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The unconsolidated mineral and organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: 1) climate; 2) macro- and microorganisms, conditioned by 3) relief, acting on
Organic matter encompasses all organic components of a soil:
Decomposing organic matter
Stable organic matter
Soil has more CO 2 but less O 2 than the atmosphere.
Due to the lag time in diffusing gases into and out of the soil.
Respiring organisms in the soil consume O 2 and produce CO 2 .
Soil air always has a relative humidity near 100%.
Respiration releases water which evaporates more slowly in the soil than on or above the soil.
Soil quality - the capacity of soils within landscapes to sustain biological productivity, maintain environmental quality, and promote plant and animal health.
Causes of Soil Quality Degradation
Mechanisms of soil quality degradation
Rocks and Minerals
Mineral – a naturally occurring substance which has a characteristic internal structure of its component atoms and the fairly definite chemical composition and physical properties.
Rocks – extensive mineral bodies, composed of one or more minerals in varying proportions.
Classes of Rocks
Igneous – from molten magma.
Metamorphic – recrystallized in the solid state from heat and pressure.
Sedimentary – formed from particles of other rocks or from solution.
Classification of some igneous rocks in relation to mineralogical composition and rock texture. Rock Texture Light-colored Minerals Dark-colored Minerals Feldspars, muscovite Quartz Hornblende, augite, biotite Coarse Granite Diorite Gabbro Peridotite/ Horblendite Inter-mediate Rhyolite Andesite Basalt Fine Felsite/Obsidian Basalt glass
Some of the More Important Sedimentary and Metamorphic Rocks and the Dominant Minerals Dominant Mineral Sedimentary Metamorphic Calcite CaCO 3 Dolomite CaCO 3 .MgCO 3 Quartz SiO 2 Clays Variable Variable Limestone Dolomite Sandstone Shale Conglomerate Marble Marble Quartzite Slate Gneiss Schist
The More Important Minerals Found in Soils Listed in Order of Decreasing Resistance to Weathering (1) Primary Minerals Secondary Minerals Quartz SiO 2 Muscovite K Al 3 Si 3 O 10 (OH) 2 Microcline K AlSi 3 O 8 Orthoclase K AlSi 3 O 8 Goethite Fe OOH Hematite Fe 2 O 3 Gibbsite Al 2 O 3 .3H 2 O Clay minerals (Al silicates)
The More Important Minerals Found in Soils Listed in Order of Decreasing Resistance to Weathering (2) Primary Minerals Secondary Minerals Biotite K Al( Mg,Fe ) 3 (AlSiO 3 O 10 )(OH) 2 Albite NaAlSi 3 O 8 Hornblende Ca 2 Al 2 Mg 2 Fe 3 Si 6 O 22 (OH) 2 Augite Ca 2 (Al, Fe) 4 ( MgFe ) 4 Si 6 O 24 Anorthite Ca Al 2 Si 2 O 8 Olivine ( Mg,Fe ) 2 SiO 4 Dolomite ( Ca CO 3 . Mg CO 3 ) Calcite ( Ca CO 3 ) Gypsum ( CaS O 4 .2H 2 O)
Weathering of rocks and minerals yields solubilized elements.
Among these are nutrient elements that are essential to plant growth.
Essential Elements Macronutrients Micronutrients from soil solids
Mostly from air and water :
Macronutrients from soil solids :
Iron Manganese Boron Zinc Copper Chlorine Cobalt Molybdenum Nickel
Essential Element, Macro- and Micronutrients
Essential element – required for the normal growth of plants.
Macronutrient – necessary in large amounts (usually 50 mg/kg in the plant) for the growth of plants.
Micronutrient – necessary in only extremely small amounts (<50 mg/kg in the plant) for the growth of plants.
Rocks Weather to Soil
Weathering is the process by which all rocks at the earth's surface get broken down.
Weathering occurs by both chemical (decomposition) and mechanical processes (disintegration).
Exfoliation - the peeling away of outer layers.
2)Abrasion by Water, (Ice), and Wind
3) Plants and animals
Common chemical weathering processes are:
Parent material - the unconsolidated and more or less chemically weathered mineral or organic matter from which the solum of soils is developed by pedogenic processes.
The effect of parent material on a soil include: soil texture, pH, and mineral constituents.
Residual - Soil formed from bedrock.
