Introduction to AgricultureSoil Science

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

Soil Horizons

Uppermost layer = O HorizonUndecomposed organic matterDecomposed organic matter

Second layer down = A HorizonTopsoil 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 feetThird layer (beneath the A horizon) = E Horizon

Topsoil has had all the organic material leached outLighter in color because clays and humus have been

leached out

Soil Horizons

B Horizon, or subsoilRegion 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

Soil Horizons

Final horizon = R horizonsBedrock The layer upon which

the other soil horizons rest

Soil Horizons

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

Soil Ecosystem

Plant LifeDepend 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

RhizosphereIs the zone where the plant receives water

and the nutrients it needs

Plant Life

Within the rhizosphere live billions of microorganisms of different types

Many organisms live off the roots of plantsAs plants live and grow, the roots are constantly

oozing materials out their rootsOozing material contains protein and other

nutrients the microorganisms feed onWhen the root cells mature and die, the

microorganisms decompose themWhen the entire plant dies, decomposition returns

the nutrients the plant took back to the soil CARBON CYCLE

Microorganisms

Most abundant are bacteriaIn only one teaspoon of soil, as many as 500

million bacteria may liveSeveral types of bacteria live in a symbiotic

relationship with the plantsCommon type of symbiotic bacteria are

nitrogen-fixing bacteria, RHIZOBIALive on the roots of legumes (beans, clovers,

alfalfa, peanuts, and peas)

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

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

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)

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

Microorganisms

NematodeOne of the most important microscopic animal

groupsAre worms in class of NematodaSmooth round bodies are not segmentedVery abundant… in one spade-full of soil, there

may be more than a million nematodas Most abundant multi-celled animal in the soil

Microorganisms

Three basic groups of nematodes:1. Those that consume decaying

organic matter2. Those that eat other

microorganisms3. 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

Microorganisms

Organisms that are visible to the unaided eye

EarthwormsEnhance 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

Macroorganisms

Insect speciesSome live their entire lives in the soil, other

just a portionGrub worms

Larval stage of beetles that laid their eggs in the soil

Most grubs feed on plant roots and some species are very destructive

Macroorganisms

Larger animalsDig 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

Macroorganisms

http://www.youtube.com/watch?v=U4xuZMpmXtc&feature=related

Caddy Shack Gopher Montage