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discoveryschool com
  • What’s the difference between soil and dirt? Dirt is what you find under your fingernails. Soil is what you find under your feet. Think of soil as a thin living skin that covers the land. It goes down into the ground just a short way. Even the most fertile topsoil is only a foot or so deep. Soil is more than rock particles. It includes all the living things and the materials they make or change.
discoveryschool com cont d
discoveryschool.com, cont’d
  • There is no soil on Mars or Venus. How come? Those planets have plenty of rocks. Mars has windstorms that erode rocks into dust. Venus has an acid atmosphere that cooks rocks into new chemicals. But there's still something missing. Without life, there is no soil. Living things haven't just made a home in the soil on our planet. Life actually made the soil as we know it.
definitions hillel introduction to soil physics 1982
Definitions, Hillel, Introduction to Soil Physics, 1982
  • Soil refers to the weathered and fragmented outer layer of the earth’s terrestrial surface.
  • The soil is a heterogeneous, polyphasic, particulate, disperse, and porous system, in which the interfacial area per unit volume can be very large. The disperse nature of the soil and its consequent interfacial activity give rise to such phenomena as adsorption of water and chemicals, ion exchange, adhesion, swelling and shrinking, dispersion and flocculation, and capillarity.
definition soil taxonomy 2 nd ed
Definition, Soil Taxonomy, 2nd ed.
  • The lower boundary that separates soil from the nonsoil underneath is most difficult to define. Soil consists of horizons near the earth's surface that, in contrast to the underlying parent material, have been altered by the interactions of climate, relief, and living organisms over time. Commonly, soil grades at its lower boundary to hard rock or to earthy materials virtually devoid of animals, roots, or other marks of biological activity. For purposes of classification, the lower boundary of soil is arbitrarily set at 200 cm.
Unsaturated System

Zone of Aeration

Vadose Zone

Unsaturated Zone

Three phase system – air, water, rock

infiltration percolation rates
Infiltration & Percolation Rates
  • The rate that water enters (infiltrates) a soil and then moves through the soil profile (percolates) depends partly on soil structure.
    • Rapid infiltration with granular and loose, single grained structureless soil.
    • Moderate infiltration with block-like and prismatic structure.
    • Slow infiltration with platy and solid, massive structure-less structure.
internal drainage
Internal Drainage
  • If permeability is slow or very slow, water stays in the pore spaces and air cannot enter.
  • Soils with poor internal drainage and aeration are mostly gray with some red or yellow streaks.
  • Soils with fair to good internal drainage and aeration are yellow-brown or reddish-brown with some gray spots (mottlings)

The process of water entry into the soil

Partitions water at surface between storm runoff and recharge

Plants depend on infiltrated water

Infiltration rate

volume (flux) of water per unit area

that enters the soil, per unit time

Infiltration capacity

the maximum rate at which

infiltration can occur under specific

moisture conditions.

