What is soil?

Merriam-Webster Online Dictionary • Main Entry: 3soil, noun

1 : firm land : EARTH2 a : the upper layer of earth that may be dug or plowed and in which plants grow b : the superficial unconsolidated and usually weathered part of the mantle of a planet and especially of the earth3 : COUNTRY, LAND <our native soil>4 : the agricultural life or calling5 : a medium in which something takes hold and develops

Definitions, Byers, et al., Formation of Soil, 1938*

• Soils are natural media for the growth of plants. They are mixtures of fragmented and partly or wholly weathered rocks and minerals, organic matter, water, and air, in greatly varying proportions, and have more or less distinct layers or horizons developed under the influence of climate and living organisms…Soils are dynamic in character …the product of the action of climate and living organisms upon the parent material, as conditioned by the local relief.

• *Yearbook of Agriculture

Definition, Soil Science Glossary, SSSA, cont’d

• (ii) The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.

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.

Soil Classificationsand; 2 - 0.5mm silt; 0.002 - 0.5mm clay; <0.002mm

Soil Texture =proportion of sand,silt,clay

Soil Texture ->water retentionorganic matterstabilization

O-Horizon• Soil horizons are the

layers in the soil. The top layer is the O horizon. The O horizon is the organic material. It’s only about an inch thick, but it is very healthy. However, it may not always be present.

4 processes contribute to horizon formation:1. transformations: soil components are

modified, destroyed, synthesized

Soil Development

Soil horizons are distinct layers in the soil with particular properties

2. translocations: movement of materials

3. additions: materials added to soil from outside

4. losses: materials are removed from soil

O -- organic material

Soil Master Horizons

A-- contains organic material. Minerals are leached from here and moved downward (eluviation)

B -- contains little organic material. Minerals and clay from above are deposited here (illuviation)

C --unconsolidated rock

E -- leached horizon where secondary minerals and organic matter are removed

Soil Master Horizons

Master Horizon ModifiersBt= accumulation of silicate clays

Bo=accumulation of Fe,Al oxides

Ba=accumulation of organic matter

Soils are classified based on morphological properties: surface horizons, subsurface horizons

Soil Classification: USDA Soil Taxonomy

Presence/absence of particular soil horizons is important for classification

Climate regime

Orders 12Classification Structure

Aridic UstipsammentTypic DurixeralfTypic Duri-xer-alfsub group, great group, sub-order, order(modifiers: texture, mineralogy, climate)

Suborders 63Great Groups 250Sub Groups 1400Families 8000Series 19000 (in US)

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

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 have distinct properties

Dark Gray Luvisol

Orthic Humo-FerricPodzol

Orthic BlackChernozem

US Spodosol Distribution

Soil Reaction

• Soil reaction is the degree of acidity or alkalinity of a soil, usually expressed as a pH value.

• Soil pH = -log [H+]

• Soil pH is an indicator of physical, chemical and biological properties in soil.

• Soil pH is also related to the cations present on the exchange complex.

Descriptive terms for Soil pH ranges

Particle size & pore space

LargeParticle

Pore

2 x 2 x 2 = 8

radius = 4r

16 r

Particle size & pore space

MediumParticle

Pore4 x 4 x 4 = 64

radius = 2r

16 r

Particle size & pore space

SmallParticle

Pore 8 x 8 x 8 = 512

radius = r

16 r

Pores and particles in soil (Pawluk)

Finding and Describing HorizonsSoil 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.

Auger Technique

1. Identify an area where you can dig four holes where the soil profiles should be similar.

3. Assemble a profile of the top 1 meter of the soil by removing successive samples with the auger and laying them end-to-end as follows:

2. Spread a plastic bag, tarp, board, or other surface on the ground next to where you will dig your first hole.

4. Identify each horizon and measure its thickness using the depth of the auger hole.

Finding and Describing Horizons

Surface Sample Technique

1. Use a garden trowel or shovel to carefully remove the top 10 cm of soil from a small area and set it on the ground.

In situations where it is not possible to expose the top meter of soil, another option is to use the top 10 cm of soil as a horizon sample for soil characterization.

2. Treat this sample as a horizon and proceed to characterize its properties.

Finding and Describing Horizons

Soil Has Texture

The way a soil feels when you touch it is called its “texture”.

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

Gritty

A soil that feels “gritty” is made up of sand, the largest pieces of soil.

Soft Soil

A soil that feels soft, silky, or “floury” is made of the medium size soil pieces called silt.

Sticky Soil

The smallest size soil pieces are clay. Clay feels “sticky” and is hard to squeeze.

The amountof sand, silt, and clay tellsus a storyabout the soil.

First, it tellsus howmuch watercan bestored inthe soil.

Second, it tellsus what kind of creatures livesin the soil.

Third, it tells us what kind of plants

can grow.

Next, it tells us how deep

plant roots can grow.

Then, it tell ushow muchwater can passthrough.

Finally, it tells us how

the soil has changed over time.

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

What is a “Ped?”

A ped is a single unit of soil structure. A ped will vary with different types of soil.

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