Transported Parent Materials
Water - Rivers = Alluvium
Wind - eolian = sand or silt (loess)
Gravity = colluvium
Ice = Glacial Drift
Origin of residual materials and soil texture Origin Texture Basalt, andesite, volcanic tuff Clayey soils Granite, coarse sandstones Loamy and sandy Siltstones, fine-grained sandstones Silt loam and silty clay loam textures
Temperature - Warmer = Faster Cooler = Slower --> Soil Development
Leaching Zone - determined by location of CaCO 3 in the soil profile
Leaching Index = Pcpt. - Evapotranspiration = the amount of effective rainfall that can cause soil leaching
Topography modifies the effects of other factors.
Modifies climate by affecting the smoothness of the surface and also the angle at which the soil surface orients towards the sun.
Topography also affects the amount of rainfall that infiltrates in a given parcel of soil .
Factors that retard soil profile development
high lime content
high clay content
high water table
mixing by animals
What happens to a soil with time
Loss of nutrients ( bases) = lower pH or soil becomes more acid
Increase in concentration of iron or soil becomes redder
Increase in clay content or old soils have more clay
Deeper weathering into the parent material
Soil Pedon & Soil Profile
Soil Pedon - the smallest volume of that can be called a soil.
Soil Profile - a vertical section of the soil from the surface through all its horizons, including C horizon.
Soil horizon – a soil layer approximately parallel to the surface with distinct physical and chemical characteristics.
Eluviation & Illuviation
Eluviation – movement of materials (usually clay and humus) out of a horizon.
Illuviation – deposition of materials (usually clay and humus) into a horizon.
O - horizon - organic material (no mineral materials) 1) forest litter 2) organic soil or peat soils, or muck
Oi - undecomposed
Oe - moderate decomp.
Oa - decomposed
Mineral Soil Horizons
A – surface horizons that accumulate O.M.
E - Translocation out - Zone of E luviations -
Leaching out; lighter in color than horizons above or below
B - below an A, E, or O with an accumulation of clay, iron,
humus or carbonates (CaCO 3 ); zones of illuviation
- or alteration of the original parent material, development of
color or structure
C - little affected by pedogenic processes and lack properties
of O-A-B-E; the Parent Material
R - hard rock
Lowercase letter symbols to designate subordinate distinction within master horizons.
a - organic matter - highly decomposed
b - buried soil horizon
e - hemic - mod. decomp. - organic soil
f - frozen soil - permanently frozen, permafrost
g - gleyed soil - gray color due to low O2 - reduction
h - accumulation of illuvial humus
i - slightly decomposed organic matter
k - accumulation of calcium carbonate (CaCO3)
m – an indurated layer, or hardpan, due to silicaion or
n - sodium accumulation p - plowing - only used with A q - silica accumulation - very weathered or old soil r - soft rock - used with C or Cr s – an accumulation of illuvial iron t – accumulation of illuvial clay w - color or structure development (Bw) x - Fragipan - hard, dense layer that developed with time y - gypsum accumulation (CaSO 4 ) z – accumulation of soluble salts Lowercase letter symbols (cont’d).
< 2 mm to > 0.05 mm
Rounded or angular in shape
Sand grains usually quartz if sand looks white or many minerals if sand looks brown,
Some sands in soil will be brown, yellow, or red because of Fe and/or Al oxide coatings.
< 0.05 mm to > 0.002 mm
Quartz often dominant mineral in silt since other minerals have weathered away.
< 0.002 mm
Flat plates or tiny flakes
Small clay particles are colloids
If suspended in water will not settle
Large surface area
Pores spaces are very small and convoluted
Movement of water and air very slow
Water holding capacity
Tremendous capacity to adsorb water- not all available for plants.
Soil strength - shrink/swell affects buildings, roads and walls.
Chemical adsorption is large
Determining Soil Texture – Hydrometer Method
The velocity of settling (V) is proportional to the square of particle diameters (d)
Bigger particles settle more quickly
Density of the water (due to suspended silt and clay) holds up hydrometer
V = kd 2
Structure – refers to the arrangement of primary soil particles into groupings called aggregates or peds.
Granular or crumb structure - often found in A horizons
Platy – E horizons
Blocky, prismatic or columnar – Bt horizons
Massive or single grain – occurs in very young soils
Soil Particle Density (PD) is the weight per unit volume of soil solids .
