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of soil Soil is the collection of
bodies in the earth's surface, in places modified or even made by man,
earthy materials, containing living matter and supporting or
capable of supporting plants out-of-doors. Its upper 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 near the surface that differ from
the underlying rock material as a result of interactions, through time,
of climate, living organisms, parent materials and relief. In the few places
where it contains thin cemented horizons that are impermeable to roots,
soil is as deep as the deepest horizon. More commonly, soil grades at its
lower margin to hard rock or to earthy materials, virtually devoid of roots,
animals or marks of other 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)
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 these compounds
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.
macro organisms: insects,
grubs, earthworms. Earthworms help decompose organic matter, releasing
plant nutrients, aerating soils and improving drainage. The insect-like
organisms have hard skins and hard jaws, helpful for dissecting woody substances.
and nematodes, plant parasitic nematodes as well as harmless ones present
in most soils.
autotrophic, aerobic, anaerobic, facultative) oxidise S and N. Decompose
organic matter and may cause some plant diseases. Rhyzobium and Bradyrhyzobium
bacteria form symbiosis with legume plants to fix nitrogen from the air,
and are particularly important in tropical soils.
damping-off, stem fungi, crown fungi, root fungi, rot) important in decomposition
of organic matter, but may cause some plant diseases. 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.
in the decomposition of organic mater.
algae: autotrophic organisms.
Blue-green algae fix some nitrogen. they need sunlight and can live only
on the surface.
unweathered primary materials:
have little capacity to hold water and nutrients and are relatively chemically
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 a result of weathering.
Silica and alumina sheets are formed by recrystallisation. Amorphous clays
in warm climates form oxides of iron and aluminium. Clays have a high cation
exchange capacity (CEC) because they are negatively charged and can attract,
retain and exchange cations. Their water holding capacity is very high
because of their large surface area per unit mass.
three-layer clays (Si-Al-Si
lattice) montmorillonite and illite, have high CEC.
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.
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.
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 aeration and a high water holding
capacity. Poor structure can result in high water holding capacity but
poor aeration. Structure can be influenced by tillage. Puddling occurs
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.
pore space: soil is 40-60% pores. The pore size influences water holding
capacity and aeration.
capillary pores: retain
water against the pull of gravity
noncapillary pores: contribute
to soil aeration
profile: a vertical cut through the soil shows its layering, characteristic
of its mineralogy and history.
O horizon: organic matter,
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.
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). The Permanent Wilting Percentage
(PWP) is the water potential below which plants are unable to draw sufficient
water for life, let alone growth. Each plant species or group of plants
has its own specific 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 at PWP=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.
held in soil capillaries:
water retained agains the pull of gravity because off cohesive and adhesive
forces. (Cohesive= attraction of unlike charged polar water 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
-0.03 MPa: soil is at field
capacity, saturated. The upper limit of soil water.
-0.2 MPa: most soil water gone.
-1.5 MPa: there is virtually
no plant-available water below this pressure.
free water that moves downward through the soil profile, by percolation.
Little of it is available to plants.
movement of soil moisture:
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 soil water
of water through the soil profile. Good structure and coarse texture results
in rapid movement. Movement decreases by poor soil structure, high clay
content, high soil water content.
capillary movement: slow
redistribution of water in soil capillaries; important where subsurface
irrigation is used and as crops withdraw water. Water can move towards
roots by capillary action.
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), a solution 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 areas to basic 8.0 in low rainfall areas. Most plants
thrive in slightly acidic soils pH=5.5-7.0, which also promotes the formation
of new soil and the availability of nutrients. Positively charged ions
like H+ are cations, whereas negatively charged ions like OH- are anions.
causes of acidity:
hydrogen ions H+: common
aluminium ions AL+++:
react with water to form H+ : Al + H2O = Al(OH)3 + 3H+
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. These change the pH by introducint H+ ions.
acid rain: nitrates (NO3-)
and sulphates (SO4--) rain down, which causes 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 their requirements. 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 of minerals to plants.
acidic soil (pH <5.5):
reduced availability of Ca, Mg. Fixation of PO4. Reduced availability of
B, Mo. The levels of soluble Al, Mn, Zn, Fe increase to the point where
they may be toxic to plants.
alkaline soil: reduces
availability of B, Fe, P, Mn, Zn, Cu. If the Na concentration is high,
then Ca and Mg are reduced too.
amending soil pH: depends
on soil texture.
raising pH: use ground
lime, coral, dolomite (contains both Ca & Mg). Finely ground lime reacts
more quickly than coarse material. Addition of CaCO3 (lime) replaces H+
by mass action, raising soil pH.
