seafriends.org.nz - 05.3 soil chemistry

8/14/2019 Seafriends.org.Nz - 05.3 Soil Chemistry

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Classification of common rocks and soils and moreby Dr J Floor Anthoni (2000)

www.seafriends.org.nz/enviro/soil/rocktbl3.htm

On this page

Rock and soil chemistry.

Properties of soil: soil components, texture, structure, pore space, moisture, pH, CEC

and more.

Soil degradation: a comprehensive summary of the many ways soil degrades and is lost

in both quantity and quality

Soil time scales: tectonic movement, profile formation, soil formation, and more.

Rock and soil chemistry: Bowen series, solid solution, cation exchange capacity and

more.

Back to the rock table contents table

.

-- seafriends home -- all about soil -- Rev 20001010,20001112,20051122,20070718,

Properties of soilSoil is the collection ofnatural bodies in the earth's surface, in places modified or even made by man, ofearthy materials, containing living matterand supporting or capable of supporting plants out-of-doors. Itsupper limit is air or shallow water. At its margins it grades to deep water or to barren areas of rock or ice.Its lower limit to the not-soil beneath is perhaps the most difficult to define. Soil includes the horizons nearthe surface that differ from the underlying rock material as a result of interactions, through time, ofclimate, living organisms, parent materials and relief. In the few places where it contains thin cementedhorizons that are impermeable to roots, soil is as deep as the deepest horizon. More commonly, soilgrades at its lower margin to hard rock or to earthy materials, virtually devoid of roots, animals or marks ofother biologic activity. The lower limit of soil, therefore, is normally the lower limit of biologic activity, which

generally coincides with the common rooting depth of native perennial plants. (US Soil Survey staff, 1975)

soil components

mineral fraction; 45-50%. Mineral particles from 95-99% of solid fraction.

organic matter: 0.5-5%, made up of different substances that are gradually broken down

by microorganisms. Includes carbohydrates, proteins, lignins, fats, waxes. Many of thesecompounds do not decompose completely and are transformed to humus, a dark,complex, non-defined colloidal material.

water: 25% of soil volume.

air: 25% of soil volume.

organisms: a small but important fraction of soil.

macr o organisms: insects, grubs, earthworms. Earthworms help decomposeorganic matter, releasing plant nutrients, aerating soils and improving drainage.The insect-like organisms have hard skins and hard jaws, helpful for dissectingwoody substances.

microorga nisms: protozoa and nematodes, plant parasitic nematodes as wellas harmless ones present in most soils.

bacteria: (heterotrophic, autotrophic, aerobic, anaerobic, facultative) oxidise Sand N. Decompose organic matter and may cause some plant diseases.
http://www.seafriends.org.nz/enviro/soil/rocktbl.htmhttp://www.seafriends.org.nz/index.htmhttp://www.seafriends.org.nz/enviro/soil/index.htmhttp://www.seafriends.org.nz/enviro/soil/rocktbl.htmhttp://www.seafriends.org.nz/index.htmhttp://www.seafriends.org.nz/enviro/soil/index.htm


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Rhyzobium and Bradyrhyzobium bacteria form symbiosis with legume plants tofix nitrogen from the air, and are particularly important in tropical soils.

fungi: (mycorrhyzal, damping-off, stem fungi, crown fungi, root fungi, rot)important in decomposition of organic matter, but may cause some plantdiseases. Mycorrhyza form symbiosis with plants to facilitate absorption of P, S,Zn and perhaps water too. They are important in soils low in phosphorus (P) but

can't extract P where it does not exist. actinomycete s: important in the decomposition of organic mater.

algae: autotrophic organisms. Blue-green algae fix some nitrogen. they needsunlight and can live only on the surface.

soil texture

unweathered primary materials: have little capacity to hold water and nutrients and are

relatively chemically unreactive.