When rain hits a dry soil,

surface effects between the soil

and water exert a tension that

draws moisture into the soil

As the capillary forces

diminish with increased

soil-moisture content,

the infiltration capacity


As more water infiltrates,

the amount of water that

can be infiltrated during

the latter stages of a

precipitation event is less

than at the beginning

how soils are formed
How Soils are Formed
  • Soil is formed from rock (Parent material)
    • Rock is slowly broken down or fragmented by
      • Weathering processes
        • Biological
        • Chemical
        • Physical
    • Topography helps control how fast parent material is broken down.
      • Steep slopes – very little or no soil
      • Moderate slope – deep soil formation
soil composition
Soil Composition
  • Soil is composed of four major parts:
    • Mineral particles
    • Organic matter
    • Water
    • Air
  • When organisms die, bacteria and other soil organisms decompose the dead material, returning the nutrient minerals to the soil.
mineral portion
Mineral Portion
  • Comes from weathered rock
  • Constitutes most of what we call soil.
  • Mineral content is determined by the type of parent material.
  • Age of soil affects its mineral content.
  • Older soils are more weathered and have poorer mineral nutrient content.
  • Young soils formed in the area of volcanoes have many essential nutrient mineral ions available.
organic matter
Organic Matter
  • Organic material is composed of:
    • Litter
      • Dead leaves
      • Branches
    • Animal dung
    • Dead remains of plants, animals and microorganisms.
  • Microorganisms (bacteria and fungi) decompose the materials into basic nutrients in the soil
  • Organic matter increases the soil’s ability to retain moisture.
  • Humus is the black or dark brown organic material remains after much decomposition.
water and air
Water and Air
  • Soil has numerous pore spaces around and among the soil’s particles.
  • The pore spaces occupy roughly 50% of a soil’s volume and are filled with varying proportions of water (soil water) and air (soil air).
  • Both are necessary to produce a moist but aerated soil.
soil water
Soil Water
  • Soil water originates as precipitation which drains downward until it reaches the groundwater level.
  • Soil water contains low concentrations of dissolved nutrient mineral salts that enter the roots of plants.
  • Water not bound to soil particles or absorbed by plant roots, percolates through the soil taking the mineral nutrients with it.
soil water1
Soil Water
  • Leaching is the removal of dissolved materials from the soil by water percolating down through it.
  • Illuviation is the deposition of leached material in the lower layers of the soil.
soil air
Soil Air
  • Soil air contains the same gases as atmospheric air, although the are usually present in different proportions.
  • As a result of cellular respiration by soil organisms, there is more carbon dioxide and less oxygen than in atmospheric air.
  • Nitrogen is used by nitrogen-fixing bacteria.
  • Carbon dioxide is turned into carbonic acid and is used in weathering of soil and bedrock.
6 – 12 inches of water are added to the hole

and the time per inch of decline is measured

Time can’t be too long (>60 min) or too short (<3 min)

soil horizons
Soil Horizons
  • A soil profile is a vertical section from the surface to parent material.
Gley soil in soil science is a type of hydric soil which exhibits a greenish-blue-grey soil color due to wetland conditions. On exposure to the air, gley colors are transformed to a mottled pattern of reddish, yellow or orange patches. During gley soil formation (a process known as Gleying), the oxygen supply in the soil profile is restricted due to soil moisture at saturation. Anaerobic micro-organisms support cellular respiration by using alternatives to free oxygen as electron acceptors. This is most often the case when the sesquioxide of iron, ferric oxide is reduced to ferrous oxide by the removal of oxygen. These reduced mineral compounds produce the gley soil color.
light colored, leached horizon

typically present only in forests


zone of accumulation

Iron bearing leached from above

and precipitated in B

Yellowish brown to strong brown color

finding and describing horizons
Finding and Describing Horizons

Soil Pit Technique

Starting from top, observe profile to determine properties and differences between horizons.

Place golf tee or marker at the top and bottom of each horizon to clearly identify it.

Look for: different colors, shapes, roots, the size and amount of stones, small dark nodules (called concretions), worms, or other small animals and insects, worm channels, and anything else that is noticeable.

Soil formed under very dry or arid conditions in New Mexico, USA

finding and describing horizons1
Finding and Describing Horizons

Exposed Profile (Road Cut) Technique

Obtain permission to take samples from the road cut, excavation, or other soil profile exposed by others. Obey any and all safety precautions requested.

Expose a fresh soil face by scraping approximately 2cm off of the vertical surface of the soil profile.

Follow Soil Pit Technique directions.

