Soils and Environment

Soils and Environment

WEATHERING

Physicaland

ChemicalEffects

WEATHERING, EROSION, TRANSPORTATION

• Weathering- Physical disintegration and chemical decomposition of rocks

• Erosion- Physical removal• Transportation- Movement of eroded particles• Chemical vs. Physical Weathering• Effects of weathering

– Surface alteration of outcrops– Spheroidal weathering– Differential weathering

Mechanical Weathering

• Freeze-Thaw Weathering

• Salt Weathering

• Wetting and Drying

• Insolation Weathering

• Pressure Release

• Stress Corrosion Cracking

CHEMICAL WEATHERING• Decomposition of rock to form new substances

– Changes in Equilibrium

• Water– Congruent solution (limestones) vs incongruent solution

(clay minerals)

• Carbon Dioxide- changes in pH change solubility of minerals

• Role of Oxygen– Fe in ferromagnesian minerals becomes oxidized

• Hematite and Limonite

Chemical Weathering

• Solution of ions and molecules

• Production of new materials– clay minerals– oxides– hydroxides

• Release of residual unweathered materials– quartz and gold

Chemical Weathering of Silicates

Na, KCa, Mg

hydrolysis

Hydration, solutionSolutions of Na+, K+, Ca2+, Mg2+

Interlayer Cations

Al 3+ & Si 4+

solution

Hydrolysis, hydration

Silicic acid (H4SiO4)

Secondary minerals, e.g. clays

Aluminosilicate sheets (e.g. as part of feldspars)

BruciteAlumina

Fe2+

Oxidation, hydrolysis

hydration

hydrolysischelation

Chelate complexes

Hydrous oxides, e.g. FeO(OH)

Brucite and alumina sheets and incorporated ions, e.g. Fe2+

Results of Weathering: Clay Mineral

• Clay minerals give information about weathering conditions

• Kaolinite: humid, acid conditions, alteration of K-Feldspar

• Illite: weathering of feldspars and micas under alkaline conditions where leaching of mobile K does not occur

• Montmorillonite: weathering of basic igneous rocks under alkaline conditions with a deficit of K+ ions

Clays• Kaolinite

• Illite

• Montmorillonite

• Chlorite

• Mixed-layer clays

Soils: Definitions• Loose unconsolidated material composed of regolith

and partially decayed organic matter, water and gases• A soil profile is a vertical face of soil that can be

exposed and includes all the layers (horizons) from the surface to the parent rock (bedrock)

• The solum is that part of the profile that is influenced by plant roots

• A pedon is a 3-D representation, the smallest volume that can be called a soil

Soils and Food Production

• Roots of Agriculture– Middle East (Iraq) origins

• Roman techniques of soil fertility

• Terrace building in Meso-America and South East Asia

• Increasing world population reliance on pesticides and fertilizers

Useful Properties of Soils• Provides water, nutrients and anchorage for vegetation• Provides habitat for decomposers, essential in carbon

cycle and mineral cycling• Acts as a buffer for temperature changes and for the

flow of water between atmosphere and groundwater• Because of its cation exchange properties, acts as a

pH buffer, retains nutrients and other element loss by leaching and volatilization

Soils as part of the Ecosystem• An ecosystem is a community of interacting

organisms and their physical-chemical environment that function as a self sufficient whole

• Soils are an essential part of the Carbon cycle due to the effects of microorganisms

Atmospheric CO2

PrimaryProducers

Decomposers

Organic Compounds

Soils and Geologic Time• Soils could only exist after the colonization of land by

organisms, in particular vegetation• First land plants in the Ordovician (450 my)• By the Devonian the land had been colonized (370 my)• By the Carboniferous (300 my) extensive forest habitat

generated soils similar to today• Properties of soils determined by climate, organisms,

relief, parent material and time, thus we can extrapolate the conditions that formed paleosols

Soils and Humans• Cultivation of soils began about 10,000 BP in Mesopotamia (Tigris

and Euphrates rivers of Iraq)• The land was a porous friable silt loam that required irrigation.• The civilization ended due to wars, floods, infilled irrigation channels,

erosion (gullying), salinization, loss of food production and famine• Other centers of agriculture in the fertile Nile valley, Indus and the

river valleys of China• In Europe soil erosion instigated colonization of other lands and

remains the worst problem facing humans.• In other areas terracing became the primary farming technique

(Southeast Asia, Peru)

The Green Revolution: An Idea of the 1960’s

• Increase world population demands the increase of food production

• Increases in land under cultivation, more intensive agriculture (mechanization) or both

• The introduction of fertilizers, pesticides, irrigation, varietal seeds (seed banks), Population growth 2% WHILE food growth 4%.