gravel: 2 - 4mm

sand: 0.05 - 2.0 mm

silt: 0.002 - 0.05 mm

weathered secondary materials

clay: is a secondary mineral less than 0.002 mm in diameter. It is formed as aresult of weathering. Silica and alumina sheets are formed by recrystallisation.Amorphous clays in warm climates form oxides of iron and aluminium. Clayshave a high cation exchange capacity (CEC) because they are negativelycharged and can attract, retain and exchange cations. Their water holdingcapacity is very high because of their large surface area per unit mass.

three-layer clays (Si-Al-Si lattice) montmorillonite and illite, have highCEC.

two-layer clays (Si-Al lattice) like kaolinite have low CEC.

amorphous clays are composed of oxides of Fe and Al and have very

low CEC. textural classification:

infiltration of water: rapid in sands, slow in clays.

drainage: rapid in sands, slow in clays.

aeration: sand has rapid gas exchange; clay slow.

fertility: sand has low fertility. Clay high, depending on type.

soil structure

appearance: granular, blocky, platy, prismatic, columnar.

larger-sized aggregates: held together by divalent cations, organic residues, other

cementing agents. influences on structure:

aeration: good soil structure is very important to provide for both god aerationand a high water holding capacity. Poor structure can result in high water holdingcapacity but poor aeration. Structure can be influenced by tillage. Puddlingoccurs when soil is tilled when too wet, or by compaction.

infiltration of water: the capacity of water to enter soil.

percolation of water downward: water seeps downward through the soil profile.


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soil pore space: soil is 40-60% pores. The pore size influences water holding capacity andaeration.

capillary pores: retain water against the pull of gravity

noncapillary pores: contribute to soil aeration

soil profile: a vertical cut through the soil shows its layering, characteristic of its mineralogy and

history.

O horizon: organic matter, leaf litter.

A horizon: zone of leaching. Plow layer in agricultural soils

B horizon: zone of accumulation: weathering from below + leaching from above.

C horizon: zone of weathering, between B and bedrock R

R horizon: the bedrock.

soil moisture: Soil moisture is expressed as osmotic pressure, required to extract it from the soil.Commonly quoted in bar or in MPa pressure. To convert: 10 bar= 1 MPa (megapascal). ThePermanent Wilting Percentage (PWP) is the water potential below which plants are unable todraw sufficient water for life, let alone growth. Each plant species or group of plants has its ownspecific PWP. Conventionally a PWP of -1.5 MPa is an average for most plants, but xerophilous(dry-loving) plants wilt at PWP=-3 MPa, whereas hygrophilous (moisture-loving) plants wilt atPWP=0.7 MPa.

chemically combined water: part of rock composition, hydrated minerals. It is

unavailable to plants.

hygroscopic water: a thin layer of water that coats soil particles. It is unavailable to

plants. (-5MPa)

capillary water:

held in soil capillaries: water retained agains the pull of gravity because offcohesive and adhesive forces. (Cohesive= attraction of unlike charged polarwater molecules to each other) (Adhesive= attraction between polar water

molecules and other polar materials such as glass, soil particles, cellulose)

plant available water: soil water available to plants, water potential ranging from-0.03 to -1.5 MPa.

-0.03 MPa: soil is at field capacity, saturated. The upper limit of soilwater.

-0.2 MPa: most soil water gone.

-1.5 MPa: there is virtually no plant-available water below this pressure.

gravitational water: free water that moves downward through the soil profile, bypercolation. Little of it is available to plants.

movement of soil m oisture:

infiltration: movement of water into a soil. Enhanced by good soil structure,coarse texture, presence of organic matter, but little is held in mulches.Decreased by soil compaction, poor soil structure, high clay content, high soilwater content.

percolation: movement of water through the soil profile. Good structure andcoarse texture results in rapid movement. Movement decreases by poor soilstructure, high clay content, high soil water content.