  • Texture – physical property of soil referring to the relative amounts of sand, silt, and clay; how a soil feels.
  • Sand – largest individual soil particle
  • Silt – medium sized individual soil particle
  • Clay – smallest individual soil particle
  • Peds – natural grouping of soil particles
  • Clods – artificial grouping of soil particles
  • Infiltration – movement of water into the soil
  • Mechanical analysis – process of separating a soil into its various parts to permit study.
  • Mottlings – indication of internal drainage & aeration; soil exhibits spots of color.
  • Percolation – movement of water through the soil.
  • Permeability – characteristic of soil which permits variations in the speed of air & water movement.
soil texture
Soil Texture
  • Soil texture = proportions of sand, silt and clay
  • Property of the soil controlled by the size of individual grains or particles
  • Soil is usually made up of particles of widely varying sizes.
  • Soil texture expresses the average or combined effect of all these grain sizes
  • At the most basic level, soil texture can be determined by feel & described as one of the following:
    • Coarse
    • Moderately Coarse
    • Medium
    • Moderately Fine
    • Fine
specific soil textures from coarsest to finest determined by mechanical analysis
Specific Soil Textures (from Coarsest to Finest) determined by Mechanical Analysis
  • Sand (Coarse)
  • Loamy Sand (Moderately Coarse)
  • Sandy Loam (Moderately Coarse)
  • Loam (Medium)
  • Silt Loam (Medium)
  • Silt (Medium)
  • Sandy Clay Loam (Medium)
specific soil textures continued from coarsest to finest
Specific Soil Textures continued (from Coarsest to Finest)
  • Clay Loam (Moderately Fine)
  • Silty Clay Loam (Moderately Fine)
  • Sandy Clay (Moderately Fine)
  • Silty Clay (Fine)
  • Clay (Fine)
specific soil textures determined by feel
Specific Soil Textures determined by Feel
  • Sand – Dry = no clods. Moist = easily crumbled ball, does not ribbon, does not stain fingers.
  • Loamy Sand – Dry = very weak clods. Moist = easily crumbled ball, does not ribbon or stain fingers.
felt textures cont
Felt textures cont…
  • Loam (most difficult) – Dry = clods slightly hard to break. Moist = forms firm ball, ribbons poorly, poor fingerprint
  • Silt – Dry = clods moderately difficult to break; ruptures suddenly. Moist = Smooth, slick feel; forms firm ball, slight ribbon; good fingerprint.
felt textures cont1
Felt textures cont…
  • Silt Loam – Dry = clods moderately difficult to break; ruptures suddenly. Moist = Smooth, slick feel; forms firm ball, slight ribbon; good fingerprint.
  • Sandy Clay Loam – Dry = clods break with some difficulty. Moist = forms firm ball that dries moderately hard; ½” ribbon.
felt textures cont2
Felt Textures cont…
  • Clay Loam – Dry = clods break with difficulty. Moist = forms firm ball that dries moderately hard; ½” ribbon.
  • Silty Clay Loam – Dry = resembles clay loam, only stickier. Moist = shows good fingerprint; forms firm ball drying moderately hard. ½” thin ribbon.
felt textures continued
Felt Textures continued…
  • Sandy Clay – Dry = clods broken with extreme pressure. Moist = forms very firm ball drying quite hard. Thin, long, somewhat gritty ribbon.
  • Silty Clay – Dry = clods broken with extreme pressure. Moist = forms very firm ball drying quite hard. Thin, long, smooth ribbon.
  • Clay – Dry = clods often can’t be broken. 2”-3” ribbon
feel that soil
Feel that soil!
  • Sand: In the moist condition sand should feel gritty and will be loose and single grained. Squeezed when wet, it will fall apart when the pressure is released
  • Clay: when moist is quite plastic and sticky when wet. When the moist soil is squeezed out between the thumb and fore finger, it will form long flexible ribbons.
  • Silty Soil: when dry and pulverized will feel soft and floury. When wet the soil readily runs together and puddles. When squeezed between the thumb and finger it will ribbon but the ribbon will appear checked and cracked.
soil structure
Soil Structure

The combination or arrangement of primary soil particles into secondary particles, units, or peds (which are separated from adjourning aggregates by surfaces of weakness)

soil aggregation
Soil Aggregation

The cementing or binding together of several soil particles into a secondary unit, aggregate, or granule

**clods are different – they are caused by some disturbance such as plowing or digging


The resistance of a material of deformation or rupture – the degree of cohesion or adhesion of the soil mass

structureless soil
Structureless soil

Soil where the particles of coarse soil fail to cling together, when fine soil breaks into large clods, or when the soil is massive, a single compacted substance

platy structure
Platy Structure

Soil aggregates developed along the horizontal direction: flaky

prismatic structure
Prismatic Structure

A soil structure type with a long vertical axis that is prism shaped, vertical faces are well defined, without rounded caps

columnar structure
Columnar Structure

Vertically oriented, round-topped structural prisms – rounded caps

granular structure
Granular Structure

A natural soil ped or aggregate – have plane or curved surfaces which have slight or no accommodation to the faces of surrounding peds

types of soil structure
Types of Soil Structure
  • Platy
  • Prismatic
  • Columnar
  • Angular Blocky: Block-like – three dimensions of same magnitude
  • Subangular blocky: same as angular except the vertices are more rounded
  • Granular
  • Crumb: similar to granular except the peds are porous
class of structure
Class of Structure

