- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7400) Unit Title: SOILS ____________________________________________________________________________ (CLF7406) Topic: ORGANIC MATTER Time Taught in Year(s) IN THE SOIL 2 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (G-15) - Identify sources of organic matter in the soil and list ways they benefit the soil. Special Materials and Equipment: References: Donahue, Roy L. SOILS. (1983). Pages 139 to 162. Brady, Nyle C. THE NATURE AND PROPERTY OF SOILS. Pages 254 to 281. Evaluation: Quiz by instructor TOPIC PRESENTATION: ORGANIC MATTER IN THE SOIL A. Soil organic matter influences the chemical and physical properties of soils far out of proportion to the small amounts present. 1. Most of the life in the soil is dependent on organic matter for energy and nutrients. 2. The accumulation of organic matter reaches an equilibrium in non-tilled soils. 3. The organic matter content of soils can be temporarily increased, thus benefiting the soils in our farming and gardening enterprises. B. Soil organic matter is made up of living and dead plant and animal residue. This includes insects, earthworms and other macro and microorganisms. 1. Most organic matter comes from plants. a. By weight about 90 percent of plant material is made up of carbon, hydrogen and oxygen. b. The moisture content of plant residues varies from 60 to 90 percent, 75 percent being a good average. c. Other elements found in plants include nitrogen, sulfur, phosphorous, potassium, calcium, and magnesium plus micronutrients. d. These elements, though small in amount, play an important role in plant nutrition and meeting requirements for soil microorganisms. _________________________ | | { | | { | | { | | 75% water { | | { | | { | | { | | { | | { | | { | | { | | { |_________________________| { | Carbon 11% | { | | { |_________________________| 25% solids { | Oxygen 10% | { |_________________________| { | Hydrogen 2% | { |_________________________| { | Ash 2% | { |_________________________| 2. Animals are usually considered as secondary sources of organic matter. a. They consume the original plant tissues, then leave their waste products and bodies as they die. b. Some animals (earthworms, centipedes, and ants) play an important role in the movement of plant residues. 3. Bacteria, fungi, and other microorganisms constitute the bulk of soil organisms. a. They aid in the breakdown of complex plant and animal residues. b. A portion of the organic matter in the soil comes from these organisms as they die and their bodies decompose. C. After extensive decomposition of organic matter a complex, amorphous, colloidal substance called humus remains in the soil. 1. It is resistant to further decomposition. 2. This is the material that helps to improve soil structure. 3. This is what imparts a dark color to the soil containing it (usually the top 10 inches). 4. It increased the soils water-holding capacity. D. When one uses organic matter for improving soils we must be aware of the effect this added material will have on the carbon : nitrogen ratio. 1. There is a close relationship between the amount of carbon and nitrogen in the soil. 2. Under natural conditions this ratio is fairly constant at 12 parts carbon to 1 part nitrogen. 3. When incorporating high nitrogen material into the soil it will change the soils c:n ratio. 4. Soil organisms (fungi and bacteria) multiply rapidly. 5. These organisms need large amounts of nitrogen for their own growth. 6. As the available soil nitrogen is used by these microorganisms, a temporary nitrogen deficiency develops for the current crop. 7. Finally, as most of the added organic matter is decomposed, the food supply for the microorganisms is exhausted, the organisms die, and some of the nitrogen contained in their bodies is again available to the crop. E. The benefits of having organic matter in the soil are varied and extensive and depend on the composition of the organic matter. 1. Organic matter is the source of from 90 to 95 percent of nitrogen present in unfertilized soils. 2. It is the major source of available phosphorous and sulfur. 3. It supplies the major soil aggregating cements. 4. It contributes to the cation exchange capacity of the soil. 5. Organic matter increases a soils water holding capacity. 6. It increases both air and water flow rates through the soil. 7. It increases the availability of micronutrients to plants by its chelating characteristics. 8. Organic matter furnishes the carbon supply for organisms that perform beneficial soil functions. 9. As a mulch, organic matter reduces erosion, shades the soil, prevents rapid water loss, and keeps the soil temperature more even (cooler in summer, warmer in winter). F. There are some detrimental effects of soil organic matter. 1. Organic matter is an energy source for many disease organisms, ensuring their survival in soils. 2. Excessive amounts of plant residues are difficult to incorporate into the soil and hinder some planting operations. 3. Some plants contain phytotoxins (plant poisons) which make them undesirable as organic matter. 4. The addition of organic matter into a soil may temporarily increase the carbon: nitrogen ratio and thus limit the available nitrogen to the current crop. 7/25/90 sg #%&C