Soil and Earth Resources

1. Geology & people
• minerals & ores
• all matter on earth is composed of combinations of one or more elements; each element represents a more or less unique type of atom
• the atoms of the natural elements are numbered 1 through 92, in order of increasing number of protons/electrons (roughly speaking, in order of mass)
• most elements are quite rare in the earth's crust with 10 elements making up 99 percent of the matter

Element	Abundance (atom fraction)	Abundance (weight)	Atomic Weight
Oxygen	61.2	46.6	16.0
Silicon	20.7	27.7	28.1
Aluminum	6.3	8.1	27.0
Sodium	2.6	2.8	23.0
Hydrogen	2.1	0.1	1.01
Calcium	1.9	3.6	40.1
Iron	1.9	5.0	55.8
Magnesium	1.8	2.1	24.3
Potassium	1.4	2.6	39.1
Titanium	0.2	0.4	47.9

• atoms combine into molecules; elements combine into compounds
• in a compound, one or more elements are chemically bound in molecular or crystalline form
• some compounds can form ions especially in water
• particles consisting of one or more atoms having an excess or deficit of electrons and thus electrically charged
• mineral = a naturally occurring element or compound, usually crystalline or with some other unique orderly internal arrangement
• for example, naturally occurring iron sulfide is called pyrite
• minerals have various properties including crystal habit, cleavage, hardness, specific gravity, color...
• an ore is a form of mineral from which it is economically feasible to extract some resource
• there are more than two thousand minerals, though a much smaller number make up most of the rocks
• the minerals can be broadly classed on the basis of their chemistry:
• native elements
• gold, diamond
• halides
• halite, sylvite
• oxides
• bauxite, magnetite, ice
• sulfides
• pyrite
• carbonates
• dolomite
• sulfates
• gypsum
• silicates (more than 90% of the rock forming minerals are these); composed of silicon, oxygen and one or more metals
• quartz, feldspar
• asbestos
• rock formation-rock is the most common material on earth
• a rock consists of a mixture of one or more minerals
• three types of rock:
• igneous = ("fire") form from cooled and solidified magma
• magma may cool rapidly, as at the midoceanic ridges
• or, may cool slowly underground (over thousands or millions of years
• visible when erosion exposes it
• sedimentary = usually made up of the particles of other rocks broken up and moved by wind, water, ice or gravity; sometimes formed from the direct deposition of minerals
• characteristically found in layers
• metamorphic = formed from igneous or sedimentary rocks under the influence of heat, pressure or chemical agents within the earth
• rocks go through a rock cycle:

2. Structure of the Earth
1. the outer layer of the earth is called the crust; it is composed of minerals; the processes of the crust and the upper layer of mantle on which is floats give rise to the minerals we use in our commerce
• minerals are natural, inorganic substances with a definite and regular chemical and physical structure
• mineral deposits that can be economically mined are called ores
2. the inner portion of the earth is the inner and outer core
• these are solid and semisolid iron, nickel and other elements
• radioactive decay here produces the heat that keeps the core molten
• convection currents here give rise to the earth’s magnetic field
3. The mechanics of the crust-continental drift
4. the notion the the terra firma of the earth’s crust was not so solid was first proposed by Wegener (a German meteorologist) in 1915 in a book called The Origin of Continents and Oceans
• Wegener noticed that the profile of the Americas seemed complementary to the profile of Europe and Africa
• he also had geological and fossil evidence that that continents were once together
• folks thought he was a nut
5. Harry Hess and geologists from Lamont Geological Observatory mapped the midatlantic ridge in 1947
• Hess speculated that the ridge was the site of new sea floor formation, one edge of a sort of sea floor conveyor belt
• sea floor spreading was the name given to this process by Robert Dietz
• magnetic orientation in minerals was dramatically observed in zebra stripes of bands of opposing orientations running parallel to the mid-ocean ridge on the sea floor
• Vine & Matthews (Cambridge) explained this as the signature of the process of new ocean floor formation at the ridges
6. the midoceanic rift
• molten basalt is forced up from the upper mantle (asthenosphere) and breaks out forming a split or rift in the crust
• the new rock cools and solidifies
• since it is dense, it settles to a low point in the crust, ie, the sea floor
• the material forms a ridge on either side of the rift
• as new material oozes out, the older material spreads away from the rift/ridge
