Soils

Mr. Smith

Components of the soil profile

•  The "A" horizon

•  The "B" horizon

•  The "C" horizon

Characteristics of the soil horizons

• The "C" horizon– Generally called parent material– It is the deepest of the three major horizons– Usually very low in organic matter– Usually no structure

Characteristics of the soil horizons

• The "B" horizon– Generally called the subsoil– Usually lower in organic matter and lighter in

color than "A" horizon– Usually red or yellowish in color– Structure is less desirable than the "A" horizon;

it may have a blocky or prismatic structure– Frequently heavier texture than "A" horizon

Characteristics of the soil horizons

• The "A" horizon– It includes the upper part of the profile in which

life is most active; generally called the topsoil– It is the most productive horizon because of its

normally high organic matter content and is usually dark colored

– May be from a few inches to a foot or more deep

– Lighter in texture than the "B" or "C" horizons– More likely to have granular structure than the

other horizons

Importance of soils

• Plants grow in and on soil• Plants support animal life• Plants and animals support human life• World population is rapidly increasing

and/or has inadequate nutrition• Supply of productive soil is limited• Improved soil management could feed

more people

Function of soil as related to plant growth, development and maintenance

• Media for seed germination• Media for support of plants• Storehouse of plant nutrients• Storehouse of water for the plant

Soil composition

Soil composition

• Solids--Approximately 50%– Mineral matter– Organic matter– Living organisms

• Pore space--Approximately 50%– Water– Air

Factors affecting soil formation

• Parent materials – Residual

• Igneous--Derived from molten materials in the center of the earth's crust (granitic, basaltic)

• Metamorphic--Formed from the pre-existing rocks through the action of extreme heat and pressures (quartzite, schist)

• Sedimentary--Formed from sediments deposited by wind, water or ice (shale, sandstone, limestone)

Factors affecting soil formation

• Parent Material– Transported

• Wind (loess)• Water (alluvial)• Glaciers (glacial drift)• Gravity (colluvial)

Factors affecting soil formation

• Decomposition by weathering– Physical weathering

• Wind• Plants and animals• Heating and cooling• Freezing and thawing• Wetting and drying

– Chemical weathering--Chemical reactions of water, oxygen and carbon dioxide

– Biological weathering--Micro-organisms secrete a gummy substance which aids in decomposing rocks

Factors affecting soil formation

• Climate– Temperature– Rainfall

• Vegetation and organisms– Plant--Lichens, mosses, weeds, grasses,

shrubs, trees– Animal--Bacteria, fungi, large animals (cattle,

horses, etc.) birds, man

Factors affecting soil formation

• Slope and drainage– Hillsides

• Thin topsoil due to soil loss by erosion• Reduced plant growth• Low organic matter• Less leaching (due to runoff)

– Flat lands• Deeper topsoil• More vegetation• High organic matter• Greater leaching

Explore the physical properties of soil

• A. Soil texture

• B. Soil structure

 • C. Soil depth

 • D. Soil color

Soil particles

• Sand

– Diameter--2.00 to 0.05 mm

– Coarse and gritty

– When moist, individual grains can be seen

– Its presence decreases water-holding capacity

– Its presence decreases nutrient holding capacity

Soil particles

• Silt

– Diameter--.05 to .002 mm

– Its presence increases water-holding capacity

– Its presence increases nutrient holding capacity

– Moderate to high exchange capacity

– Feels smooth and velvety

Soil particles

• Clay

– Diameter--less than .002 mm

– Its presence increases water-holding capacity

– Its presence increases nutrient holding capacity

– High to very high exchange capacity

Determine soil texture

• Mechanical analysis

– A mechanical analysis of a soil reports the percentage of each of the soil particles (sand, silt and clay)

– Percentages can be applied to the texture triangle to determine the texture of a soil

Determine soil texture

• Feel method– Texture is determined by moistening the soil

and rubbing between thumb and fingers– The wet sample is worked into a ball and

placed between thumb and index finger, the thumb is pushed gradually forward in an attempt to form the soil into a ribbon (clayey soil)

– If the wet sample will not form a ribbon, evaluate for grittiness (sandy soil)

– Evaluate wet sample to determine if it feels velvety and slick, but will not ribbon (silty soil)

Soil Texture

• Descriptions of soils of different texture using the feel method– Sandy soil

• Coarse and gritty• When moist, individual grains can be seen• Called a "light" soil

– Silty soil• Feels smooth, flowing when dry• Feels velvety or slick when wet

– Clayey soil• Sticky and will form a ribbon when wet• Very hard when dry• Called a "heavy" soil 

Soil Structure

• Granular (sphere shaped) -- Ideal for plant growth

Soil Structure

• Blocky (sharp and angular faces)– Water storage good– Circulation of air and water is poor

Soil Structure

• Platy (flat, horizontal, plate-like)--Poor permeability

Soil Structure

• Prismatic and columnar (column-like)--Poor air-water relationship

Soil Structure

• Massive

Color and importance

• Color is an important characteristic used in the identification of soil conditions that affect the value of land for agricultural uses

• Influenced mainly by organic matter content; benefits of organic matter include– Makes soil porous – Supplies nitrogen and other nutrients to the

plant– Holds water in the soil– Reduces leaching– Improves soil structure

Soil colors

• Dark brown to black--Regarded as the most productive; usually contains a higher organic matter content

•  Red or reddish brown--Usually less fertile than black or dark brown soils; may contain a high iron content

 • Yellow or gray--Usually caused by

imperfect drainage

Acidity or alkalinity

• The acidity or alkalinity of the soil solution is determined by the relative number of hydrogen (H+) ions and hydroxyl (OH-) ions

• When a soil solution contains more H+ ions than OH- ions, it is acidic.

• When the OH- ions are more abundant, the solution is alkaline.

• A neutral solution has an equal amount of H+ and OH- ions

pH

• pH ranges on a scale from 1 to 14– 1 to 7--Acidic soil – 7--Neutral soil– 7 to 14--Alkaline soil

Liming soils

• Soil acidity can be corrected by adding lime to the soil.

• The function of lime is to neutralize the hydrogen (H+) ions that cause soil acidity

Liming soils

• Amount of lime to apply depends on– The degree of acidity of the soil– The crops to be grown– The grade or purity of the lime materials– The frequency of application– Soil texture– Soil exchange capacity

Types of alkali soils

• Saline--Soils in which there has been an accumulation of soluble salts, for example: NaCl (table salt); these are referred to as "white alkali" soils

 • Sodic--Soils in which there has been an

accumulation of sodium (Na); sodium affected soils have low permeability to water

• Saline-sodic--Both salty and sodic affected

Reclamation of alkali soils

• Saline– Flood with water and leach out salts– Install drainage tiles to remove accumulated

salts– Grow salt tolerant crops

• Sodic– Apply gypsum (CaSO4)– Grow sodic tolerant crops

• Saline-sodic– First correct sodic– Then correct salt problem