Size of individual ped

good soil structure
Good Soil Structure
  • Good Soil Structure:
  • Necessary for good water penetration into the soil
  • Water holding capacity
  • Ease of working the soil
  • Good root penetration
  • Favorable movement of soil air
  • Availability of plant nutrients
  • Good internal drainage
binding agent in the soil
Binding Agent in the soil
  • Organic matter converted to humus is the chief binding agent for stable soil structure.
  • Continuous cultivation and never plowing under any organic matter tends to destroy structure
soil structure is fragile
Soil structure is fragile.
  • Soil structure can be damaged or destroyed by:
    • Working soil that is too wet
    • Repeated movement of heavy objects or animals over the soil
    • Use of equipment at the same depth of the soil
    • Continual flooding of the soil
improving soil structure
Improving soil structure
  • Leave it alone. Given enough time nature will repair damaged structure
  • Planting green manure crop
  • Incorporate plant residue into the soil
Soil Taxonomy


Taxonomic Names

Soil Orders

Alfisols-- high to med. base saturation. Older landscapes but not extensively leached. Not as weathered as Ultisols

Andisols--volcanic parent material. Non crystalline clays, high organic matter

Aridisols -- Arid environment. Light colored A horizon. Some B horizon development.

Entisols-- young soil, lacking horizon development

Gelisols-- showing freeze/thaw mixing

Histosols-- large organic horizon

Soil Orders

Inceptisols -- young, weakly weathered. Few diagnostic horizons

Mollisols--well developed horizons, high in organic matter and calcium. High base saturation.

Oxisols--highly weathered soil, Fe, Al oxides. Low CEC

Spodosols--leached E horizon. B horizon with organics, Al, Fe oxides

Ultisols --Low base saturation. Weathered soils, but not as much as Oxisols

Vertisols --high shrink/swell clay content. Cracks

soil orders in florida
Soil Orders in Florida
  • Entisols – little development, usually A-C horizons, ochric epipedon
  • Inceptisols – A little more development, Bw horizons, ochric, umbric epipedon
  • Alfisols – Argillic/Kandic horizon (Bt) less than 2 m, base saturation is > 35%
  • Ultisols – Argillic/Kandic horizon (Bt) less than 2 m, base saturation is < 35%
  • Mollisols – Mollic epipedon, dark, high organic matter
  • Spodosols – Spodic horizon (Bh), illuvial O.M., ochric, umbric epipedon
  • Histosols – Organic soil, histic epipedon
State Soil of Florida: Myakka Series

Sandy, siliceous, hyperthermic

Aeric Alaquods

Surface layer: gray fine sand

Subsurface layer: light gray fine sand

Subsoil: dark reddish brown fine sand with organic stains

Substratum: brown and yellowish brown fine sand



soil orders
Soil Orders

Weathering and





Histosols Inceptisols Andisols Gelisols

Aridisols Vertisols

Alfisols Mollisols

Ultisols Spodosols


The Florida state soil is a spodosol: Myakka fine sand

Extent of Florida Soil Orders

Spodosols 8.4 million acres

Entisols 7.5

Ultisols 6.9

Alfisols 4.6

Histisols 4.0

Inceptisols 1.0

Mollisols 1.0

Myakka fine sand


The last syllable in the taxonomic

name indicates the soil order

Apopka loamy, siliceous, hyperthermic grossarenic paleudult

Ledwith fine, smectitic, hyperthermic mollic albaqualf

-ent -oll -od -ept

soil taxonomy
Soil Taxonomy



Great group

Sub group




Soil forming processes / diagnostic horizons

Genetic similarity: Wetness, climate, vegetation






Moisture Conditions




Aquic – poor aeration, reduced iron

Udic- dry < 90 total days

Ustic - limited but is present

Aridic- moist <90 total days

Xeric - dry



Great group

Sub group



Temperature Conditions




Cryic – icy cold

Frigid – lower than 8oC

Mesic – between 8 and 15oC

Thermic – between 15 and 22oC

Hyperthermic - > 22oC


moisture, diagnostic horizons

Suborder Examples



Great group

Sub group



Aquod very wet spodosol

Udult wet ultisol

Udoll wet mollisol

Xeroll dry mollisol

Psamment sandy entisol




Great Groups

Based on diagnostic horizons

and their arrangements or

other features like age, color, texture



Great group

Sub group



Arg - argillic horizon present

Pale - old

Kand - kandic horizon present

Hapl - minimum horizonation

quartzi – quartz sand

Hum - humid

SuborderGreat Group

Udult paleudult

Aquoll argiaquoll

Udalf paleudalf

Udult hapludult



Great group

Sub group



Aquic – poor aeration, reduced iron

Udic- dry < 90 total days

Ustic - limited but is present

Aridic- moist <90 total days

Xeric - dry

Sub group

Expresses the core concept of the great group

Moisture, sandiness, depth, color



Great group

Sub group



Typic (typifies the great group)