• this spreading goes on at the breakneck pace of a few centimeters per year
7. plate tectonics
• the spreading sheets of basalt form large tectonic plates
• where two plates collide, one slides under the other back into the mantle, a process called subduction
• in subduction zones, deep ocean trenches are found
• the continents are formed from bits of lighter rock that float atop the heavier sea floor
• like the bits of stick and so forth that gather at an obstruction in a stream
• the bits of land are called terranes and can have widely different geological histories
• the crust is thicker at the continents (think of two floating blocks of wood; the bigger block extends both higher above and lower into the water)
• the Atlantic Ocean formed some 140 million years ago from the rift in Pangaea, the supercontinent
• at the rift, there is an outpouring of minerals
• seawater convects through the heated rock here producing hot springs called black smokers
• chemosynthetic bacteria here use the minerals and hydrogen sulfide to produce fixed carbon used by a food chain not dependent on the sun
• zinc, lead and copper sulfide deposit near these in formations called massive sulfides
3. Volcanoes and earthquakes
1. movement at the rifts and subduction zones cause earthquakes
• the Pacific plate moves to the northwest with respect to the North American plate along the San Andreas fault
2. types of volcanoes
• seafloor or "hot spot"
• a plume of hot mantle flickers like a match under the moving piece of paper of a plate
• the hot spot leaves a trail of volcanoes as the plate moves over (Hawaii is at the southeastern end of a trail of extinct volcanoes)
• the magma is rather fluid, so it flows readily and frequently (every few years) without a lot of explosion
• subduction zones or "rim of fire"
• hot magma bubbles back to the surface as a plate goes down (Japanese islands, aleutian islands)
• this magma doesn't flow as easily, so the volcanoes erupt less often (several centuries) but look out when it does
• eg Mt St Helens
• steam loosened the top forming a crater
• the top of the mountain swelled up over 300 feet
• earthquakes shook material loose from the top and eventually triggered a landslide that was like pulling the pop top on a can
• the top 1300 feet blew off the mountain
• trees were levelled almost 10 miles away
• anything living above ground (including people) were wiped out
• junk rained down into the midwest
• only a trace of dust remained in the stratosphere, giving us good sunsets but no climate change (in contrast with the eruption of Tambora in 1815, which led to "the year without a summer")
• ecosystem recovery from eruptions
• a more complex process than had been thought
• some plants survived the blast underground and began an immediate comeback
• animals began wandering in within a few weeks
• about 90% of plant species back on the job within three years
• lakes were heavily sedimented but are recovering
4. What is soil?
1. soil is a mixture of
• mineral particles
• organic matter
• detritus
• living things (detritus feeders & decomposers)
2. soil is not inert, it is a dynamic factor within ecosystems
• provides nutrients and is the site of many nutrient-cycle reactions
• provides a physical substrate for plants, provides moisture etc
3. Weathering & Soil Formation
• pedogenesis = the formation of soil or the study thereof
• see drawing of soil horizon on page 188 in Nebel
• exposure to wind, rain, ice and sun breaks down rock into smaller pieces
• smaller pieces are acted on by plants
• topsoil = mixture of small rock particles and organic matter; this zone contains living roots, worms, bugs, bacteria, and fungi as well as burrowing animals
• nutrients leach out of this zone under the influence of infiltrating rainwater
4. Soil fertility
• eight of the ten elements essential for plants are taken up from the soil (carbon and oxygen are taken from the air, though oxygen is also taken up from the soil as water)
• these elements are often present in mineral form in total quantities that exceed plant needs; however, the amount available to plants varies greatly depending on other factors
• the elements, typically in the form of some ion, have to be dissolved from the mineral grains by water
• this process of leaching may make the elements available to plants or may wash the nutrients out of the topsoil, into the subsoil or into ground- or surface water
• once present in the topsoil, the availability of the nutrients is affected by:
• moisture
• dependent on soil texture, affected by grain size, humic content, degree of aeration etc
• pH (pH must also be within the neutral range or plants will be killed by the extreme acidity or alkalinity)
• what is pH you ask?