lowering pH: usually
by adding elemental sulfur (S), which is oxidised to sulphuric acid H2SO4
by soil bacteria.
exchange capacity (CEC): a measure of the negatively charged sites
in a soil that can attract, hold and exchange positively charged ions (cations)
Ca, K, Mg, NH4, H, Na. The more negatively charged sites a clay contains,
the more cations it can hold and the higher its CEC.
units: expressed as centimoles
of negative charge in 1.0 kg of dry soil (centimol/kg). The CEC is equivalent
to the centimoles of H+ that will combine with 1.0 kg of dry soil.
percent base saturation:
the proportion of CEC satisfied by Ca, Mg, K and Na, but H and Al are excluded.
cation exchange: exchange
of cations between the soil solution and exchange sites are based on two
the lyotropic series:
the relative capacities for cations to replace one another if present in
equivalent quantities. The order is Al > H > Ca > Mg > K > Na, where the
weakest-bound ones (left) are the easiest to replace. Sodium (Na) is usually
very 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, and large 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 in concentration of a nutrient or
High CEC enhances nutrient retention
range of soil CEC: zero
to over 100 centimole/kg; kaolinite clay= 10; montmorillonite clay= 100;
organic matter = 150-300.
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 a detrimental
effect on plants due to high concetration of salts. The concentration can
be reduced by leaching. Salts can accumulate also due to too much fertiliser,
poor drainage and salty irrigation water, and soil water evaporation.
sodic soils: 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.
base rock is broken into smaller pieces but does not change chemically.
base rock falls apart in parts that are chemically different, some being
rainfall: slightly acidic
rainfall dissolves and leaches minerals fom parent material. Water acts
as a catalyst.
temperature: higher temperature
hastens the rate of all chemical reactions, so the parent material weathers
time: weathering of parent
material is a very slow process.
organisms acidify their environment and hasten the rate of dissolution
of parent material. Near roots, the concentration of ions can be very high.
degradation This systematic classification
of the many ways that soil can be lost, is not only interesting but also
shows that sustainable farming is like walking a tight-rope. Managing agricultural
soil can be improved considerably by paying attention to each of the factors
loss in quantity
creep: Soil slowly creeps
down-hill. Particularly clayey soils do this because clay, especially three-layer
clays (Montmorillonite) can hold water up to equal their volume (100%).
During droughts, the clay shrinks and during rains it expands again. 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 scar behind and producing
large amounts of loose soil on top of down-hill soil. Slips are natural
but occur more so on agricultural land. Clay soils may slip over sloping
bedrock after long periods of gradual rainfall. This softens the clay,
which lubricates the bedrock. A slight disturbance like an earth shock
can then set off masses of slips. Trees reduce slips considerably.
slump: very large slips
where whole hill sides move, are called slumps.
raindrop impact damage:
Water drops hit bare soil and loosen clay and sand particles. This is the
largest source of clay runoff. Raindrop impact can be eliminated by covering
the soil with vegetation or mulches.
sheetwash: Water runs
over the soil like a sheet. It was thought that this caused most 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 cut through a cropland,
transporting loose soil particles. At this stage, the water flows fast
and has sufficient volume to exert some pressure on the land. Sand, silt
and clay are transported. Rilling can be prevented by contour ploughing
and planting, and by reducing the uphill size of the field. Allowing steeper
uphill ground to revegetate 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 form outside the fields.
They can be controlled by fencing and revegetating their sides and by constructing
tunnelling: Water runs
through cracks to the bedrock and scours out tunnels between bedrock and
soil. Eventually such tunnels slump, causing gullies that can erode quickly.
This form of erosion is hard to control because it happens out of reach
and out of sight. Fencing, revegetating and planting spaced trees uphill
prevents 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 earlier rain storms and are
easily eroded. Rivers need to flow freely during rain storms. Tree roots
cannot prevent this. Riparian (riverside) planting has little effect and
attention must be paid to the uphill sources of erosion. Riparian fencing
helps to keep cattle out.
Wind without rain is surprisingly erosive. Clay particles become air-borne
and can blow vast distances. The soil selectively loses its most fertile
components. Clay disappears and sand remains. Leaving stubble on the field
helps. Planting shelter belts is very effective over a large distance.
glaciers: Glaciers are
rivers of ice, formed from snow. Under pressure, snow compacts to ice.