Arenic (sandy)

Grossarenic (deep sandy)

Aquic (aquic moisture)

Rhodic (red color)


Grossarenic quartzipsamment


Properties important to growth of plant roots

Particle sizemineralogytemperature oC







Fine loamy


Frigid < 8

Mesic 8-15

Thermic 15-22

Hyperthermic > 22


Horizon number, order, thickness, texture, structure,

Color, Organic matter, pH, accumulations

Order Sub-order G. Group Sub-group Family Series

Mollisol Aquoll Argiaqoll typic Argiaquoll typic argiaquoll Brookston

loamy siliceous Cordova


Horizon Properties

Soil Structure

Soil structure is the shape that the soil takes based on its physical and chemical properties. Each individual unit of soil structure is called a ped. Possible choices of soil structure are:

With Structure:



Actual size

Actual size




Actual size

Single Grained



See hands for relative size

Pencil is 19 cm

Horizon Properties

Soil Color

Munsell Notation

The Munsell code below each color in the GLOBE color chart is a universal notation that describes the soils’ color.

7.5 YR 4/3

The first set of number and letter symbols represents the hue.

F Hue represents the position of the color on the color wheel (Y=Yellow, R=Red, G=Green, B=Blue, YR=Yellow Red, RY=Red Yellow).

Horizon Properties

Soil Color (continued)

Munsell Notation

The Munsell code below each color in the GLOBE color chart is a universal notation that describes the soils’ color.

7.5 YR 4 / 3

The number before the slash is the Value.

The number after the slash is the Chroma.

F Value indicates the lightness of a color. The scale of value ranges from 0 for pure black to 10 for pure white.

F Chroma describes how the “intensity” of a color. Colors of low chroma values are sometimes called weak, while those of high chroma are said to be highly saturated, strong, or vivid. the scale starts at zero, for neutral colors, but there is no arbitrary end to the scale.




Horizon Properties

Soil Color (continued)

Munsell Notation



Soil Color Chart Pages


Horizon Properties

Soil Color (continued)

1. Take a ped of soil from each horizon and note on the data sheet whether it is moist, dry or wet. If it is dry, moisten it slightly with water from your water bottle.

3. Break the ped and compare the color of the inside surface with the soil color chart.

2. Stand with the sun over your shoulder so that sunlight shines on the color chart and the soil sample you are examining.

Note: Sometimnes, a soil sample may have more than one color. Record a maximum of two colors if necessary, and indicate (1) the Main (dominant color) and (2) the Other (sub-dominant color).

Factors of Soil Formation

Parent Material





• Horizons are split or

differentiated by changes

in color, texture, roots,

structure, rock fragments

soil ph
Soil & pH

(Remember pH is a scale of 0-14 that measures acidity)

  • Soil solution has a unique pH
  • Plants grow best at a certain pH
  • Nutrients are not available to plants at certain pH levels
plant ph needs
Plant pH Needs
  • Goldenrod 5.0-7.5
  • Sugar Maple Tree 6.0-7.5
  • Soybean 6.0 and 7.0
  • Apple Tree 5.0-6.5
  • Blueberry Bush 4.0-5.0
some soil organisms
Some Soil Organisms
  • Fungus
  • Worms
  • Insects
  • Bacteria
In one tablespoonful of soil, there are more bacteria than there are people on the entire planet!
soil organisms what can they do
Soil Organisms: What can they do?
  • Mix organic matter from the surface deeper
  • Break down large compounds into plant food
  • Make space in the soil for air and water
  • Soil provides support for plants and is a major site of decomposition.
  • Soil has an abiotic and a biotic part.
  • Tons of Fungus, Bacteria, Insects, and Worms live in the soil.
  • Soil organisms can mix organic matter deeper into the soil, break down large compounds, and make space for air& water in the soil.