. 2) Soil crusts and other compacted layers can have their structure improved by tillage. e. Adding organic matter usually improves soil structure. 3. Color: Soil color can serve as an indicator of many soil properties. a. Dark soil usually indicates a soil high in organic matter. b. Red or orange soil usually indicates fast drainage and a high iron content. c. Gray or mottled soils are often poorly drained. 4. Depth: The rooting zone available to plants is restricted by the depth of a soil. This essentially restricts the amount of water and nutrient a plant has available to it (in a field setting). E. Chemical Characteristics 1. Soil pH: One of the most important chemical characteristics of soil is its pH. The acidity or alkalinity of soil, unlike texture, can vary from time to time depending on, for instance, the chemical balance of fertilizers or amendments being used in the soil and the amount of water being applied. a. pH is measured on a scale of 0 to 14. 7.0 is considered "neutral." b. High pH (i.e., pH 10.0) is considered alkaline (or "sweet" in the parlance of the farmer). c. Low pH (i.e., 4.0) indicates an acid (or "sour") soil. d. Most plants prefer a slightly acid soil environment, about pH 5.5 to 7.5. e. pH has a profound effect on availability of nutrients. 1) When pH is too high, for instance, iron becomes less available to plants. 2) When pH is too low, molybdenum, for example, becomes unavailable to plants. Phosphorous doesn't like a pH that is either too high OR too low. 3) Even though there may be plenty of these materials in the soil, the pH condition causes them to be bound to the soil instead of in the soil solution, available to plants. 2. Nutrient Levels: Soil provides 14 of the 17 essential chemical nutrients required by plants. a. Nutrients obtained from the soil fall into three categories: 1) Primary macronutrients, used in the largest quantity: nitrogen, phosphorous and potassium (the N-P-K supplied by commercial fertilizers). 2) Secondary macronutrients, also used in large quantity: sulfur, magnesium, and calcium. 3) Micronutrients, used by plants in minute quantities: boron, copper, chlorine, iron, manganese, molybdenum, zinc, and nikel. b. The general nutritional characteristics of soils are determined mainly by the parent material from which the soil originated and the climate the soil was formed by and is subject to. c. Of course, the biological activity surrounding a soil has a profound effect on which nutrients are available in what quantity. d. For a complete discussion on plant nutrients see unit (CLF6350) Plant Nutrition. 3. Salinity: The level of soluble salts in the soil has a strong effect on its ability to support the growth of plants. a. Salts compete with plants for water and for certain nutrients, and at too high a level salts are toxic. b. The sodium ion (Na+), particularly, can have a detrimental effect on soil structure if present at too high a level. Soils high in sodium are termed sodic or alkali soils. 4. Cation Exchange Capacity: Certain clay particles and organic humus provide soil with the ability to hold positively charged ions (plant nutrients such as potassium [K+] and ammonium [NH4+]) and then release them in exchange for other cations. a. This ability is essential if plant nutrition is to proceed successfully. b. Cation exchange capacity (CEC) tends to be lower in acid soils. F. Water Relationships: Water relationships in soils are extremely important where plant growth is concerned, but go beyond the scope of this discussion. See Topic Titles (CLF7409) Soil Moisture Behavior and (CLF7410) Soil Water for excellent coverage of this topic. 1/6/91 MH/clh #%&C