PRINCIPLES OF SOIL SCIENCE

Soil Science 1To my Baby Sister “shannon kamille Matalog”..

Hehe… :D

Definition

• SOIL - A dynamic natural body composed of mineral and organic materials and living forms in which plants grow.

• … dynamic body - this means that its composition and properties change with time.

Definition

• 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

4) parent material over a period of

5) time.

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5 – Functions5 – Functions• 1) Medium of plant growth

• 2) Regulating water

• 3) Habitat of soil organisms

• 4) Recycler of raw materials

• 5) Engineering medium

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4 Major Components4 Major Components• 1) Air

• 2) Water

• 3) Mineral matter

• 4) Organic matter

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4 Soil-Forming Processes4 Soil-Forming Processes• 1) Additions

• 2) Losses

• 3) Translocations

• 4) Transformations

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5 Factors of Soil Formation5 Factors of Soil Formation• 1) Climate

• 2) Living organisms

• 3) Relief

• 4) Parent material

• 5) Time

Composition of soil by volume

• Mineral – 45%

• Organic matter – 5%

• Pore space – 50%Ideal soil: air – 25%

water – 25%

SOM

• Organic matter encompasses all organic components of a soil:– Fresh residues– Decomposing

organic matter

– Stable organic matter

– Living organisms

Soil Air

• Soil has more CO2 but less O2 than the atmosphere.Due to the lag time in diffusing gases into and out

of the soil. Respiring organisms in the soil consume O2 and

produce CO2.

• 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

• 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

• Overgrazing

• Deforestation

• Agricultural activities

• Overexploitation

• Industrialization

Mechanisms of soil quality degradation

• Water erosion

• Wind erosion

• Chemical degradation

• Physical 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 CaCO3

Dolomite CaCO3.MgCO3

Quartz SiO2

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 SiO2

Muscovite KAl3Si3O10(OH)2

Microcline KAlSi3O8

Orthoclase KAlSi3O8

Goethite FeOOH

Hematite Fe2O3

Gibbsite Al2O3.3H2O

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 KAl(Mg,Fe)3(AlSiO3O10)(OH)2

Albite NaAlSi3O8

Hornblende Ca2Al2Mg2Fe3Si6O22(OH)2

Augite Ca2(Al,Fe)4(MgFe)4Si6O24

Anorthite CaAl2Si2O8

Olivine (Mg,Fe)2SiO4

Dolomite (CaCO3.MgCO3)

Calcite (CaCO3)

Gypsum (CaSO4.2H2O)

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:

Carbon

Hydrogen

Oxygen Macronutrients from soil solids:

Nitrogen

Phosphorus

Potassium

Calcium

Magnesium

Sulfur

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).

Physical Weathering

• 1) Temperature

• Exfoliation - the peeling away of outer layers.

• 2)Abrasion by Water, (Ice), and Wind

• 3) Plants and animals

• 4) Unloading

Chemical Weathering

• Common chemical weathering processes are:

1) hydration

2) hydrolysis

3) dissolution

4) carbonation

5) oxidation-reduction

6) complexation

Parent Materials

• 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.

Parent Materials

• Residual - Soil formed from bedrock.

• Transported Parent MaterialsWater - Rivers = AlluviumWind - 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

Climate

• Temperature - Warmer = Faster Cooler = Slower --> Soil

Development • Precipitation - higher rainfall = greater

leaching

• Leaching Zone - determined by location of CaCO3 in the soil profile

• Leaching Index = Pcpt. - Evapotranspiration= the amount of effective rainfall that can cause soil leaching

Topography

• 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.

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Factors that retard soil profile development

• low rainfall• high lime content• high clay content• steep slopes• cold temperature• severe erosion

• low humidity• high quartz• hard rock• high water table• constant deposition• 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.– Exit

• Illuviation – deposition of materials (usually clay and humus) into a horizon.

Organic Horizons– 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 Eluviations - 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 (CaCO3); 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 of Fe• 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

calcification

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 (CaSO4)

z – accumulation of soluble salts

Lowercase letter symbols (cont’d).

Sand• < 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.

Silt

• < 0.05 mm to > 0.002 mm

• Quartz often dominant mineral in silt since other minerals have weathered away.

Clay

• < 0.002 mm

• Flat plates or tiny flakes

• Small clay particles are colloids– If suspended in water will

not settle

• Large surface area

Clay

• 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

Stokes Law

V = kd2

Soil Structure

• Structure – refers to the arrangement of primary soil particles into groupings called aggregates or peds.

Soil Structure

• 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

Particle Density

• 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/m3; g/cm3 or ton/m3

– Quartz, feldspar, micas, and colloidal silicates• Assumed PD of typical mineral soils = 2.65 Mg/m3 .

Bulk Density

• Bulk Density (BD) is the weight of a volume of bulk soil (soil particles + pore space) – BD = Weight/volume (Mg/m3)

• BD - always measured on oven dry soil • BD - changes as the pore space changes • BD of common surface soils = 1.1 - 1.4 Mg/m3

• BD of common subsoils = 1.3 - 1.7 Mg/m3

Bulk Density

• Fine-textured soils have lower bulk densities than sandy soils.

• Cultivated clay and silt loams = 0.9 – 1.5 Mg/m3.

• Cultivated sandy loams and sands = 1.3 – 1.8 Mg/m3.

• Generally, subsoils have higher bulk densities than the surface soils.

Porosity• Porosity is the volume of the pores divided by the

bulk volume.• % Pore Space = [1 – (BD ÷ PD)] * 100

• Water which fills all or parts of the pores is soil water.

