- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7400) Unit Title: SOILS ____________________________________________________________________________ (CLF7405) Topic: BENEFITS OF Time Taught in Year(s) SOIL ORGANISMS 5 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (G-14) - List the benefits of earthworms and microorganisms in the soil. Special Materials and Equipment: Potato Dextrose Agar, petri dishes, Erlenmeyer flasks, wax pencil, rubber bands, microscope slides, microscope. References: Donahue, Roy L. SOILS. Pages 113 to 138. Evaluation: Quiz by instructor TOPIC PRESENTATION: BENEFITS OF SOIL ORGANISMS A. Living organisms in the soil include macroorganisms (the larger living things) and microorganisms (very much smaller living things). 1. Soil organisms act both chemically and physically. 2. The kind and amount of soil organisms present in a given soil depend on temperature and soil moisture climate, vegetation, soil pH, and fertility. 3. Not all microbes are beneficial to plants and crop production. Many are detrimental. 4. There are however, many ways in which soil organisms increase crop productivity. 5. Beneficial organisms far exceed the detrimental ones in overall effect. B. Lets consider the ways that soil organisms increase crop productivity. 1. Roots of higher plants are good sources of organic matter. 2. The decomposition of these roots form organic acids and glue-like materials that bind soil particles together. This aids in the formation of aggregates necessary for good soil structure. 3. As roots decompose, open channels are left in the soil, improving aeration and drainage. 4. Many of these organisms also decompose other plant parts, stems, leaves and other crop residues. C. To better understand the activities and interrelationships between these organisms lets look at them individually and study some of the specific effects. 1. Earthworms aerate and stir the soil. a. This allows better water infiltration and root penetration. b. Soil particles and organic matter pass through the worms' digestive tract. Digestive enzymes and grinding action within the animal increase the availability of plant nutrients (especially N,P,K,Ca, and Mg). c. They increase both the size and stability of the soil aggregates. 2. Nematodes are found in almost all soils, often in surprisingly large numbers. a. Nematodes are microscopic, unsegmented worms. b. Some nematodes live on decaying organic matter while others are predaceous (prey on other soil organisms). c. A third group, parasitic nematodes, are particularly harmful to nearly all trees, field crops and vegetable crops. (Note: These are discussed as a disease organism in another unit.) 3. Soil algae are microscopic, chlorophyll-bearing organisms. a. Algae are not important as decomposers but are producers of new photosynthetic growth. b. Under the right conditions (wet and light) they will produce considerable amounts of organic material. c. Some symbiotic algae associate with one of several fungi in forms called "lichens". (They help weather rock surfaces.) 4. Fungi are organisms without the ability to use the sun for energy. a. They live on dead or living plant animal tissue. b. Fungi hasten nutrient recycling. c. The type of organic matter present in a soil determines the particular fungi that prevail. d. Fungi are vigorous decomposers of organic matter and readily attack cellulose, (woody materials) lignins, gums and other complex compounds. e. Fungi also secrete substances that aid in the formation of soil aggregates. f. They continue the decomposition process after bacteria and actinomycetes have essentially ceased to function. 5. Mycorrhizae are fungi that form a mutually beneficial (symbiotic) relationship with plant roots. a. Some form a kind of sheath around the root giving it a hairy, cottony appearance. b. The fungi aid in transmitting nutrients and water to the plant roots. c. They increase the plant seedlings tolerance to drought and high temperature, and resistance to infection by disease fungi. d. The increased nutrient availability from mycorrhizae is thought to be due to the additional absorbing surface provided by the fungi. (It has been calculated to be 10 times that of uninfested roots.) 6. Bacteria are unicellular microorganisms; one of the simplest and smallest forms of life known. a. Their strong suit is their ability to multiply. They can adjust their population quickly in responses to environmental changes. Their numbers in the soil usually exceed all other microorganisms. b. Bacteria, as a group, are responsible for several important enzymatic transformations, nitrogen oxidation, sulfur oxidation and nitrogen fixation. Lets look at them individually: 1) Nitrogen oxidation (nitrification) is the conversion of ammonium ions into nitrates through the activities of certain bacteria. Diagrammatically the bacterial nitrification process would look like this: NH4+ bacteria NO2- bacteria NO3- -----------------> -------------------> Ammonium Nitrate Nitrate (utilized by (Transitory and (utilized by plants) toxic) plants) Nitrification is a great concern to the quality of our environment, because the conversion of NH4+ to N03+ permits movement of N03+ into ground water. When in the drinking water this can cause severe health problems to babies of mammals. The advantage is the higher levels of nitrates in the soil which is beneficial to plants. 