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capillary movement: slow redistribution of water in soil capillaries; importantwhere subsurface irrigation is used and as crops withdraw water. Water canmove towards roots by capillary action.

soil pH: The pH value measures the number of H+ ions in solution as inverse powers of ten.Thus a solution with pH=8 has 10 times less H+ ions than one with pH=7. At pH=7 (pure water), asolution is neutral with as many acidic H+ ions as basic OH- ions. A pH greater than 7 is basic

(alkaline), whereas a pH less than 7 is acidic. Soil pH ranges from acidic 3.5 in high rainfall areasto basic 8.0 in low rainfall areas. Most plants thrive in slightly acidic soils pH=5.5-7.0, whichalso promotes the formation of new soil and the availability of nutrients. Positively chargedions like H+ are cations, whereas negatively charged ions like OH- are anions.

causes of acidity:

hydrogen ions H+: common acidity.

aluminium ions AL+++: react with water to form H+ : Al + H2O = Al(OH)3 + 3H+

soil acidification:

leaching of cations: Ca++, Mg++, K+ by water.

crop removal of cations: crops use cations as nutrients.

use of acid-forming fertilisers such as ammonium sulfate. Thesechange the pH by introducint H+ ions.

acid rain: nitrates (NO3-) and sulphates (SO4--) rain down, whichcauses aluminium to leach from clay, forming poisonous compounds.Forests leach excessive amounts of acid. Soils, rivers and lakes acidify.

significance of soil pH

effect on plants: most plants grow best at PH 5.5-7.0 but plants vary in theirrequirements. pH less than 4.0 or greater than 9.0 can be toxic to roots.

influence on nutrient availability: pH 4-5 and 8-9 influences the availability ofminerals to plants.

acidic soil (pH


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the lyotropic series: the relative capacities for cations to replace one another ifpresent in equivalent quantities. The order is Al > H > Ca > Mg > K > Na, wherethe weakest-bound ones (left) are the easiest to replace. Sodium (Na) is usuallyvery strongly bound.

law of mass action: adding large amounts of one cation will replace others,regardless of their relative capacity for replacement. Since H will replace Ca,

when they are present in equivalent amounts, excess Ca will replace H, andlarge amounts of it must be added to soil in order to raise its pH.

importance of soil CEC:

High CEC increases soil buffering capacity, the resistance to the change inconcentration of a nutrient or pH.

High CEC enhances nutrient retention in soils.

range of soil CEC: zero to over 100 centimole/kg; kaolinite clay= 10; montmorillonite

clay= 100; organic matter = 150-300.

problem soils:

saline soils: soils having less than 15% of CEC satisfied by Na, and a pH 7.0-8.5, and

an excess of Ca, Mg, and Na as salt (NaCl) or as sulphate (Na2SO4). It has adetrimental effect on plants due to high concetration of salts. The concentration can bereduced by leaching. Salts can accumulate also due to too much fertiliser, poor drainageand salty irrigation water, and soil water evaporation.

sodic s oils: soils having more than 15% of CEC satisfied by Na, and a pH 8.5-10 cause

detrimental effects to plants due to high pH and Na concentration but not because of salt(NaCl). It can adversely affect soil structure. Soils can be reclaimed by applying CaSO4(gypsum), wich displaces Na to produce soluble Na2SO4. (sodium sulfate).

soil formation factors:

mechanical weathering: base rock is broken into smaller pieces but does not change

chemically.

chemical weathering: base rock falls apart in parts that are chemically different, some

being soluble.

rainfall: slightly acidic rainfall dissolves and leaches minerals fom parentmaterial. Water acts as a catalyst.

temperature: higher temperature hastens the rate of all chemical reactions, sothe parent material weathers more rapidly.

time: weathering of parent material is a very slow process.

biological weathering: organisms acidify their environment and hasten the rate of

dissolution of parent material. Near roots, the concentration of ions can be very high.

soil typeCECcmole/kg

leachingof minerals

naturalfertility

watercapacity

sandy soils

sandy loamloamsilt loamclay/ clay loamorganic soil

2 - 4

2 - 177 - 169 - 304 - 6050 - 300

fast

fastfast to moderatemoderatelowvery low

very low

very lowlowmoderatehighhigh

low

medium

highvery low


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Soil degradationThis systematic classification of the many ways that soil can be lost, is not only interesting but also showsthat sustainable farming is like walking a tight-rope. Managing agricultural soil can be improved

considerably by paying attention to each of the factors detailed below.

