Soils

Formation of Soils• Physical Weathering

• Rain, wind, abrasion• Chemical Composition is not altered• Influenced by climate

• Chemical Weathering• Acid action / Dissolution – reaction with carbonic, sulfuric, and nitric acid• Hydrolysis – reaction with water

• Impacts calcium carbonate rock heavily• Oxidation – reaction with oxygen

• Biological Weathering• Influenced by living organisms• Can be physical and/or chemical• Chemiosynthetic bacteria• Respiration from roots• Moss and lichen acids• Roots breaking rock into smaller pieces

• Time scale - Formation of 2.5 cm of topsoil takes 200 - 1000 yrs.

Soil Characteristics

Understand what soil is and how it forms. Compare and contrast the characteristics of different soils. What type do you have around your house?

1) clay = “layer silicates that are formed as products of chemical weathering of other silicate minerals at the earth's surface. They are found most often in shales, the most common type of sedimentary rock.”

2) silt = rock worn into tiny pieces (coarser than clay, but finer than sand). usually 1/20 millimeter or less in diameter

3) sand = quartz or silica worn down over time. grains with diameters between 0.06 mm to 2 mm

4) organic matter (humus)

5) Loam = soil containing a mixture of clay, sand, silt and humus. Good for growing most crops.

Soil Porosity and Permeability

1) Clay - porosity permeability, retention

2) Silt - porosity, permeability, retention

3) Sand - porosity, permeability, retention

4) Organic matter - porosity, permeability, retention

Porosity - volume of water that “fits between” the soil particles

Permeability - rate of flow of water through soil

% retention - how much water is “trapped” by soil

Porosity and Permeability are directly related; when one is high, the other is high as well. % water retention is inversely related to both.

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Soil Texture

Loam - ideal agricultural soil

40% sand (larger - structural support, aeration, permeability)

40% silt (smaller - holds nutrient minerals and water)

20% clay (even smaller - holds nutrient minerals and water)

Plants grown in sandy soils

More susceptible to mineral deficiencies and drought

Plants grown in clay soils

More susceptible to waterlogging, and oxygen depletion (think small pore size, low porosity)

Soil Texture Triangle

Soil Acidity (pH)

pH - concentration of H+ ions

Scale - From 0 (very acidic) to 14 (very basic, or alkaline)

pH of most healthy soils - 4 - 8

pH matters because it affects solubility of nutrient minerals

Aluminum and Manganese are more soluble in low pH - roots sometimes absorb too much (toxic levels)

Soil pH affects leaching - high pH increases leaching of important ions such as K+

Causes of changes - acid rain, decomposition, leaf litter, mining (acid sulfate soils)

Remediation

- Too high pH, add acidic leaf litter

- Too low pH, add lime

Soil Layers• Soils develop into layers – • All layers may not be present due to human activity• O = Organic / Humus. It consists of fragments of leaf litter,

twigs, roots, and other organic material lying on the surface of the soil. This layer is not present in cultivated fields.

• A = Topsoil. It is usually darker than lower layers, loose, and crumbly with varying amounts of organic matter. This is generally the most productive layer of soil. Most roots don’t penetrate below.

• B = Subsoil. It is usually light colored, dense, and low in organic matter.

• C = Weathered Parent Material. The unconsolidated organic and mineral material in which soil forms.

• R = Bedrock. The solid rock that underlies the soil and other unconsolidated material