• water ionizes:
• H2O <=> H+ + OH-
• at pH 7, [H+] = [OH-] = 107
• increase the [H+] tenfold, and the pH goes to 6, the solution becomes more acidic, and the [OH-] decreases to one tenth of what it was
• pH has a big effect on availability of soil nutrients, mineral formation, toxicity of metals, and other bio/geochemical processes
• chemical state of the element
• redox potential
• the soil ecosystem (bacteria and other microdecomposers, worms and other macrodecomposers) has an effect on all these
• use of chemical agents (fertilizers or toxins) can disturb the natural fertility of the soil
• the elemental nutrients are usually present as ions, hence the ion-exchange capacity of the soil is roughly the same as its nutrient-holding capacity (a soil’s ability to retain nutrients)
• things on earth came out of the lithosphere (or fell from outer-space) at some point, hence the parent bedrock is the ultimate source of the elemental nutrients; however, this source is not the immediate source for most plants
• the process of leaching is slow
• many rocks do not contain much of the needed elements
• the nutrient cycles provide most of the elements that plants use
5. Water not only affects the availability of the elemental nutrients, it is vital in itself for plant growth
• it is required for photsynthesis; this water is a small fraction (<1%) of the total water taken up by plants
• most of the water serves to keep the tissues hydrated and to carry material from the soil to the leaves
• the plants "circulatory system" is powered by the loss of water at the leaves through a process called transpiration
• each water molecule lost at the leaves lowers the chemical activity (ie concentration) of water within the plant, allowing the osmosis of water into the roots
• note that the presence of salt in the soil lowers the chemical activity of water in the soil and so reduces or destroys the ability of plant roots to take up water
• to grow an acre most crop plants require 1 to 2 acre-feet of water per year (some crops require quite a bit more); the amount of rainfall required is generally higher for reasons we will mention in a moment; typically, at least 20 inches of rainfall are needed to grow crops without irrigation
• rainfall is not the only factor that determines how much water is available to plants
• many factors affect how much water will soak in to the soil, or infiltrate, and how much will runoff; water which runs off is not available
• water that does infiltrate must be held within the topsoil to be available to most plants
• the size distribution of soil and its humus content determine the water-holding capacity of the soil
• if water is not held by the soil, plants need frequent rains
• evaporation depletes the supply of available water
• factors which reduce evaporation may increase the amount of water available to plants
• too much water in the soil can be a problem for plants, whose roots need oxygen to carry on respiration
• plants which thrive in marshes or other flooded lands have adapted so that air can diffuse down their stems
• most plants depend on the ability of soil to allow oxygen to diffuse to their roots; factors such as compaction which reduce the pore space in the soil can suffocate plant roots
6. Fertilizers
• substances added to soil to supplement plant nutrients, usually nitrogen, phosphorus or potassium (hence NPK); these are removed when biomass is removed from the land
• traditionally, animal wastes or tissues have been used to restore soil fertility (legend has it that the Native Americans taught the first European immigrants to New England to put fish in the fields when planting; fish are a good source of nitrogen and other elements)
• required NPK addition varies with soil conditions and plant species
• fertilizers can pollute ground or surface water
• overuse of fertilizer can be a waste of money, or may even cause greater damage to the soil
• modern synthetic ("inorganic") fertilizers require the use of fossil fuels, contributing to the energy subsidy of modern agriculture
5. Soil Properties
1. The two components of the soil (mineral and organic matter) each have properties that determine the overall role of the soil in an ecosystem
• the size distribution of the mineral particles determines the soil texture (USDA Soil Triangle--Nebel p 189)
• the mineral substrate has particles that range in size from boulders (bigger than about 1 foot), through cobbles (~1 to 12 inches), gravel (2 mm to several inches), sand (0.05 or 0.1 mm to 2 mm), silt (0.002 mm to 0.05 mm), to clay (<0.002 mm)
• the distribution of sizes can be determined by sieving or sedimenting (measuring rate of settling in water)
• what size distribution is present in a soil is a function of what happened when the soil was formed
• the type of parent material
• the weathering processes that broke the soil off the bedrock
• the other processes of water, wind, etc, that carried soil to its present location (these processes tend both to shape and sort the soil grains)