loss in quantity

gravity

creep: Soil slowly creeps down-hill. Particularly clayey soils do this because clay,especially three-layer clays (Montmorillonite) can hold water up to equal theirvolume (100%). During droughts, the clay shrinks and during rains it expandsagain. This causes cracks in summer and also moves the soil slowly down-hill.

slip: A slip is usually a small area suddenly sliding down-hill, leaving a scarbehind and producing large amounts of loose soil on top of down-hill soil. Slipsare natural but occur more so on agricultural land. Clay soils may slip oversloping bedrock after long periods of gradual rainfall. This softens the clay, whichlubricates the bedrock. A slight disturbance like an earth shock can then set offmasses of slips. Trees reduce slips considerably.

slump: very large slips where whole hill sides move, are called slumps. water

ra indrop impact damage: Water drops hit bare soil and loosen clay and sandparticles. This is the largest source of clay runoff. Raindrop impact can beeliminated by covering the soil with vegetation or mulches.

sheetwash: Water runs over the soil like a sheet. It was thought that this causedmost runoff, but at this stage, the speed and pressure of the water is low.Sheetwash can be stopped by dense planting and soil cover.

rilling: As water collects downhill, small streams or rills are formed. They can cutthrough a cropland, transporting loose soil particles. At this stage, the water flowsfast and has sufficient volume to exert some pressure on the land. Sand, silt andclay are transported. Rilling can be prevented by contour ploughing and planting,

and by reducing the uphill size of the field. Allowing steeper uphill ground torevegetate naturally can serve as a water trap.

gullying: Surface gullies are formed in steep terrain where water flows fast,eroding the soil underneath until the bedrock is reached. Gullies normally formoutside the fields. They can be controlled by fencing and revegetating their sidesand by constructing check dams.

tunnelling: Water runs through cracks to the bedrock and scours out tunnelsbetween bedrock and soil. Eventually such tunnels slump, causing gullies thatcan erode quickly. This form of erosion is hard to control because it happens outof reach and out of sight. Fencing, revegetating and planting spaced trees uphillprevents further tunnelling.

river bank erosion: Swollen rivers exert pressure and friction on river banks,

particularly when saturated with mud. Most river banks were deposited by earlierrain storms and are easily eroded. Rivers need to flow freely during rain storms.Tree roots cannot prevent this. Riparian (riverside) planting has little effect andattention must be paid to the uphill sources of erosion. Riparian fencing helps tokeep cattle out.

wind: Wind without rain is surprisingly erosive. Clay particles become air-borne and can

blow vast distances. The soil selectively loses its most fertile components. Claydisappears and sand remains. Leaving stubble on the field helps. Planting shelter belts isvery effective over a large distance.


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ice

glaciers: Glaciers are rivers of ice, formed from snow. Under pressure, snowcompacts to ice. Glaciers exert enormous pressure but move slowly. They grindrocks to finer particles, which are deposited where they end, producing a sil (bar)in the valley. Because their pressure increases with depth, glaciers scour deepU-shaped valleys.

frost-heave: Under special conditions of moisture and repetitive frost, the soilcan expand suddenly, pushing its surface up and damaging roads.

rock cracks: When water freezes, it expands with enormous force, enough tobreak large rocks. On mountains, erosion is highest where snow becomes water,then freezes again.

drought

deep cracks: Deep cracks occur particularly in clay soils. Clay absorbs waterand expands. During droughts, it contracts and forms cracks. Cracks dry the landmore quickly. Water from the first rains, runs into these cracks, causing erosion.

creep: Consecutive expansion and contraction of the soil causes land to creepdown-hill.

down-hill degradation

nutrients/chemicals in rivers: Depending on the amount of rainwater and theamounts of fertiliser applied, rivers can exceed safe levels of nutrients, causingmortality to fish and other river life. Biocides in low concentrations are sufficientto also cause damage.

nutrients/chemicals in aquifers: Farm nutrients and persistent chemicals oftenend up in aquifers where they may remain and accumulate for hundreds of years.Aquifers also provide drinking water, becoming poisonous in the process.

