chapter 15

b horizon (illuvial, subsoil)

accumulation of minerals transported from above

spodosols

acid filter and humus light colored and acidic humus and iron and aluminum compounds more thoroughly leached eluvial horizon because higher rates of precipitation under coniferous forests --> less organic material --> thinner o layer acidic nature of parent material and organic material from coniferous trees increases amount of minerals that are dissolved as water percolates through soil profile

oxisols

acidic light colored humus iron and aluminum compounds mixed with clay thin o layer and small amounts of nutrients limited potential for use in agriculture under tropical rain forests --> high rates of net primary production and hefty amounts of biomass per unit area --> lots of organic material --> warm temp and plentiful rain cause organic material to decay quickly abundant rainfall leaches many minerals from upper layers weathered over long periods of time

mollisols

alkaline, dark, rich in humus accumulation of clay and calcium compounds formed below grasslands deep a horizon because large net primary production that is consumed by grass land herbivores added to surface layer as fecal material deep soils possible because low levels of precipitation and cold winters (slow decay of organic materials)

what effects cation exchange capacity

anions: negatively charged particles common to clay sands, silts, clays pH - measure of acidity surface area of soil particles (extensive particle surface area holds more cations)

no till agriculture

avoiding tilling because tilling breaks soil structures that resist water and wind erosion

saltation erosion

bounces particles along the ground in a series of short hops

storing nutrients

cation exchange capacity: soils ability to hold positively charged nutrients rooting depth: depth of plant roots

factors of soil formation

climate living organisms parent material topography time

types of soil water

decreasing water soil: gravitational water: drains right through; macropores capillary water: water held in micropores; plant roots absorb available water hygroscopic: remaining water strongly adheres to soil particles

storing water

depends on how porous the soil is some hold water very well but not available for plants some hold water loosely but dont have much capacity to store water

aridosols

desert pavement weak humus-mineral mixture dry,brown to reddish-brown with variable accumulations of clay, calcium carbonate, and soluble salts old alluvium from eroded uplands eluvial horizons undergo little leaching because little precipitation if irrigated, can support high levels of agricultural production

two types of weathering

disintegration: physical/mechanical changes decomposition: chemical changes

soil formation (3 chemical processes)

dissolution - water+carbon dioxide+calcite dissolve into calcium ion and bicarbonate ion hydrolysis - water ions bind with and break down mineral structures in the parent material oxidation - oxygen combines with compounds

strip planting

farmers divide their land into strips and plant crops on alternate strips

contour plowing

farmers plant crops in rows that cut across slopes (no slope along each row); slows surface flow and erosion

hunting and gathering vs farming

food from edible energy flows of natural ecosystems population density of hunter-gatherer societies is low

soil formation (above and below)

formed above by biological actions formed below from breakup of underlying bedrock

erodibility factor

how easily soil particles can be detached and transported horizontal water movements detach and transport soil particles

sheet erosion

if rainfall exceeds rate at which water percolates into soil, film of water will build up on surface as this film moves downhill, it will scrape and transport topsoil in even layer

gully erosion

if rills are concentrated into deeper channels most visible form of erosion not as much soil lost as sheet or rill erosion

shelterbelts

in strip planting, plant row of trees that line every second strip

effect of climate on soil formation

influences types of organisms that live in an area species determine nature of organic material (o horizon) and how organic material decays

why don't farmers use optimal soil conservation

lack of understanding of costs of erosion soil erosion is an externality population growth and best first principle lack of resources, tenant farming arrangements, etc

e horizon (eluvial)

light-colored mineral particles, zone of eluviation and leaching minerals leached as water percolates through soil leaching causes concentration of of nutrients and minerals to decrease

o horizon

loose and partially decayed organic matter three layers (deciduous forests): 1. surface layer (leaves and twigs) 2. middle layer (partially decomposed organic material) 3. bottom layer (humus: partially decomposed plant or animal matter)

pore space and particle size

macropores: soils that consist of large particles have a few very large pores micropores: soils that consist of many small particles have many smaller pores smaller pores represent more space than few large pores

parent material

mineral material from which soil forms

a horizon (topsoil)

mineral matter mixed with some humus most of material is mineral (as opposed to organic in o horizon)

field capacity

moisture remaining 2/3 days after rain

types of soil

mollisols spodosols oxisols aridosols

creep

movement of larger soil particles along the surface occurs when during saltation, particles hit larger particles

soil horizons

o horizon (humus) a horizon (topsoil) e horizon (eluvial) b horizon (subsoil) c horizon r horizon

oxidation

oxygen combines with compounds in parent material weakens a material --> more vulnerable to weathering

effect of parent material on soil formation

parent material that consists of softer rocks/soluble minerals tend to disintegrate faster than parent material that consists of harder rocks/insoluble minerals

c horizon (substratum)

partially altered parent material regolith) supports little biological activity little weathering

wilting point

point at which plants will wilt due to lack of water

universal soil-loss equation

rainfall soil erodibility slope length land cover erosion-control practices

terraces

ridges and channels that are constructed across the slope to prevent rainfall runoff from accumulating and causing serious erosion

soil particles

sand: largest soil particles; gritty silt: can only be seen under a microscope; smooth clay: can only be seen with electron microscope; sticky loam soil: equal mixture of clay, sand, and silt; ideal mixture of macro and micro pores (greatest amount of water available to loam soil)

loam soil

sandy soils have low wilting point --> large portion of water in sandy soil is available to plants total amount of water in sandy soils is low due to small amount of pore space small particles in clay soils create a lot of pore space, but holds water tightly

types of soil erosion by water

sheet erosion rill erosion gully erosion

rill erosion

sheet erosion concentrated in small channels (rills) relatively harmless because rills can be smoothed by plowing, but cannot fix fact that soil is lost

impacts of soil erosion

social costs: soil transported beyond farm and causes damage where it is deposited (eutrophication, siltation of streams, siltation of dam reservoirs) private costs: incurred by individual who causes erosion, occurs on the farm, and farmers suffer their effects

conserving soil

soil conservation techniques: increase crop residues left on soil after harvest contour plowing terraces strip planting shelterbelts reduce plowing (plowing creates channels for surface flow)

wind erosion equation

soil erodibility climate roughness vegetative cover

suspension erosion

soil particles lifted high into the air and carried long distances generated dirt storms associated with dust bowl

effect of living organisms on soil formation

soils associated with grasslands have higher organic content than soils associated with forests grassland soils have greater nutrient content and thicker A horizon because they are inhabited by a greater population of nonsymbiotic nitrogen-fixing bacteria

effect of topography on soil formation

soils on steep slopes shallower than soils in flatter areas steep slopes encourage erosion --> slows accumulation of surface layers small depressions accumulate soil --> retard plant growth and slow soil formation

soil erosion policy

subsidies stand in the way of true policy in the US in the developing world, USAID and other agencies consider environmental issues like soil erosion (assistance trumps environment)

how much wind speed and turbulence affect soil is determined by...

surface roughness: irregularieties in soil surface field width vegetative cover

types of soil erosion by wind

suspension saltation creep

effect of time on soil formation

time enhances formation and depth of soil

regolith

unconsolidated material that lies above bedrock --> parent material

r horizon (bedrock)

unweathered parent material

soil

upper portion of regolith that has been changed both chemically and biologically

carbonation and solution

water combines with carbon dioxide to form weak acid (carbonic acid) acid converts some minerals such as calcium and potassium to carbonates --> dissolve in water and open spaces within rock

second best problems

when the theoretical conditions that underlie a policy are not satisfied, implementing the policy may make things worse

turbulent flow

wind that moves across surface but also has an up-and-down component