2) Sulfur Oxidation: Elemental sulfur is not immediately available to higher plants. It must first be oxidized to the sulfate form. Thiobacillus bacteria bring this about through a complicated series of reactions. 3) Nitrogen fixation: This is the changing of atmospheric nitrogen (N2) to nitrogen compounds. This is done by symbiotic and nonsymbiotic bacteria. 4) Symbiotic bacteria: (Symbiosis is the growing together of two organisms with mutual benefit.) From an agricultural standpoint, this is the most important bacteria capable of fixing free nitrogen from the air. It is accomplished by the rhizobium bacteria. These grow in the nodules or legume roots supplying the legumes with some of its nitrogen. The following crop also benefits when the nodules disintegrate and decompose. 5) Nonsymbiotic bacteria: These bacteria live in the soil independent of higher plants. They have the ability to use atmosphere nitrogen in the synthesis of their own body tissues. A dozen or more of these bacteria have been found but the two most studied are the genera Azotobacter and Clostridium. Azotobacter grows best in aerated soil containing abundant organic matter, whereas Clostridium bacteria are anaerobic and do better in a more acid environment. 7. Actinomycetes are a group of organisms with characteristics intermediate between the simple bacteria and the true fungi. a. They decompose organic matter, especially cellulose and other resistant molecules. b. Like fungi, they secrete non-water soluble gummy substances which aid in the formation of desirable soil structure. c. Recently it has been found that they form symbiotic nitrogen fixing relationships with even more plant families than the Rhizobia bacteria. d. In soils their numbers rank second behind bacteria. e. They have attracted much attention lately because they produce many useful antibiotics. Over 500 have been isolated including streptomycin, auremycin, terramycin and neomycin. __________________________________________________________ ACTIVITY: 1. Have students prepare petri dishes with potato dextrose agar (P.D.A.) and then inoculate them with soil from various locations. Here is a suggested procedure for students to use: a. Prepare agar. To rehydrate medium: 1) Suspend 3.9 grams of potato dextrose agar in 100 ml. of cold distilled water in an Erlenmeyer flask. 2) Heat to boiling to dissolve. Stir to prevent bottom burning. (When the solution clears it is about ready to boil). 3) The medium will have a final reaction of pH 5.6. 4) Pour immediately into petri dishes dividing the medium evenly into 7 separate dishes. 5) Label each dish for identification (wax pencils work well on plastic petri dishes). Five for inoculation and two which are left closed and serve as a contrast to test for contamination. b. After the medium has solidified (cooled) inoculate each of the five petri dishes with a sample of soil from a different location. Use damp topsoil and spread on the top of the agar. Use only a small amount and sprinkle it onto the agar (like you would salt. In your notes record information (what such as the type of soil usedand the location of soil collection. To save space the 5 petri dishes can be stacked and tied with 2 rubber bands. Turn the stacks upside down. This will help prevent drying out as the water will condense onto the agar and continually rehydrate it. c. Place the inoculated petri dishes in a location where they can be kept very warm. (up to 100 degrees F for several days). d. Record the daily changes that take place within the petri dishes. Some observations would include: 1) Any changes in color of the agar or the growth on it. 2) Changes in aroma. 3) Changes in reflectance (shiny or dull) on the agar surface. 4) The variety of colonies. Each type of fungus has spores of specific size, shape and color. Bacterial colonies will generally appear to be smoother and glossier. e. After about 3 days small amounts of the fungus colonies can be transferred to a glass slide and studied under a microscope. A ten power lens will reveal many identifiable characteristics of each type of fungus. 2. Have students collect a damp soil sample and view it under a microscope. Many of the soil microbes that we have discussed will be visible even under low magnification. There are procedures also for extracting nematodes from soil samples and isolating and staining bacteria for identification. 3. Set up a demonstration to show students the various types of soil organisms. (Prepared stained slides of many of these microorganisms are available from several firms.) Some are best shown by stereoscopic microscopes; others by compound microscopes. __________________________________________________________ 7/24/90 sg #%&C