PRINCIPLES OF SOIL SCIENCE Soil Science 1 To 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 -4-4-5 1) Medium of plant growth 3) Habitat of soil organisms
4) Recycler of raw materials
5- 4 -4-5
5-4- 4 -5
5 Factors of Soil Formation
Composition of soil by volume
SOM Organic matter encompasses all organic components of a soil:
Decomposing organic matter
Soil Air 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 : Iron Manganese Boron Zinc Copper Chlorine Cobalt Molybdenum Nickel
Macronutrients from soil solids :
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 Exfoliation - the peeling away of outer layers.
2)Abrasion by Water, (Ice), and Wind
Common chemical weathering processes are:
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 Materials Water - Rivers = Alluvium
Wind - eolian = sand or silt (loess)
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 CaCO 3 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 .
Factors that retard soil profile development
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
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 C - little affected by pedogenic processes and lack properties
of O-A-B-E; the Parent Material
Lowercase letter symbols to designate subordinate distinction within master horizons. a - organic matter - highly decomposed 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).
Sand 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.
Quartz often dominant mineral in silt since other minerals have weathered away.
Clay Flat plates or tiny flakes Small clay particles are colloids
If suspended in water will not settle
Clay Pores spaces are very small and convoluted Movement of water and air very slow 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
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 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/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 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
Bulk Density Fine-textured soils have lower bulk densities than sandy soils. Cultivated clay and silt loams = 0.9 – 1.5 Mg/m 3 . Cultivated sandy loams and sands = 1.3 – 1.8 Mg/m 3 .
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/cm 3 BD = weight of dry soil/volume of soil solids + pore space (bulk vol.), g/cm 3
% Pore Space = [1 – (BD ÷ PD)] * 100
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: 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.
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 0 bar -0.33 -15 -100 -10000 Saturated Field Cap Wilt point air dry oven dry AWC
-15 to -100 bar = stages of air dry
Some Points on Water Movement 1) Pore size is one of the most important fundamental properties affecting how water moves 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
Capillary forces are greater in small pores than in large pores.
3) Gravitational and capillary forces act 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 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 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 100
Calculating Soil Moisture The volume of water in a given volume of soil (cm 3 of water per cm3 of soil)
Pv = Vol H 2 0 cm 3 ÷ Vol soil cm 3 x 100
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: a. Gravimetric moisture content b. Volumetric moisture content
c. Depth of soil water if the depth of soil is 1.0 m.
Sample Problems 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
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 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 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:
the brown of maghematite.
Causes of Soil Colors
Other minerals that sometimes give soils distinctive colors are:
Soil 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.
Soil 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