rivers silting up: As water flows down-hill, it meets more water. Rills becometorrents, then rivers. Rivers flow swiftly in some places, slower in others, allowingsediment to settle out. This may change the downslope profile of the river,causing repeated flooding.

eutrophication of lakes: Nutrients from farms and fertilisers can wash out intorivers, ending up in lakes where they cause dense plant and plankton blooms.When plant matter sinks to the bottom, decomposing bacteria use up all oxygenand the lake becomes poisonous to life.

eutrophication of the sea: Nutrients from agriculture wash into the sea,fertilising the waters and causing excessive plankton blooms. Poisonousplankton species take over, posioning coastal fisheries and killing marinespecies. Decomposing bacteria take over, spreading disease and death.

pollution of the sea: Clay particles soil the sea, causing filter-feeding animals tosuffocate; Clay settles on plants and shades their leaves, while inhibiting their lifeprocesses. Clay clouds the water and shades deeper plants, who die. Clay formspans on the bottom, killing bottom life and changing vast areas of sea bottom

habitats. Persistent biocides are concentrated in marine animals, disrupting foodchains.

disappearing beaches: when beaches become polluted by nutrients (plankton),sewage (bacteria) or fine soil particles (mud), they won't dry out anymorebetween high tides. As a result, their self-repair mechanism is lost and theygradually retreat and disappear.

lowering of groundwater levels: By pumping groundwater for irrigation, its levelgradually drops, affecting areas down-hill and around. It may affect naturalstands of vegetation and wetlands.


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loss in quality

rain: Soluble nutrients are dissolved in water and transported. Carbondioxide CO2,

dissolved in rain forms a weak carbonic acid H2CO3 (H.HCO3) which can dissolve anumber of elements like calcium (Ca.(HCO3)2). Sulphur from the atmosphere formssulphuric acid H2SO4 which binds strongly with cations like Ca++ to gypsum CaSO4 andothers.

leaching out (e luviation): The leaching out of nutrients and minerals is greatlyaccelerated by ploughing. Soils lose their fertility.

leaching in (illuviation): Dissolved nutrients can be transported through the soilinto the groundwater but can also react with the soil minerals in the B horizon.Impenetrable iron pans can be formed or layers with carbonate compounds(CO3--) ('effervescence'). Acidity can prevent the formation of three-layer clays.

drought

loss of soil biota: During drought, the soil biota shrivel and die. The soil canlose its fertility easily.

ferrugination: Long droughts prevent clays from forming. Instead, ironsesquioxides are formed, which adhere firmly to sand and gravel, giving them a

red colour, and may cement them to form a subsurface iron pan. It forms a soilwith low fertility because nutrients are leached out and downward. Ferruginationmay occur after deforestation or because of poor farming practices.

rubification: when soil is thoroughly dried from time to time, precipitates of ironand organic matter cannot accumulate and the organic matter disappears bydecay, causing irreversibly dehydrated iron sesquioxides to form. Soils are calledcinnamonic (red).

farming practices affecting soil

burning: Almost all biomass (90%) is lost to the atmosphere, including allnitrogen and carbon compounds to feed the soil biota. In addition, soil biota arecooked and the top layer baked, losing its valuable humus and water-bindingstructure. The charcoaled wood won't decompose.

ploughing: Deep ploughing and shallow harrowing serve to render soil morefriable and porous. It removes weeds and produces a fine structure suitable forseeding and planting. Harrowing is also used to 'mulch' soil to prevent excessiveevaporation.

nutrient loss: Water can flow more freely through the A horizon into theB and C horizons and into the water table, taking nutrients with it. Onlyabout 15-50% of nitrogenous fertiliser is taken up by plants. Someevaporates, but as much as 40-50% washes into groundwater andaquifers.

humus loss: Organic matter is lost because soil biota decompose itmuch faster since the loose soil brings all the oxygen they need. Evenpersistent humus is lost this way.

soil biota loss: Soil biota are lost because not enough organic matterremains to feed them. In this way fertility is lost with them. The fertility ofa natural soil is kept inside the bodies of the soil organisms!