• In 1970, Norman Borlaug received the Nobel Peace Price• However, not a panacea >> potential realized, need for

irrigation,constant inputs of fertilizers, pesticides, and energy intensive mechanized labor, benefit large land holders, detrimental to most 3rd world countries

Soils

• Pebbles, gravel and sand particles

• Aggregates (mm to cm) of clays

• Roots

• Partially decayed to totally decayed vegetation (Humus)

• Organisms (earthworms, arthropods)

• Pore spaces filled with air and gases

• Water

SOIL Texture• Texture: Relative proportions of sand, silt and

clay–Dominant size fraction as a descriptor [clay, sandy

clay, silty clay]– If no dominant fraction then >> Loam {40% sand;

40% silt; 20% clay}

• Clay-sized particles vs. clay minerals– type of clay not just % clay

• Texture is an indicator of other properties (ease of cultivation)

Soil Textural Classification

Soil Structure

• Arrangement of soil particles into cemented aggregates

• Aggregates are secondary units or granules composed of many soil particles held together by organic substances, iron oxides, carbonates, clays and/or silica

• Natural aggregates are called PEDS• A CLOD is a coherent mass of soil broken

apart by artificial means

Bulk Density

• Density of soil minerals ranges between 2.6 to 2.7 g/cm3

• When dry, the bulk density is about half the above value, because voids are filled with air

• Defined as b = M/V;

• Commonly 1.0-1.6 g/cm3

• Varies over small distances due to weather, cultivation, compression by animals

• Increases with depth

Core sampler for determination of bulk density. The sampleryields a core of a fixed volume. The core is dried and weighedThe weight divided into volume gives the bulk density of soil

Porosity

• Calculated from the dry bulk density and the particle density– e = 1 - (b / s) x 100 = % porosity

• Where s is usually between 2.6-2.7 g/cm3

• The pore space is occupied by water and air• Transmission pores >50m• Storage pores 0.5-50 m• Residual pores <0.5 m

Relationship Between Texture Bulk Density, and Porosity

TexturalClass

BulkDensity

Porosity

Sand 1.55 g/cm3 42%Sandy Loam 1.40 g/cm3 48%Loam 1.20 g/cm3 55%Silt Loam 1.15 g/cm3 56%Clay Loam 1.05 g/cm3 59%

SOIL

• Various definitions– Unconsolidated material above bedrock– weathered material & organic matter

• supports plant life [air, water, organic matter & mineral material]

• Loam {40% sand; 40% silt; 20% clay}• Clay-sized particles vs. clay minerals• Soil Horizons• Residual Soil (on bedrock)• Transported Soil (alluvium)

SOIL

• Parent Rock, Time, and Slope

• Organic Activity

• Soils and Climate– Pedalfer- aluminum and iron rich clays– Pedocal- calcium rich– Hardpan- crusts (Fe and caliche)– Laterites- tropical soils

• Bauxite- Principal ore of Al

• Buried soils

Soil Horizons

• Identified, named, by symbols consisting of upper and lower case letters

• Each symbol recognizes a formational property– O- organic material– A (A1)-accumulation of decomposed org. matter– E (A2)- mineral layer, loss of silicate, eluviation (leaching)

horizon– B (B2)-Illuviated humus, silicate clay or hydrous oxides– C (C)- mineral horizon above parent– R (R)- Consolidated Bedrock

Soil Structure: Pan Structures

• Dense layers or pans

• Interference with root and water penetration

• Produce shallow soils

• Due to compaction, filling of pores with clays or chemical cements

• Very firm layers are called hardpans

Types of Pans• Claypan

– Dense soil layers produced by downward migration of clay and accumulation in subsoil as a B-horizon material

• Duripan– Layers cemented by precipitates of silica, alone or in combination

with iron oxides or calcium carbonates

• Fragipan– Fragilis (brittle), dense subsoil layers (50-60 cm beneath surface)

bonded into a hard, brittle form by clay

• Caliche– Hard lime-cemented white crust in arid regions

• Plinthite– Laterite, precipitated sesquioxides as cements. Weathered soils of

the tropics formed at depth

• Plowpan– Artificially produced, due to compaction by plows.

