• defn - "soil" - a dynamic natural body, made up of fine materials covering the earth’s surface;
• plants grow in this material;
• it is comprised of both mineral & organic matter
• the mineral component of soils is ultimately derrived from the physical & chemical weathering of bedrock (parent rock)
• this weathered material, along with organic material, is the "parent material" from which soil develops
• soils evolve over time & develop physical characteristics that distinguish them from their parent materials
• the ability of soil to support plant productivity is referred to as "soil fertility"
• soil fertility is a function of its organic material content & clay mineral content
• these clay minerals & organic materials absorb water & key elemental ions

(soil erosion is a worldwide problem - 35% of farmlands are losing soil faster than it can form)

(geographers study the spatial patterns of soil types, and the factors responsible for these patterns)

• one way to characterize soils, is to talk about their profile (a soil profile) a "pedon"
• a pedon is a soil profile extending form the ground surface to the lowest extent of plant roots or to the depth where regolith or bedrock is encountered

(a pedon is imagined as a hexagonal column encompassing form 1 to 10 square meters in surface area forming the basic soil sampling unit)

• a collection of pedons in an area is referred to as a "polypedon"
• is composed of many pedons
• it is the basic soil unit used in preparing soil maps
• a polypedon has distinctive characteristics differentiating it from surrounding polypedons
• each layer in a pedon is referred to as a "soil horizon"
• these are the layers exposed in a pedon; & are roughly parallel to the ground surface
• each horizon (layer) has characteristics distinctly differetn from the layers directly above and below
• the following properties differentiate individual horizons: color, mineral content, soil consistency, texture, structure, porosity, moisture, & operating chemical processes
• "O" horizon - found at the top of the pedon; high in organic content (20-30%) - good water absorbing ability & high in nutrients; derrived from plant and animal material (litter) deposited on the surface - this material is transformed into "humus" - a mixture of decomposed organic debris in the soil - dark in color
• "A" horizon - is usually darker & richer in organic content than lower horizons; contains some humus & clay particles
• "E" horizon - more pale, comprised of coarse sand, silt, and resistant minerals; clays & aluminum oxides are leached (by water) from the E horizon & moved to lower layers - called "eluviation"

("eluviation" - the downward removal of finer particles and minerals from the upper horizons of soil; the > the precip. the > the eluviation)

• "B" horizon - clays, Al, & Fe (and perhaps some humus) accumulate in this layer ("illuviation") - this layer has a reddish or yellowish hue due to presence of mineral & organic oxides

A, E, & B horizons are referred to as the "solum" - are considered to be the true soil of a pedon

• "C" horizon - weathered bedrock or weathered parent material - the "regolith" - no biological activities (plant roots or soil microorganisms) present
• "R" horizon - the bottom of the soil profile; consists of either unconsolidated material or consolidated bedrock of granite, sandstone, limestone, or other rock

• soil color reflects composition & chemical makeup
• red & yellow soils are common in the southeastern United States - because of their high iron oxide content
• the soils of plains states are dark colored because of their high organic content
• lighter colored soils result form higher levels of Al oxides & silicate minerals

• refers to the size & organization of particles in a soil
• individual mineral particles (grains) are referred to as "soil separates"
• particles smaller than 2 mm are considered as part of the soil; particles larger than 2 mm are called pebbles, gravel, or cobbles
• most soils are not composed of a single separate
• sand, silt, & clay, respectively, have different particle sizes
• "loam" - is a mixture of sand, silt, & clay in almost equal proportions, with no one texture dominating
• a sandy loam with clay content < 30% is considered good for farming due to its water-holding characteristics & cultivation ease
• soil texture is important in determining water retention & transmission rates

• refers to the arrangement (architecture) of soil particles
• the "ped" - an individual unit (collection) of soil particles - it determines the structural type that the soil assumes - pore space exists between peds - moisture is stored here

(4) Soil Consistence (soil cohesion)

• reflects a soil’s resistance to manipulation under varying moisture regimes
• wet soils - variably sticky when soils are held between thumb & forefinger
• moist soils - from noncoherent to firm
• dry soil - from loose, to soft, to extremely hard

• pores control the flow of air and water

porosity is increased through biologic activity - plant roots, animal activity, human activities (plowing, adding sand)

• major gaseous constituents in soil are:
• nitrogen (same amounts as in the atmosphere)
• oxygen (less than in atmosphere)
• carbon dioxide (more than in atmos.)
• water present in soil pores is said to be the soil solution
• this water (solution) is the source of nutrients for plants
• "soil colloids" - tiny negatively charged (anions) clay and organic particles in the soil that are important for the retention of cations in the soil
• these cations are critical to plant growth
• these cations are attracted to the negatively charged soil colloids
• if not for the soil colloids, the cations would be leached away & would be unavailable to the plant roots
• cations attach to the surface of the soil colloids through a process called "adsorption"

soil acidity & alkalinity:

• acid soils are created when the soil is rich in hydrogen cations
• an alkaline (or basic) soil is rich in base cations
• a major factor in increasing the acidity of soil is through the addition of acidic precipitation (pH ~ 3.0)
• crops are sensitive to pH’s in soil below 6.0 and therefore must be treated to raise the pH - by adding bases (e.g., calcium carbonate)

three soil-forming factors:

(1) dynamic factors - climatic & biologic

• soil types correspond closely with climate types worldwide
• the regimes of moisture, evaporation, and temperature (which are functions of climate) determine the chemical reactions, organic activity, and eluviation rates of soils
• the organic content of soil is determined by the type of vegetation growing in that soil;
• and by the animal and bacterial activity present
• the chemical makeup of the vegetation present contributes to the acidity or alkalinity of the soil solution

(2) passive factors - parent material, topography & relief, & time

• through slope & orientation, landforms also affect soil formation - steep slopes don’t allow for full soil development - erosion & gravity
• flat slopes have poor drainage - which affects plant growth & plant type present
• slope orientation - south facing slopes are warmer than north facing slopes -affects soil moisture & plant type

(3) human factor

• humans & their advanced farming techniques can act to conserve soil resources - e.g., contour & terrace farming; wind breaks & shelter belts
• all of the above soil formation & management factors require time to operate
• ~ 500 years for a few centimeters of soil to form
• these same soils are lost at a rate of a few centimeters per year
• to solve the problem of soil erosion, international cooperation is needed