Soil Science Simplified

Soil Science Simplified

Soils/Land Use Objectives This ppt will cover the following objectives: • U. Recognize soil as an important resource. • V. Describe basic soil properties (and formation factors) • X. Determine basic soil properties and limitations, such as mottling and permeability, by observing a soil pit or soil profile

Why Study Soil? • Great Integrator! – – – – – – – –

Medium of crop production Filter water and waste Producer and absorber of gases Home to organisms Medium for plant growth Waste decomposer Source material for construction, art, medicine, etc. Snapshot of geological, climatic, biological, and human history – Essential natural resource.

Nature and Function of Soil • Soil – the unconsolidated cover of the earth, made up of mineral and organic components, water and air and capable of supporting plant growth. Most important function: GROW PLANTS • Photosynthesis – plant ability to combine CO2 and H2O from the ground into sugar (C6H12O6). Light furnishes the energy for this reaction. Nitrogen, sulfur and phosphorus are required for synthesis of plant proteins too.

• As a medium for plant growth, soil performs four functions: – – – –

Anchors roots Supplies water Provides air Furnishes minerals for plant nutrition

• The pore space between the solids is taken up by water and air. • Air takes up part of the pore space not occupied by water. As the water increases, the air content decreases. In respiration (opposite of photosynthesis), plant roots use oxygen and give off carbon dioxide. So, soil usually contains less oxygen and more carbon dioxide than atmospheric air does. • Millions of microbes live in each ounce of fertile soil. Without them, soils would become inactive and lose their ability to support plants.

Physical Properties of Soils • Soils consist of solid, liquid, gaseous and biotic components. • Sand and silt are merely broken down rock frags (consists of quartz, feldspar, mica, or other minerals). Chemically they are essentially inert compared with clay and organic matter, which are responsible for most of the chemical reactions in the soil.

Soil fraction Gravel

Sand Silt Clay

Diameter Larger than 2 mm 0.05 - 2 mm 0.002 – 0.05 mm Smaller than 0.002 mm

Descriptio n Coarse

Gritty Floury Sticky when wet

• Clay particles are plastic and sticky when wet. They are highly adsorptive of water, gas, and dissolved substances. Clays are minute, plateshaped, aluminosilicate crystals consisting of silicon, aluminum, iron, magnesium, oxygen, and hydrogen. • There are several types of clay. Two of the most important ones are kaolinite and smectite. Smectite clays have the ability to swell on wetting and shrink when dry. Smectite enter chem rxns more than kaolinitic clays.

• Clay is a negatively charged colloid. This negative charge is the reason that positively charged cations surround each clay particle. The individual cations can be exchanged for each other. • If the cations can get close to the surface of the clay, the neg charge on the clay is largely neutralized and the clay particles will cling together or flocculated. Calcium and magnesium are dominant in this area. They are small and effective in holding clay particles together.

Soil Texture • Texture – relative proportion of the various grain sizes in a soil. • To describe soil texture, names, such as loamy sand, silt loam, clay loam and silty clay are used. • The best soils are generally those which contain 10 to 20% clay, with silt and sand in approximately equal amounts, and a fair amount of organic matter.

• The content of sand, silt, and clay for the twelve main soil texture classes can be found on this triangle. Ex. Point A is in the sandy loam texture class with 65% sand, 25% silt, and 10% clay. Not that soils with relatively small clay contents (