• Soil porosity directly influences soil water movement.

PD, BD & Porosity

• PD = Weight of dry soil/volume of soil

solids (particle vol.), g/cm3

• BD = weight of dry soil/volume of soil

solids + pore space (bulk vol.), g/cm3

• % Pore Space = [1 – (BD ÷ PD)] * 100

Soil Aggregates

• Soil aggregates are groups of soil particles that bind to each other more strongly than to adjacent particles.

What influences aggregate stability?

• The stability of aggregates is affected by: – soil texture – dominant type of clay– extractable iron– extractable cations– amounts and type of organic matter – type and size of the microbial population

What influences aggregate stability?

• Expansion and contraction of clay particles

• Calcium ions associated with clay generally promote aggregation

• Sodium ions promote dispersion• Soils with over about five percent iron

oxides tend to have greater aggregate stability

What influences aggregate stability?

• Soils that have a high OM content have greater aggregate stability.

• Additions of OM increase aggregate stability - after decomposition begins and microorganisms have produced chemical breakdown products or mycelia have formed.

What influences aggregate stability?

• Soil microorganisms produce many different kinds of organic compounds, some of which help to hold the aggregates together.

• The type and species of microorganisms are important.– Fungal mycelial growth binds soil particles

together more effectively than smaller organisms, such as bacteria.

• Aggregate stability declines rapidly in soil planted to a clean-tilled crop.

Adhesion & Cohesion

• Adhesion - attraction of water molecules to solid surfaces.

• Cohesion - attraction of water molecules to each other.

Capillary Water

• Capillary water in soils refers to the water in small pores that is connected to a free water surface, or water in a dish, a lake or the water table.

• The smaller the pores, the higher the water will rise above the water table.

• The higher the rise, the tighter the water will be held to soil particles to overcome the force of gravity.

Water Potential

• Soil water potential = amount of work that must be done per unit quantity of water in order to transport a quantity of water from a pool of pure water to the soil water.

Soil Water Classification

• 0 to -0.3 bar = Gravitational

• -0.3 to -15 bar = FC & PWP

• -15 to -100 bar = stages of air dry

• -10,000 bar = oven dry

0 bar -0.33 -15 -100 -10000

Saturated Field Cap Wilt point air dry oven dry

AWC

Some Points on Water Movement

• 1)  Pore size is one of the most important fundamental properties affecting how water moves

through soil. – Larger pores as in sand conduct water more

rapidly than smaller pores in clay.

• 2) The two forces that allow water to move through soil are gravitational forces and capillary forces.

– Capillary forces are greater in small pores than in large pores.

•  3)  Gravitational and capillary forces act

simultaneously in soils.

Capillary action involves two types of attractions, adhesion and cohesion.

Gravity pulls water downward when the water is not held by capillary action.

Thus, gravity influences water in saturated soils.

•  4) Sandy soils contain larger pores than clay

soils, but do not contain as much total

pore space. – Sandy soils do not contain as much

water per unit volume of soil as clay soils.

• 5) Factors that affect water movement

through soil include: texture, structure,

organic matter, and bulk density. – Any condition that affects soil pore size and shape

will affect water movement.

– Examples include compaction, tillage, decayed root channels and worm holes.

Calculating Soil Moisture• Gravimetric

– The mass of water in a given mass of soil (g of water per g of oven dry soil).

• Pw = Percent water by weight or

%MC = (weight of wet soil – weight of oven dry soil) X 100weight of oven dry soil

Calculating Soil Moisture

• Volumetric– The volume of water in a given volume of

soil (cm3 of water per cm3 of soil)

– Pv = Vol H20 cm3 ÷ Vol soil cm3 x 100

• Pv = Percent volumetric

• Pv = Pw x bulk density

Calculating Soil Moisture• Cm of water per depth of soil …. or how

many cm of water are in a specified depth of soil.

• Cm water = Pv x depth of soil

Sample Problem

• A soil sample was taken for soil moisture determination and the following data were obtained:Fresh weight: 40 gOven dry weight: 30 gBulk density: 1.3 g/cm3

• Calculate:a. Gravimetric moisture contentb. Volumetric moisture contentc. Depth of soil water if the depth of soil is 1.0 m.

Sample Problems

• DATA:• Soil Core Volume = 250 cm3 (for each soil

core below)• Weight of soil core at -1/3 bar (FC) = 420 g

(July 4, 2008)• Weight of soil core at -15 bar (PWP) = 350 g• Weight of soil core at present field condition

= 395 g (on July 9)• Weight of Oven dry soil core = 300 g

• 1) What is the Bulk Density? • Answer: B.D. = 300 g/250 cm3 = 1.2

g/cm3 (remember, always use oven dry weight)

• 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 • by volume • 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

• Gravimetric

• Electrical Resistance Blocks

• Neutron Scattering

• Time-Domain Reflectometry

Soil Water Measurement - Water Potential

• Tensiometer

• Thermocouple Psychrometer

• 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

Wet Consistence

• Two attributes of soil behavior are measured:

• Stickiness - the quality of adhesion of the soil material to other objects.

• 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

Soil Color

• HueHue is the dominant spectral color of the rainbow - yellow, reds, orange.

• ValueValue is the relative darkness or lightness.

• Chromahroma is the purity or strength of the color.

Soil Color

• ValueValue is expressed as the numerator of the fraction. • ChromaChroma 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

Fe3+

Well-aerated soils

Soil Colors and Soil Attributes

Soil Color Soil Attributes

Gray, bluish-green

Fe2+

Poorly drained soils

White to gray Accumulation of salts

White to gray Parent material: marl, quartz