Soils 205
Lecture 1

Video Sessions                                                Pages in Text.

Soil as a Natural Body

Terminology

The soil vs. a soil-
If "the soil" is thought of like a forest, "a soil" would be an individual tree in that forest

Soil, Regolith, and Bedrock:

Regolith- unconsolidated layer above hard, unweathered, bedrock

Solum- upper portion of the regolith that has been altered through biochemical and physical processes.  The material between the solum and bedrock is referred to as the C horizon.  It is slowly changing into solum.

Pedon- a 3-D sampling that displays the full range of properties that are characteristic of a soil (1-10 m²).

Soil Profile- one vertical face of a pedon. 

Soil Horizons- horizontal layers, differentiated by color or texture, described within a profile

A, B, and C horizons                a profile is one face of a  pedon (3-D)
in a soil profile

Master Horizons- this is a brief introduction, we will return to the master horizons near the end of the course

The Functions of Soil

1. Medium for Plant growth

- provides anchorage

- ventilation, soil pores allow CO2, formed through root respiration escape to the atmosphere and O2 to be  replenished

- absorption and storage of water

- temperature modification

- supplies nutrients

            Essential Elements

            C, H, O - supplied mostly from air and water

            N, P, K, Ca, Mg, S, Fe, Mn, B, Zn, Cu, Cl, Co, Mo, Ni - supplied by the soil

            C. HOPK_iNS CaFe M_anag_ed B_y M_in_e CouZ_in MoNi_que Cl_ark

_{2.  Regulator of water supplies}

_{There is a growing concern over the quality of the nations water supplies and increasing awareness of the relation between soil and water quality.  Remember that almost every drop of water in our river, lakes, estuaries and aquifers has traveled through or over the soil at some point in time.}

_{- storage of water}

_{- water purification/filtration}

_{3.  Recycler of raw materials}

_{- soil processes take great quantities of organic waste and recycle it to plant available forms.  The recycling of plant material (decomposition) returns C in the form of CO2 to the atmosphere.  In this capacity, soils play a large role in global cycles and are important to issues such as the Greenhouse Effect.}

4.  Habitat for soil organisms

- a handful of soil may be home to billons of organisms belonging to a thousand species

- soils harbor much of the world's genetic diversity

- provides niches

5.  Engineering medium

Reliable construction requires knowledge of the diversity and variability of soil properties across time and space.

Composition of Soil

Mineral-

Consists of sand (0.05 - 2 mm), silt (0.02-0.05 mm), and clay (<0.02 mm) sized particles.  The mineral fraction includes silicate clays, Fe/Al oxides, and primary minerals such as quartz, feldspars, and micas.

    Primary minerals-

    1.  Quartz- SiO₂- the most common primary mineral.  It is very resistant to breakdown and is nonreactive

    2.  Feldspars-

            Orthoclase- KAlSi₃O₈
            Anorthite- CaAl₂Si₂O₈
            Albite- NaAlSi₃O₈

   3.  Micas

                Muscovite- KAl₃Si₃O₁₀(OH)₂
              Biotite- KAl(Fe,Mg)₃Si₃O₁₀(OH)₂

    Secondary minerals- these minerals are the weathering products of primary minerals

            Hematite            Fe₂O₃
            Calcite              CaCO₃
            Gypsum            CaSO₄
            Clay minerals    Al silicates (kaolinite, smectites, vermiculite, etc.)

    Click here to view the 3-D structures of some of the minerals listed above:

Organic-

Contains a wide range of C-containing substances including living and dead organisms and organic compounds from metabolism.  Small %, but it has a large effect on soil properties.

Soil Water-

Held in pores with varying tenacity (depends on the size of the pore) and contains hundreds of dissolved organic and inorganic substances.  Please note:  the total pore space in a soil averages 50 %.  In the figure above, the pore space is half filled with water and half filled with air (this is an ideal situation).  There is a finite amount of pore space, therefore as the water content increases, the amount of air decreases-- There is an inverse relationship between soil water and air content.

Acidity- the H⁺ concentration in soil water is what is measured with a pH me

Acidity (H⁺) or alkalinity (OH^-)

* pH = -log [H⁺] = log(1/[H⁺])

[H⁺] = concentration (activity) of H⁺

* H₂O ¬ ¾ ® H⁺ + OH^-

neutrality is pH = 7
(H⁺ = OH^-)

* as acidity ­ , pH ¯

* pH range in soils

Soil Air-

-very different that the atmosphere; it is variable due to the gases produced and consumed by plant roots and microbes.

-relative humiditity ~100%

-higher CO₂ and lower O₂ concentrations than found in the atmosphere

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