Soil and Its Uses

Chapter 13

Outline:

• Land or Soil ?

• Soil Formation

• Soil Properties

• Soil Profile

–Horizons

• Erosion

• Soil Conservation

Soil vs. Land

• Land - Portion of the world not covered by water.

• Soil - Mixture of minerals, organic material, living organisms, air, and water.

Soil Formation

• Parent Material - Ancient rock layers with recent deposits from lava or glacial flows.

• Weathering - Factors bringing about fragmentation or chemical change of parent material.

• Mechanical Weathering - Temperature changes and abrasions are primary agents. (Chain Gang)

• Chemical Weathering - Small rock fragments exposed to atmosphere may oxidize, or otherwise chemically change when exposed to other elements.

• Humus - Decaying organic material.

–Humus becomes mixed with the top layers of rock particles, and supplies some of the needed nutrients to plants.

–Humus also creates crumbly soil which allows adequate water absorption and drainage.

• Other factors influencing soil formation:

–Earthworms

–Burrowing animals

–Plant roots

–Bacteria and Fungi (decomposers)

Soil Properties

• Texture - Determined by the size of soil particles.

–Too many large particles leads to extreme leaching.

–Too many small particles leads to poor drainage.

• Structure - Refers to the way various soil particles clump together.

– In good soils 2/3 of the intra-soil spaces contain air after the excess water has drained.

• Friability - Soil’s ability to crumble

Soil Profile

• Soil Profile - A series of horizontal layers of different chemical composition, particle size, and amount of organic matter.

• Horizon - Each recognizable layer of the profile.

• A Horizon - (Topsoil) Uppermost layer - contains most of the soil nutrients and organic matter.

• B Horizon - (Subsoil) Underneath topsoil. Contains less organic matter and fewer organisms, but accumulates nutrients leached from topsoil. - Poorly developed in dry areas.

• C Horizon - Weathered parent material - very little organic material.

• Mixtures ?

–Over 15,000 separate soil types have been classified in North America. However, most cultivated land can be classified as either grassland or forest soil.

• Grassland Soils - Usually have a deep A Horizon - low rainfall limits topsoil leaching. A horizon supports most root growth.

• Forest Soils - Topsoil layer is relatively thin, but topsoil leachate forms a subsoil that supports substantial root growth. (High rainfall areas)

Soil Erosion

• Erosion - Wearing away and transportation of soil by wind and water.

–Worldwide removes 25.4 Billion tons/yr–Made worse by deforestation and

desertification.–Poor agricultural practices increase erosion

and lead to the transport of associated fertilizers and pesticides.

Soil Erosion - cont.

• Most current agricultural areas lose topsoil faster than it can be replenished.

• Wind erosion may not be as evident as water erosion, but is still common.–Most common in dry, treeless areas.–Great Plains have had four serious

bouts of wind erosion since late 1800’s.

Soil Conservation Practices

• When topsoil is lost, fertility is reduced or destroyed, thus fertilizers must be used to restore fertility.–Raises food costs, and increases

sediment loads in waterways.–Over 50% of US land is suitable for

agriculture, but only 2% does not require some form of soil conservation practice.

• Contour Farming - Tilling at right angles to the slope of the land. Each ridge acts as a small dam.

• Strip Farming - Alternating strips of closely sown crops to slow water flow.

• Terracing - Level areas constructed at right angles to the slope to retain water - good for very steep land.

• Windbreaks - Planting of trees or other plants that protect bare soil from the full force of the wind.

Tillage

• Plowing has multiple desirable effects:–Weeds and weed seeds are buried /

destroyed.–Crop residue is turned under - decays faster

and helps build soil structure.–Leached nutrients brought closer to surface.–Cooler, darker soil is brought to top and

warmed.

Problem:

• Each trip over the field is an added expense to the farmer, and at the same time increases the amount of time the soil is open to erosion via wind or water.

• Conservation Tillage :

–Plowing followed by secondary tilling

–Strip Tillage

–No-Till Farming

• All of these require less fuel and less time - but may require more herbicides.

SUMMARY

• Soil is an organized mixture of minerals, organic matter, living organisms and air.

• Soil Profiles - Horizons A-C

• Erosion is the removal of soil via air or water.

• Soil conservation techniques can reduce erosion.