waterlogging: Ploughed soil opens the path for fine particles to bewashed downward where they clog together, blocking the naturaldrainage of water.

compaction: Heavy machinery (in 1948 averaging 2.7t, in 1990averaging 7-22t) is used to work the soil, causing compaction. Becausethe soil has lost its natural structure with roots and tunnels made by soil


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organisms, it is easily compacted, leading to water logging or reducingthe soil's ability to absorb water. Even without compaction, ploughedsoils have a porosity equal to a density of 1.4, whereas no-till or naturalsoil has density 1.0 near the surface. Deeper than plough depth, theybecome equally dense.

acidity pH: Compared to no-till cropping, the ploughed soil becomes

acidic pH=4.5 at 25cm, whereas no-till soil stays less acidic pH=6.5 totwice that depth.

irrigation eluviation of nutrients: Excessive irrigation can cause loss of nutrients.

pan-forming: Hard pans of iron oxide can form between B and Chorizons when the soil is unusually acid, such as under pine forests andother forests producing resinous leaf litter. The resins decompose toacids that leach minerals down into the B horizon. Here they react withnewly weathered soil to form a hard pan, a thin layer, which isimpenetrable to water and roots. (note that this happens mainly in coldpine forests). Acids may originate from fertiliser, acid rain and lack ofcalcium. Poor drainage is also a factor.

salinisation: When land is irrigated with water from lakes, water that ranoff other land first, cropland can become salty from the salts dissolved inthe water. Normally, salt (NaCl) is not held by the soil (but enough isretained in its organisms), and it is leached downward through the soilprofile and groundwater. Salinisation thus happens in arid climates withhigh evaporation and too little rain to wash excessive salt away. It canalso be caused by a drainage problem.

ch emical application: Herbicides (plant killers), fungicides (mould killers) andpesticides (insect killers)

loss of soil biota: The soil contains bacteria, fungi, worms and insects.Like their above-ground cousins, these are also sensitive to biocides.They all work together and if one group is affected, it affects the entirefunctioning of the soil. Manufacturers' claims of chemicals 'neutralising' inthe soil, must be treated with suspicion. How do they neutralise? Bykilling soil biota?

permanent unsuitab ility: Persistent biocides may cause long-termdamage to soil biota and thus soil fertility.

grazing: Grasslands can be sustainable but many are not. Because mostmeadowland has no trees, it is sensitive to erosion (see above) but otherdangers arise:

overgrazing: Overgrazing won't leave enough organic matter for the soilorganisms, causing soils to become less fertile. Overgrazing does notleave enough leaf coverage so that rain drops cause erosion. Lack ofleaf cover also unnecessarily dries the land out. Lost nutrients are notreplaced, causing soils to degrade. The slope of the land worsens allovergrazing effects. Overgrazing also reduces plant biomass becausethe small part left above ground cannot maintain a large partunderground.

pugging/camping /tramping: Cattle left to range over the land freely,compact the soil by their small hooves, which concentrate their weightover a small surface. The soil recovers slowly but now that the grasslandhas been made far more productive than natural prairie or steppe, manymore hooves tread it per acre. Pugged areas can become waterlogged.


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atmospheric pollution: Farms do contribute to atmospheric pollution and global warming.

fossil fuel use: Modern farming uses some 7 Gcal/ha = 830 l/ha fuel each year.

CO2: Carbondioxide is produced by farm machinery.

biogenic gases:

NH3: Ammonia is produced by soil bacteria when converting wastes andfertilisers.

N2O: Laughing gas and other nitrous oxides are produced when nitrogen is inexcess, due to fertilising.

CH4: Natural gas or methane is produced by grazing animals and by rice padis.

CO2: Carbon dioxide escapes from soil as its organic matter degrades, beingburnt up by soil organisms. In the process of soil degradation and deforestation,large amounts of CO2 enter the atmosphere.