PD – not affected by pore space
Not related to particle size and arrangement of particles (structure)
Particle densities for most mineral soils = 2.60 – 2.75 Mg/m 3 ; g/cm 3 or ton/m 3
Quartz, feldspar, micas, and colloidal silicates
Assumed PD of typical mineral soils = 2.65 Mg/m 3 .
Bulk Density (BD) is the weight of a volume of bulk soil (soil particles + pore space)
BD = Weight/volume (Mg/m 3 )
BD - always measured on oven dry soil
BD - changes as the pore space changes
BD of common surface soils = 1.1 - 1.4 Mg/m 3
BD of common subsoils = 1.3 - 1.7 Mg/m 3
Fine-textured soils have lower bulk densities than sandy soils.
2)What is the % water by weight at field capacity?
Answer: 420 g - 300 g = 120 g of water
120 g water/300* g soil = 0.4 (* use oven dry weight )
0.4 x 100= 40% water by weight at field capacity
3) What is the % water by volume at field capacity?
Answer : 420 – 300/250 = 0.48
0.48 x 100 = 48% water by volume
Or BD X % water wt. = % water
1.2 X 40% water by weight = 48% water by volume
4) What is the total possible % Available Water-holding Capacity (AWC) by volume? (AWC = FC - PWP)
Answer : (420-350)/250 = 0.28 x 100 = 28% available water
5) How many cm of AWC are in the upper 1 m of soil? cm of soil x % AWC = cm of AMC
Answer: 1 m X (100 cm/m) X 0.28 = 28 cm of AWC in upper 1 m of soil .
6) How many cm of available water are left in the soil at present field condition?
Answer : Field Condition = 395 g and PWP = 350 g;
therefore: (395-350)/250 = 45/250 = 0.18 (%AWC by Vol.) and 0.18 X 100 cm (of soil) = 18 cm of water available in upper 1 m.
In other words, the soil has lost 10 cm of water (28-18) since it was at field capacity.
What Determines Plant Available Water Capacity (AWC) AWC = FC-PWP
Rooting depth a) type of plants, b) growing stage
Depth of root limiting layers
Infiltration vs. runoff (more water entering soil, more will be stored )
Amount of coarse fragments (gravel)
Soil Texture - size and amount of pores silt loam has greatest AWC, followed
by loam, clay loam, silty clay loam.
Soil Water Measurement - Water Content
Electrical Resistance Blocks
Soil Water Measurement - Water Potential
Pressure Membrane Apparatus
Soil Consistence & Consistency
Consistence – the combination of properties of soil material that determine its resistance to crushing and its ability to be molded or changed in shape.
Such terms as loose, friable, firm, soft, plastic and sticky describe soil consistence.
Soil Consistence & Consistency
Consistency – the interaction of adhesive and cohesive forces within a soil at various moisture contents as expressed by the relative ease with which the soil can be deformed or ruptured.
It is determined by the soil’s resistence to penetration by an object.
Blunt end of a pencil or thumbnail
Two attributes of soil behavior are measured:
Stickiness - the quality of adhesion of the soil material to
Plasticity – the ability of the soil material to
change shape continuously under
the influence of an applied stress
and to retain the impressed shape
on removal of the stress.
Causes of Soil Colors
Most soil colors are derived from the colors of iron oxides and organic matter that coat the surfaces of the soil particles.
Subsoil horizons, with little organic matter, often clearly display the iron oxide colors, such as:
yellow of geothite
the red of hematite, and
the brown of maghematite.
Causes of Soil Colors
Other minerals that sometimes give soils distinctive colors are:
manganese oxide - black
glauconite - green
calcite - whitish color
Hue is the dominant spectral color of the rainbow - yellow, reds, orange.
Value is the relative darkness or lightness.
C hroma is the purity or strength of the color.
Value is expressed as the numerator of the fraction.
Chroma is along the bottom, and is the denominator of the fraction.
Chroma is the relative purity or strength of the color, low chromas have dull colors, while high chromas have bright colors.
Example: A color of 10YR 3/2 has a hue of
10YR , a value of 3 , and a chroma of 2 .
Soil Colors and Soil Attributes Soil Color Soil Attributes Brown to black (surface horizon) Accumulation of OM, humus Black (subsoil) Accumulation of Mn Parent material (e.g. basalt) Yellow to reddish Fe 3+ Well-aerated soils
Soil Colors and Soil Attributes Soil Color Soil Attributes Gray, bluish-green Fe 2+ Poorly drained soils White to gray Accumulation of salts White to gray Parent material: marl, quartz