Soil Horizons

1. Soil Horizons Introduction to Agriculture Soil Science

2. Soil Horizons • As soil forms from rock or organic matter, it forms in layers that lie parallel to the earth’s surface • As layers are added they begin to take on different characteristics • Soil profile

3. Soil Horizons • Uppermost layer = O Horizon • Undecomposed organic matter • Decomposed organic matter • Second layer down = A Horizon • Topsoil that contains organic matter along with minerals • Generally darker than the horizons below and may range in thickness from a few inches to a few feet • Third layer (beneath the A horizon) = E Horizon • Topsoil has had all the organic material leached out • Lighter in color because clays and humus have been leached out

4. Soil Horizons • B Horizon, or subsoil • Region where the materials leached from the upper levels accumulate and form • Generally high in clay content • Solum • Beneath the B horizon is the C horizon • Made up of parent material… material from which the soil originated • C horizon not affected by the soil making process • Does not have the properties of the other horizons

5. Soil Horizons • Final horizon = R horizons • Bedrock • The layer upon which the other soil horizons rest

6. Soil Ecosystem • Ecosystem = all of the plant and animal life that lives in an area • Life forms depend on each other for the proper balance of food and other environmental factors • Includes many different forms of plant, animal, and microbial life

7. Plant Life • Depend on soil for existence… soil supports root systems and supplies nutrients • Roots may reach as far as several feet into the soil • Alfalfa roots have been known to grow 25 feet or more into the ground

8. Plant Life • Rhizosphere • Is the zone where the plant receives water and the nutrients it needs

9. Microorganisms • Within the rhizosphere live billions of microorganisms of different types • Many organisms live off the roots of plants • As plants live and grow, the roots are constantly oozing materials out their roots • Oozing material contains protein and other nutrients the microorganisms feed on • When the root cells mature and die, the microorganisms decompose them • When the entire plant dies, decomposition returns the nutrients the plant took back to the soil • CARBON CYCLE

10. Microorganisms • Most abundant are bacteria • In only one teaspoon of soil, as many as 500 million bacteria may live • Several types of bacteria live in a symbiotic relationship with the plants • Common type of symbiotic bacteria are nitrogen-fixing bacteria, RHIZOBIA • Live on the roots of legumes (beans, clovers, alfalfa, peanuts, and peas)

11. Microorganisms • Live in lumps on the roots of nodules… bacteria is living in the soil, and when a new plant starts, attach themselves to the root hairs

12. Microorganisms • In reaction to the bacteria, the roots form nodules • The bacteria receives all the nutrients they need to live/reproduce from the host plant… in return, the bacteria convert nitrogen from the air in the soil into a plant-usable form • Often rhizobia is introduced into fields where legumes have not grown before

13. Microorganisms • Fungi are plant-like organisms that contain no chlorophyll • Range in size from microscopic to large mushroom fungi • Grow on the surface of soil or on decaying plant material • Play an important role in breaking down and decay of plant material • Particularly important in forest soils because they breakdown lignin (a primary component of wood)

14. Microorganisms • Protozoa are one-celled organisms that live in moist soil • Aquatic organisms that live in the particles of water in soil • In dry soil, become inactive until the soil is moist again • Feed on bacteria in the soil and help to maintain a better balance of bacterial life in the soil

15. Microorganisms • Nematode • One of the most important microscopic animal groups • Are worms in class of Nematoda • Smooth round bodies are not segmented • Very abundant… in one spade-full of soil, there may be more than a million nematodas • Most abundant multi-celled animal in the soil

16. Microorganisms • Three basic groups of nematodes: • Those that consume decaying organic matter • Those that eat other microorganisms • Those that are plant parasites • Use a needle-like projection on the front of their bodies to open a tiny hole in the plant roots • Suck out the contents of the cell and damage the plant, and expose it to disease

17. Macroorganisms • Organisms that are visible to the unaided eye • Earthworms • Enhance the soil by burrowing through the layers of soil • Burrows allow passage of air and water deep into the soil • Some types of earthworms drag organic matter into the soil and the richness of the soil is enhanced • Castings

18. Macroorganisms • Insect species • Some live their entire lives in the soil, other just a portion • Grub worms • Larval stage of beetles that laid their eggs in the soil • Most grubs feed on plant roots and some species are very destructive

19. Macroorganisms • Larger animals • Dig in the ground to make nests or to find food… in doing so they open up passages for air and water to get into the soil and also turn the soil over so that organic matter from the surface is carried down below • Some are very destructive to plants growing in the soil • Others are beneficial because of the large number of insects they consume • Examples of larger animals present in a soil ecosystem – prairie dogs, moles, armadillos, groundhogs, chipmunks, shrews

20. Caddy Shack Gopher Montage http://www.youtube.com/watch?v=U4xuZMpmXtc&feature=related