CO: Carbonmonoxide is produced by agricultural soil as part of organic matterdegradation and burning.

soil timescalesSoil is perhaps the only resource that is not directly consumed. Water and air are principally inexhaustiblerenewable resources, but they are used. Water is used. The used water does not return directly, butrecycles through the atmosphere at high rates. So does air (carbon dioxide). But soil forms a noticeableexception. Every time it is used for a new crop, it is still there, afterwards. But soil degrades and is lost

gradually. Here are some timescales to remind you of its uniqueness.

tectonic movement and replenishment: 50-200 million years

soil profile formation: 10,000 years. Soil and terrestrial ecosystem have to evolve together and gothrough many thousands of years of succession until finally a stable community with its soil profileis formed. Soil without a ground cover of vegetation will never form a profile, since erosionexceeds soil formation. Soil formation: 1mm per 10 years. 1m = 10,000 years. New Zealand soilsdeveloped under very slow metabolising forests and therefore took much longer to form. Erosionrates were much lower too and NZ coastal seas very clear.

soil slip repair: 50-300 years. Slips repair relatively quickly because the soil is essentially stillthere and plants and micro organisms have to adjust to its new place. Some forests take a longtime to recover because trees over 300 years old may need to be replaced.

soil degradation through careful farming: 50-200 years, depending on the slope. Much farminghas been a hit-and-miss affair by trial and error. Modern scientific farming aims to change thistime scale.

soil degradation through careless farming: 5-50 years. Depending on slope, bad soil can be lostvery rapidly through bad farming practices.

Nitrogen cycling in the soil from bacterium to other microorganism: days

Nitrogen cycling between soil and plant: months

Nitrogen cycling from litter to soil: 1-200 years

Nitrogen cycling between soil and atmosphere: ?

Important rock and soil chemistry


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Acidic - intermediate - Basic = grouping according to the ratio of metal to oxygen atoms. Basic= highratio (less than 50% silica). Acidic = low (more than 50% silica). It is also the order of mineral formation

from a magma melt.

Solid solution = composed of various components such that the chemical formula of the rock is notunique and any combination is possible. Two or more elements can substitute for each other completely.For example, the anions Mg++ and Ca++ , which are similar in size and function, can combine CaSiO3

and MgSiO3 to (Ca,Mg)SiO3 as if the rock components were dissolved into one another. The fact thatsilicate rocks allow for substitution makes them easy to take apart through weathering.

Bowen Series: the Bowen Series orders igneous minerals by how soon they condensate out of a magmamelt, as it cools. First 'ultra-basic' minerals are formed. These have a high content of heavy elements andare correspondingly low in silica content. Likewise, the last minerals to condensate are 'acidic', havinghigh silica content and low heavy elements. A very rough rule is that the darker or denser the rock type,the more basic it is. Crustal minerals and rocks tend to be siliceous. Erupted lavas tend to be basic, and

deep-seated minerals and rocks tend to be ultrabasic. The bowen series is:

ultrabasic

olivine

pyroxene

Ca-feldspar, plagioclase

basic

amphibole

bio tite mica

intermediate

Na-feldspar, orthoclase

K-feldspar, orthoclase

muscovite mica

acidic

quartz

Lyotropic replacement series = the relative capacity for cations to replace one another if present inequivalent quantities. The order of preference is: Al+++ > H+ > Ca++ > Mg++ > K+ > Na+. Thus Al has

the weakest bond whereas Na the strongest.

Law of mass action: adding large amounts of one cation will replace others, regardless of their relativereplacement. Since H+ will replace Ca++, an excess in Ca++ must be added to soil in order to raise the

pH (make it less acidic).

Cation Exchange Capacity = a measure of soil, particularly the charged clay particles, to attract, holdand eschange cations Ca++, K+, Mg++, NH4+, H+ and Na+. The more negatively charged sites a claycontains, the more cations it can hold and the higher its CEC. A high CEC increases the soil's bufferingcapacity (its resistance to changes in pH or changes in nutrient concentrations). A high CEC enhances

nutrient retention in soils so that they can hold more. Soil CEC ranges from 0 to over 1 mole/kg. Note that'organic' soil (the soil biota) has a CEC of nearly two orders of magnitude (100x) larger than soils without

soil biota.

(See CEC table above)

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