SOILS

Excerpts of the presentations of:

Basic Soil Science &The Soil Resource Web Pages

http://www.soils.agri.umn.edu/academics/classes/soil2125/

SOIL i –

• SOIL - The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.

Soil 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:

Factors • 1) climate (including water and temperature effect

• 2) macro- and microorganisms, conditioned by

• 3) relief, acting on

• 4) parent material over a period of

• 5) time.

Soil – A Product

• soil differs from the material from which it is derived (Parent Material) in many :

• a) physical, • b) chemical, • c) biological, and • d) morphological properties and characteristics.

Soils are Dynamic

• the % air, % water, % organic matter will change with time and space.

• The Soil Ecosystem = all parts of the soil, including biotic and abiotic.

Soils and Plant Growth

• Physical support of plants• Provides water and air• Provides essential elements

– Macro-nutrients = N,P,K,Ca,Mg,S– Micro-nutrients = B,Fe,Mn,Cu,Zn,Mo,Co,Cl

Soil profile and horizons

Organic Horizons– O - horizon - organic material (no mineral materials) 1) forest litter 2) organic soil or peat soils, or muck

• Oi - undecomposed (fibris)• Oe - moderate decomp. (hemis)

• Oa - decomposed (sapric)

Processes of Mineral Soil Horizon Formation

A. Additions - H2O, organic matter,air, soil particles, salt

B. Losses - H2O, organic matter, CO2 , nutrients by plant removal

C. Transformations - Changes to soil structure, development of clay minerals, weathering of minerals to elements, chemical Rx

D. Translocations - movement from one horizon to another of O.M. Clay, Water, Iron, & Nutrients in colloidal size, (very small particles)

Mineral Soil Horizons

• A horizon - surface horizons that accumulate O.M.

• Ap = plowed soil

Ap

EB

Bt

BC

C

E Horizons

• E - (formerly A2) - Translocation out - Zone of Eluviations - Leaching out

• lighter in color than horizons above or below

AEE/B

Bt

BCC

B Horizons-two kinds:

• a) Translocation in - below an A, E, or O with an accumulation of clay, iron, humus (O.M. decomposed) or carbonates (CaCO3)

• b) or alteration of the original parent material, development of color or structure

Bt

C Horizons

• C - little affected by pedogenic processes and lack properties of O-A-B-E- or is the Parent Material

• R = hard rock

C horizon

Solum A + B

Soil Texture• Soil texture is determined by separating the

amount of sand, silt and clay in a soil and determining the % of each.

• This mineral part of the soil or the Sand, Silt and Clay particles are from the weathering of Rocks and Minerals.

• Soil texture is the single most important physical property of the soil. Knowing the soil texture alone will provide information about:

• 1) water flow potential,

• 2) water holding capacity,

• 3) fertility potential,

• 4) suitability for many urban uses.

Soil Texture

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• Physical weathering -

Rocks get broken into pieces but its chemical composition remains unchanged.

• Processes of Phys.Weathering

• 1) Freeze / thaw weathering - occurs when temperature freezes at night and rises during the day. Water expands when frozen which forces rocks open.

• 2) Biological weathering _ Roots of plants grow into cracks and force cracks open.

• 3) Exfoliation - when rock at earth's surface is worn away. • After a rock that has formed deep in the earth is exposed at

the surface it expands and gradually breaks into sheets.

Chemical Weathering• Chemical weathering -

Rock broken down by chemical change - water always plays a part. For Example:

• Carbon dioxide dissolves in rain water forming carbonic acid which dissolves limestone rock which is carried away in solution as calcium hydrogen carbonate.

• Chemical weathering is faster for limestone than sandstone and is speeded up by heat.

Chemical Weathering

• Common chemical weathering processes are: hydrolysis, dissolution, and oxidation.

• Chemical weathering produces clays on which vegetation can grow.

• A mixture of dead vegetation, clay, rock fragments of sand and silt size particles produces soil.

Chemical Weathering

Chemical Weathering

Weathering

• Weathering is controlled largely by climate. The more water available, the more likely that chemical processes can proceed.

• Additionally, in warm temperatures chemical weathering can proceed even faster.

Weathering

• In arid climates, however, weathering processes move very slowly.

• Mechanical weathering will be the dominant process in arid climates;

• however, because physical weathering relies on chemical weathering, it will also be quite slow

Soil as a Resource

• WHAT DOES SOIL DO? • Healthy soil gives

us clean air and water, bountiful crops and forests, productive rangeland, diverse wildlife, and beautiful landscapes.

• Soil does all this by performing five essential functions:

Soil as a Resource with 5 essential functions:

1) Regulating water.

• Soil helps control where rain, snowmelt, and irrigation water goes.

• Water and dissolved solutes flow over the land or into and through the soil.

2) Sustaining plant and animal life.

• The diversity and productivity of living things depends on soil.

• The more productive the soil, the more diversity in both the plant and animal community.

3) Filtering potential pollutants.

• The minerals and microbes in soil are responsible for :

• filtering, buffering, degrading,immobilizing,and detoxifying organic and inorganic materials, including industrial and municipal by-products and atmospheric deposits.

4) Cycling Nutrients.

• Carbon, nitrogen, phosphorus, and many other nutrients are stored, transformed, and cycled through soil.

• This is a good thing for it keeps them out of our water systems.

5) Supporting Structures

Buildings need stable soil for support, the bearing capacity determines the ease of stable construction.

Impact on soils in the environment

Deforestation

• The main contributors to land degradation are erosion and soil compaction, as a result of extensive removal of vegetation, exposure of the soils to heavy rainfall, increased evaporation, and later wind action.

Agriculture

• Agriculture may last for a few hundred of years or it may last for thousands of years.

• Agave production on these fields in Mexico may last for fewer than 50 years due to soil erosion which results in the loss of valuable topsoil.

Water Erosion

• Water erosion is the wearing away of soil particles.

• Raindrops detach the soil particles.

• As infiltration is reduced, water moving down slope takes the soil with it.

Wind Erosion

• Wind erosion is the detachment of soil particles by the wind and moving them to another location.

Chemical Degradati

on• Chemical spills can pollute the soil beyond which it can recover naturally.

• Soil remediation can reclaim the soil, making it useful again.