Caliche

Soil Taxonomy

• Organization into 11 orders*, 54 suborders, 238 great groups, 1922 subgroups and then families and series, each series subdivided into mapping units called phases of series– * including a tentative order andisols (soils

with over 60% volcanic ejecta)

Orders• Most general category• 5 of the orders exist in a wide variety of climates

– Histosols (organic soils); Entisols (undeveloped); Inceptisols (slightly developed); Andisols (volcanic); Vertisols (swelling-clay)

• 6 are the product of time and the microclimate in which they develop– Mollisols- naturally fertile, slightly leached, semiarid to subhumid,

grassland – Alfisols- fertile soils in good moisture regimes– Ultisols-leached, acidic soils, warm climates, low-moderate fertility– Aridisols-arid region soils– Oxisols- infertile, hot humid tropics– Spodosols- cool climate, acidic sandy

Soil Orders

• In the US Mollisols cover 25% of the land

• Worldwide distribution– Aridisols 19%– Alfisols 13%– Inceptisols 9%– Mollisols 8%– Oxisols 8%

Soil Taxonomy• Classification at 6 different levels• Level of generalization relates to the range in

properties allowed in the different classes• Soil Orders

– Suborders• Great Groups

– Subgroups» Families» Series

Soil Orders

Engineering Properties• Plasticity-water content of soil• Soil Strength-ability to resist deformation

– Cohesion-ability of particles to stick together– Friction-fnc. Density, size, shape of soil particles– Sensitivity-measures changes in soil strength, clay

soils very sensitive to disturbance (liquifaction)• Compressibility- soils tendency to consolidate

(decrease in volume), settling causes foundation cracks

• Erodibility- ease of removal by wind and water• Corrosion- function of soil chemistry• Ease of Excavation• Shrink-swell potential- gain or loose water

(expansive soils)

Soil Erosion

• Soil Erosion: Removal in part or whole of soil by wind or water

• Natural process• Erosion is slight from areas covered by

dense grasses or forest but increases dramatically in exposed steep, poorly covered soils

• Increased by human activity especially poor agricultural practices

Soil Erosion

• Soil erosion has been documented as early as 10,000 ybp in Mesopotamia due to agriculture that cleared the land (deforestation), overgrazed the land by herbivores (sheep, goats)

• Erosion in Europe is believed to have occurred 5000 ybp with clearing of woodlands

• In the us during the 1930’s (Dust Bowl) wind and water erosion left devastating effects

• On a positive note, erosion from Ethiopian highlands generated the fertile sediment for Egyptian agriculture for 1000’s of years

Erosion by water in 1961 Kentucky

Environmental Problem• Human induced activities such as over cultivating can

deplete soils. Severe erosion can exceed 200 Mg/ha/yr (90 tons/acre/yr)

• Loss of soil to support growth of crops, grasslands, forest

• Soil erosion destroys human-made structures (reservoirs), lakes and rivers and badly damages land

• Deposition of sediment in rivers can cause them to change course, variable seasonal flow and flooding

• The cost of dredging rivers and harbors each year is 15X the cost of holding the soil in place

Environmental Problem

• More than 1 million acre-feet of sediment settles annually in reservoirs lowering their capacity

• Water can become polluted. 1 ton of soil containing 0.2% N and 0.05% P will transfer 2kg N and 0.5Kg P to rivers and lakes causing eutrophication

• Air pollution: fine particles can reduce solar radiation and affect chemical processes in the atmosphere

Poorly Managed Construction Site

City of Ballinger, TX used water from this reservoir from1920-1952. By early 1970’ssoil erosion sediments filledthe reservoir to more than 35feet destroying the dams ability to hold water.

Dredging of a Sediment Filled Drainage Ditch

Eutrophication of Water Body due to Nutrient Loading

Environmental Problem• This high rate of erosion has tried to be controlled

by laws past. In 1972, US Congressed passed P.L. 92-500, The Federal Water Pollution Control Act (FWPCA).

• The Clean Water Act amended in 1977 (Section 208 of FWPCA) required states to develop plans to control ‘non-point sources’ such as sediment in waterways

• In 1981 renewed effort that targeted areas having the most severe erosion

• By 1986 some improvements were evident

Effects of Soil Loss• Amount of erosion depends on erodibility of the soil, characteristics

of the land and land use management• Universal Soil Loss Equation (USLE)A = RKLSCPA: annual soil lossR: erosivity of rainK: soil erodibility factor (easily detached particles); reference soil plot

obtained using standard plot 22.1m long on 9% slope, bare of vegetation, plowed up and down

L & S: length and angle of slope (in percent)C: crop management factor and vegetative coverP: practices for soil conservation (contour, terracing)

Agricultural Soil Erosion

Contour strip croping of hay and corn

Bench Terraces

Calkins sweep plow designed toprovide 90% stubble on soil as mulchto reduce wind erosion

Field windbreak protecting a corn crop in North Dakota

Soil Erosion

Soils and Pollution

• Pollution vs. Contamination

• All chemicals are harmful in excess concentrations

• Concentrations of chemicals are regulated by law in most industrialized countries

• Natural soils can have chemicals in excess concentrations (selenium, molybdenum, lead)

• Soils are nature’s filters and a receptacle for burial– Physical (sieve action)– Chemical (adsorption and precipitation)– Biological (decomposition of organic material)