• coarser soils (sands or sandy gravels) tend to be better aerated and have higher rates of infiltration; these soils may be so well drained that there is little available water
• silts and clays tend to be less permeable but will hold water; mineral nutrients also will stick to the large surface area of the small particles (these soils tend to have high ion-exchange capacity)
• a typical good soil is called loam, which is a mixture of roughly 2 parts sand, 2 parts silt and 1 part clay
• sandy soils are easier to plow, and so are said to be more workable; clays become hard (like pottery) when they dry out (the extreme example of this is found in the latosols of the cleared rainforest-these can reportedly turn as hard as brick)
• the organic components are critical for soil structure
• generically, the organic fraction of soil is often called humus; specifically, humus is the refractory part of detritus left after the action of detritus feeders and some decomposers
• humans will sometimes pile up organic wastes to promote this feeding & decomposing; the result is compost
• humus and compost are not particularly good as fertilizer, but their physical properties promote aeration and retention of nutrients and water by soil, they also keep the soil loose and workable
• the presence of humus gives soil a structure that enhances the properties a soil has by virtue of its texture
• a layer of detritus/humus (eg decaying leaves) also promotes infiltration and inhibits evaporation and erosion
• the topsoil supports a wide variety of organisms, including some that are symbiotic and some that are parasitic
• these organisms are essential for the maintenance of vital topsoil
• plants are dependent on this soil, and this soil is dependent on plants, since they provide the continual input of detritus required by the soils food web
• the loss of topsoil due to overgrazing, erosion, etc., causes a dramatic decrease in the ability of soil to support plant life (including crops)
6. Runoff
1. Effects of soil
• soil characteristics greatly affect the degree of runoff versus infiltration
• clay content reduces infiltration, especially after the ground is saturated with water (which swells the clay)
• sandy soil or friable loams, both of which have low clay content, have higher rates of infiltration
• more densely compacted soils have lower rates of infiltration
• note: rainfall can actually compact soil
• rainfalling directly on soil also tends to plug the opening in the ground with fines from near the surface (like espresso grind coffee in a melitta filter)
• waterlogged soil has a lower rate of infiltration than dry soil
• frozen soil has almost no infiltration (it may actually be coated with ice
• thus the timing and dynamics of a storm event can have a great influence on the proportion of runoff
• generally speaking, longer or repeated rains result in more runoff
• human development of an area usually has a dramatic effect on increasing runoff
• paving and roofs have essentially no infiltration
• one of the engineering calculations that must be done when planning a development is the excess flood flow--this must be taken into account
• diverted stormwater may have substantially poorer quality than native runoff (metals and oils etc from the streets and yards)
• commercial and industrial establishments that discharge stormwater at a point are now subject to the restrictions of categorical wastewater discharge permit
2. Effects of vegetation
• vegetation can greatly increase the rate of infiltration (more on this in a moment)
• hold water in place, buffer the release of water to the groundsurface
• shield the soil surface from some of the effects of compaction by rainfall
• a forest is particularly good, because the trees allow water to drip slowly down to ground level and their litter allows the water to slowly seep into the top of the soil itself
3. where there’s runoff, there’s erosion
• erosion is the ongoing process of wearing down and removal of rock and soil; tends toward the leveling of the high and the filling of the low
• gravity provides motive force for downhill travel
• water does some wearing and some transporting
• erosion is not necessarily bad, after all, it is critical for soil formation in the first place
• steeper slopes erode more quickly (depending on the material)
• rock steeper than sand
• dry steeper than wet
• sudden dramatic erosion is called a landslide; socially important in many places; California for example
4. Erosion and loss of cropland
• the effects of wind and water on exposed soil are destructive to the soil ecosytem
• the humus and clay particles that hold the nutrients and moisture are the first to be eroded
• a vicious circle results, since the damaged soil is less able to support a protective coat of vegetation (which also then leaves less detritus) the soil is more prone to erosion
• in semiarid areas (10 to 30 in/yr of rainfall), the loss of topsoil results in desertification (since the remaining gravel cannot hold enough water to support plants)
