What is soil?

What is soil?

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

• 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

• 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, 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.

Three phase system – air, water, rock

Unsaturated System

Zone of AerationVadose Zone

Unsaturated Zone

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• 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)

InfiltrationThe process of water entry into the soil

Partitions water at surface between storm runoff and recharge

Infiltration ratevolume (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.

Plants depend on infiltrated water

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

decreases

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• 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

Physical weathering breaks rocks into small mineral particles.

Chemical weathering dissolves and changes minerals at the Earth’s surface.

Decomposing organic material from plants and animals mixes with accumulated soil

minerals.

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• 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.

Parent material (bedrock) undergoes weathering to become regolith (soil + saprolite).

Soil is a mixture of mineral and organic matter lacking any inherited rock structure.

Soil

Saprolite is weathered rock that retains remnant rock structure.

Saprolite

Saprolite

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

• 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 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 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 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)

Infiltration Rates in Soils

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.

if more than 2-3 inches thick – probably been plowed

typically present only in forests

light colored, leached horizon

:

Yellowish brown to strong brown color

zone of accumulation

Iron bearing leached from above and precipitated in B

IDEAL SOIL HORIZON

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

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.

Follow Soil Pit Technique directions.

Finding and Describing Horizons

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

Recognizing Soil Texture & Structure

Terminology• 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

Terminology

• Clay – smallest individual soil particle

• Peds – natural grouping of soil particles

• Clods – artificial grouping of soil particles

• Infiltration – movement of water into the soil

Terminology• 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 = 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

Texture

• 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• 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)

• Clay Loam (Moderately Fine)

• Silty Clay Loam (Moderately Fine)

• Sandy Clay (Moderately Fine)

• Silty Clay (Fine)

• Clay (Fine)

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…

• 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 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 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…

• 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!• 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

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

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

Consistence

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

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

Soil aggregates developed along the horizontal direction: flaky

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

Vertically oriented, round-topped structural prisms – rounded caps

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

• 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

Size of individual ped

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

• 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 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

• Leave it alone. Given enough time nature will repair damaged structure

• Planting green manure crop

• Incorporate plant residue into the soil

Soil Taxonomy and

Taxonomic Names

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

Soil Orders

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

Inceptisols -- young, weakly weathered. Few diagnostic horizons

Soil Orders

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•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

Sandy, siliceous, hyperthermic Aeric Alaquods

State Soil of Florida: Myakka Series

Surface layer: gray fine sandSubsurface layer: light gray fine sandSubsoil: dark reddish brown fine sand with organic stainsSubstratum: brown and yellowish brown fine sand

http://soils.usda.gov http://www.fao.org

Soil Orders

Entisols

Histosols Inceptisols Andisols Gelisols

Aridisols Vertisols

Alfisols Mollisols

Ultisols Spodosols

Oxisols

slight

Strong

Weathering anddevelopment

The Florida state soil is a spodosol: Myakka fine sand

Spodosols 8.4 million acres

Entisols 7.5

Ultisols 6.9

Alfisols 4.6

Histisols 4.0

Inceptisols 1.0

Mollisols 1.0

Extent of Florida Soil Orders

Myakka fine sand

Apopka loamy, siliceous, hyperthermic grossarenic paleudult

The last syllable in the taxonomic name indicates the soil order

Ledwith fine, smectitic, hyperthermic mollic albaqualf

Taxonomy

-ent -oll -od -ept

Order

Suborder

Great group

Sub group

Family

Series

12

19,000

Soil Taxonomy

Soil forming processes / diagnostic horizons

Genetic similarity: Wetness, climate, vegetation63

250

1400

8000

Wet

Dry

Moisture Conditions

Aquic – poor aeration, reduced ironUdic- dry < 90 total days

Ustic - limited but is present Aridic- moist <90 total daysXeric - dry

cold

Hot

Temperature Conditions

Cryic – icy coldFrigid – lower than 8oCMesic – between 8 and 15oCThermic – between 15 and 22oCHyperthermic - > 22oC

Order

Suborder

Great group

Sub group

Family

Series

Family

Suborder

Suborder Examples

Suborders

moisture, diagnostic horizons

Aquod very wet spodosolUdult wet ultisolUdoll wet mollisolXeroll dry mollisolPsamment sandy entisolOchreptUmbreptAlboll

Order

Suborder

Great group

Sub group

Family

Series

Great Groups

Based on diagnostic horizonsand their arrangements orother features like age, color, texture

Arg - argillic horizon presentPale - oldKand - kandic horizon presentHapl - minimum horizonationquartzi – quartz sandHum - humid

Order

Suborder

Great group

Sub group

Family

Series

Suborder Great Group

Udult paleudult

Aquoll argiaquoll

Udalf paleudalf

Udult hapludult

Order

Suborder

Great group

Sub group

Family

Series

Aquic – poor aeration, reduced ironUdic- dry < 90 total days

Ustic - limited but is present Aridic- moist <90 total daysXeric - dry

Sub group

Expresses the core concept of the great group

Moisture, sandiness, depth, color

Typic hapludultGrossarenic quartzipsamment

Typic (typifies the great group)Arenic (sandy)Grossarenic (deep sandy)Aquic (aquic moisture)Rhodic (red color)

Order

Suborder

Great group

Sub group

Family

Series

Families

Properties important to growth of plant roots

Particle size mineralogy temperature oC

SandyLoamyFine loamyClayey

SiliceousKaoliniticSmectiticOxidic

Frigid < 8Mesic 8-15Thermic 15-22Hyperthermic > 22

Series

Horizon number, order, thickness, texture, structure, Color, Organic matter, pH, accumulations

Mollisol Aquoll Argiaqoll typic Argiaquoll typic argiaquoll Brookston loamy siliceous Cordova

Westland

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

Horizon PropertiesSoil 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: Granular Blocky

Prismatic Columnar Platy

Actual size

Actual size

Actual size

Structureless: Single Grained MassivePencil is 19 cm

See hands for relative size

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.

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

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.

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.

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

The number after the slash is the Chroma.

Horizon Properties

Chroma

Soil Color (continued)

Munsell Notation

Hue Value

Hue Value Chroma

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.

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

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

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).

Horizon Properties

Factors of Soil Formation

Parent Material

Climate

Vegetation

Topography

Time

• Horizons are split or differentiated by changes in color, texture, roots, structure, rock fragments

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

• 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

•Bacteria

•Fungus

•Insects

•Worms

The Biotic (Living) Part of Soil

•In one tablespoonful of soil, there are more bacteria than there are people on the entire planet!

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

Summary

•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.