• Soils are needed to:– Grow crops for food, animal fodder and fibers, trees for fuel and timber

and to support natural ecosystems• Increasingly human population explosion has increased the amount

of land in cultivation, so that what remains is marginal or sub-economic land

• The question is sustainable development of resources for our increasing population, estimated at 10 billion by 2050

• The answer of management is not easy since social, political, economic and cultural conditions have to be met just as the physical ones

Contamination by Nutrients: Nitrates

• Nitrate is present in soils from microbial breakdown of organic matter, manures and plant residues; fertilizers; the microbial oxidation of ammonium (NH4

+) and additions from the atmosphere as HNO3

• NO3- is not adsorbed by most soils because of its

negative charge. It remains in solution until it is either taken up by plants, leached out in drainage water or denitrified

• Loss of nitrate is undesirable because it is a health hazard, it causes economic losses (fertilizers); it causes eutrophication

Health Risks of N• A health hazard from nitrite was first recognized in 1945 in Iowa-

Methemoglobinemia (blue baby syndrome), also affects the elderly and livestock– Nitrate becomes toxic to any animal with a disrupted digestive track that

causes microbes to reduce NO3- to NO2

- in large amounts. The nitrite is absorbed into the bloodstream where it oxidizes oxyhemoglobin to methemoglobin thus suffocating the young animal, turning it blue (cyanosis), when 70% of hemoglobin is changed, death ensues

– Nitrate content of wells is regulated at 45ppm nitrate or 10ppm Nitrate-nitrogen; many rural wells exceed this by 2X

• Respiratory illnesses from PANS (peroxyacetylnitrates)and other nitrogen oxides

• Cancer (gastric) from nitrosamines from NO2- and secondary

amines in food

Pesticides• Pesticides are extensively used to control harmful populations of

insects• Since Greek and Roman times some mixtures have been used to

control insect populations and fungi (sulfur, arsenic and copper compounds)

• In 1930’s 2,4-D and DDT were found to kill weeds and insects. It was the ‘magic-bullet’. Its ‘inventor’ was awarded the Nobel Prize in Chemistry in 1948

• DDT is still an excellent insecticide (malaria control, Chagas disease, typhus ect..). However, its half-life in the environment is too long (10-25 years) and it bioaccumulates in the fat of animals (Silent Spring- Rachael Carson)

• DDT was banned in US in the early 1970’s

Pesticides

• All pesticides are organic chemicals (chlorinated hydrocarbons, organophosphorus compounds, carbamates etc…)

• About 600 commercially important ones exist and over 1500 registered for sale

• To be a ‘good pesticide’; it must be 1) short lived in the environment, 2) not carcinogenic, teratogenic or mutagenic and must 3) be effective yet be able to be handled safely

Pesticides

• Most pesticides are adsorbed to soils especially those of a high molecular mass that form positively charged ions

• Some pesticides are volatilized and all are eventually biodegraded by soil microorganisms depending on their half-life

• Groundwater contamination is one of the greatest threats we face today

Soil Degredation• Erosion: greatest long term hazard to long term maintenance of soil

fertility

• Acidification: the soil pH of a weakly buffered soil in the humid tropics can drop from 6.0 to 4.5 in 3 years when fertilized by ammonium sulfate

• Salinization and sodification: particular problems that occur in arid and semi-arid environments under irrigation

• Accumulation of toxic elements: from mining and industry

• Depletion of plant nutrients: harvesting of specific crops• Reduction of soil organic matter content

• Compaction and crusting• Waterlogging and drought: periods of wet/dry. Sahel has been in a

drought since the mid 1960’s and the onslaught of desertification

The Ultimate Pollutant: People• Pollution: the degredation of a substance or

system for people’s use• Carrying capacity (limit of an ecosystem to

support organisms without causing a catastrophe)

• The more nature is bent abnormally by more and more people, the more catastrophic will be the results, whenever we lose control

Degradation

• The American Farmland Trust stated that unless California’s agricultural problems were addressed in the next 10-20 years the state farming industry would decline– Agricultural land conversion to nonagricultural uses

• Over 17,00 ha/yr were converted to urban uses, >80% were irrigated croplands

– Soil erosion– Increasing salinity of soil and water– Diminishing water supply and diversion for

nonagricultural uses

Parting Thought

AS IMPORTANT AS TECHNOLOGY, POLITICS, LAW, AND ETHICS ARE TO THE POLLUTION QUESTION, ALL SUCH APPROACHES ARE BOUND TO HAVE DISSAPPOINTING RESULTS, FOR THEY IGNORE THE PRIMARY FACT THAT POLLUTION IS PRIMARILY AN ECONOMIC PROBLEM, WHICH MUST BE UNDERSTOOD IN ECONOMIC TERMS