• the vicious circle can be set in motion by human intervention, clear cutting a forest can increase runoff and erosion disastrously, as can overgrazing and improper cropping:
• overcultivation
• heavily cropped land tends to have a high rate of erosion
• to produce a field with a single crop, the soil is generally plowed to bury weeds; as a result the soil lays bare until the monoculture gets well along and then is often bare again after harvest
• the removal of biomass at harvest also depletes nutrients and reduces detritus
• using chemical fertilizers to replace lost elements is not effective in sustaining the humus building communities of the topsoil
• there are agricultural practices which are not so destructive to the soil:
• plowing along hillside contours (ie "conservation tillage") tends to slowdown runoff and thus decrease erosion and increase infiltration
• bands running around the hills can be cropped separately, minimizing the amount of soil which is bare at any one time
• to maintain the soil it is critical to rotate crops; for example, every third or fourth year, a field is allowed to grow just alfalfa or some other leguminous crop
• these crops actually add nitrogen to the soil and can recycle deep nutrients to the surface
• leaving all of the alfalfa biomass on the field
• overgrazing
• a classic case of the tragedy of the commonwealth--the rangelands are often available for general use or at least use by someone with no ownership interest
• the owner of livestock seeks to maximize short term gain by maximizing the number of animals
• the destruction of topsoil seems to be a threshold effect, but with ongoing insults contributing to eventually reaching the threshold: a constantly stressed grassland ecosytem can go into catastrophic failure when additional burdens are placed on it by natural or human factors
• deforestation
• as you might guess, this is a bad thing for soil too
• as I mentioned above, forests are quite effective at increasing infiltration and decreasing runoff/erosion
• clear-cutting increases runoff and erosion, but it also disrupts the mineral nutrient cycles; elemental nutrients tend to be lost for clear cut land
• the forest floor without the trees often will not support the same plant species that were present before, often the trees themselves will not grow back
• the whole forest ecosystem appears to be like a mega-organism that has evolved over time, removing the trees seems to kill the organism; replanting them is like doing a heart transplant on a dead patient
• nonetheless, well conducted forestry (including some clear cutting) can increase the amount of snowmelt which is available for human use without dangerously increasing erosion
• forest resources (ie trees) are valuable and in demand; worldwide there is economic incentive to clear forests
• overall, economically feasible management practices and/or alternatives have to be available or the overcropping, overgrazing, and deforestation will not stop
• only about 0.5 mm of soil is produced in a year, erosion on lands used by humans worldwide can be typically 1 to 5 mm or more per year; such a disparity leads to destruction of productivity and loss of farmland; new land can be cleared but that then starts to erode at the fatal rate (map on p 202--Nebel)
• the eroded material and leached nutrients can also cause a problem
• sedimentation is destructive to aquatic habitats
• excess nutrients are too
• destruction by farmland runoff can be exacerbated by irrigation, primarily through the process of salinization
• salts present in the irrigation water are left behind when the water evaporates (as a good portion of irrigation water does)
• note that irrigation also increases leaching of nutrients out of the root zone
• as water used for irrigation makes its way downstream (from one field to the next) it gets saltier and saltier
• use of water for irrigation also depletes water resources and results in the mining of groundwater (also made worse by the loss of infiltration)
• drip irrigation minimizes waste of water, salinization and leaching; alternatively, crops can be selected in keeping with the amount of rainfall in an area
7. Some other soil events that can be significant for humans include:
1. subsidence
• when water (or oil) is pumped out at a rate faster than it can be replenished by rainfall, the water table drops; the non-waterlogged material is more compressible hence the ground above subsides
• Anderson et al quote a figure of 1 foot subsidence for every 20 to 30 foot of water table drop
• midwest and far west have serious depletion of groundwater due to pumping out water for irrigation (this can also cause salt water intrusion near the sea)
• the tower in Pisa leans as a result of subsidence (due to water squeezed out rather than pumped out-but the sinking of Venice is due to overuse of groundwater-both cities are built on deltaic sediment)
• as you can imagine, this can be quite damaging to things built on (buildings, pipelines) or in (well casings, water and sewer lines) the ground in these areas
2. expansion
• clay expands when it gets wet, can crack a foundation or worse