Glaciers exert enormous pressure but move slowly. They grind rocks to finer
particles, which are deposited where they end, producing a sil (bar) in
the valley. Because their pressure increases with depth, glaciers scour
deep U-shaped valleys.
frost-heave: Under special
conditions of moisture and repetitive frost, the soil can expand suddenly,
pushing its surface up and damaging roads.
rock cracks: When water
freezes, it expands with enormous force, enough to break large rocks. On
mountains, erosion is highest where snow becomes water, then freezes again.
deep cracks: Deep cracks
occur particularly in clay soils. Clay absorbs water and expands. During
droughts, it contracts and forms cracks. Cracks dry the land more quickly.
Water from the first rains, runs into these cracks, causing erosion.
creep: Consecutive expansion
and contraction of the soil causes land to creep down-hill.
nutrients/chemicals in rivers:
Depending on the amount of rainwater and the amounts of fertiliser applied,
rivers can exceed safe levels of nutrients, causing mortality to fish and
other river life. Biocides in low concentrations are sufficient to also
nutrients/chemicals in aquifers:
Farm nutrients and persistent chemicals often end 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 become torrents, then
rivers. Rivers flow swiftly in some places, slower in others, allowing
sediment 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 into rivers, ending up
in lakes where they cause dense plant and plankton blooms. When plant matter
sinks to the bottom, decomposing bacteria use up all oxygen and 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. Poisonous plankton species take over,
posioning coastal fisheries and killing marine species. Decomposing bacteria
take over, spreading disease and death.
pollution of the sea:
Clay particles soil the sea, causing filter-feeding animals to suffocate;
Clay settles on plants and shades their leaves, while inhibiting their
life processes. Clay clouds the water and shades deeper plants, who die.
Clay forms pans on the bottom, killing bottom life and changing vast areas
of sea bottom habitats. Persistent biocides are concentrated in marine
animals, disrupting food chains.
when beaches become polluted by nutrients (plankton), sewage (bacteria)
or fine soil particles (mud), they won't dry out anymore between high tides.
As a result, their self-repair mechanism is lost and they gradually retreat
lowering of groundwater levels:
By pumping groundwater for irrigation, its level gradually drops, affecting
areas down-hill and around. It may affect natural stands of vegetation
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 a number
of elements like calcium (Ca.(HCO3)2). Sulphur from the atmosphere forms
sulphuric acid H2SO4 which binds strongly with cations like Ca++ to gypsum
CaSO4 and others.
leaching out (eluviation):
The leaching out of nutrients and minerals is greatly accelerated by ploughing.
Soils lose their fertility.
leaching in (illuviation):
Dissolved nutrients can be transported through the soil into 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.
loss of soil biota: During
drought, the soil biota shrivel and die. The soil can lose its fertility
Long droughts prevent clays from forming. Instead, iron sesquioxides 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 soil with
low fertility because nutrients are leached out and downward. Ferrugination
may occur after deforestation or because of poor farming practices.
rubification: when soil
is thoroughly dried from time to time, precipitates of iron and organic
matter cannot accumulate and the organic matter disappears by decay, causing
irreversibly dehydrated iron sesquioxides to form. Soils are called cinnamonic
practices affecting soil
burning: Almost all biomass
(90%) is lost to the atmosphere, including all nitrogen and carbon compounds
to feed the soil biota. In addition, soil biota are cooked and the top
layer baked, losing its valuable humus and water-binding structure. The
charcoaled wood won't decompose.
ploughing: Deep ploughing
and shallow harrowing serve to render soil more friable and porous. It
removes weeds and produces a fine structure suitable for seeding and planting.
Harrowing is also used to 'mulch' soil to prevent excessive evaporation.
nutrient loss: Water
can flow more freely through the A horizon into the B and C horizons and
into the water table, taking nutrients with it. Only about 15-50% of nitrogenous
fertiliser is taken up by plants. Some evaporates, but as much as 40-50%
washes into groundwater and aquifers.
humus loss: Organic matter
is lost because soil biota decompose it much faster since the loose soil
brings all the oxygen they need. Even persistent humus is lost this way.
soil biota loss: Soil
biota are lost because not enough organic matter remains to feed them.
In this way fertility is lost with them. The fertility of a natural soil
is kept inside the bodies of the soil organisms!
soil opens the path for fine particles to be washed downward where they
clog together, blocking the natural drainage of water.
compaction: Heavy machinery
(in 1948 averaging 2.7t, in 1990 averaging 7-22t) is used to work the soil,
causing compaction. Because the soil has lost its natural structure with
roots and tunnels made by soil organisms, it is easily compacted, leading
to water logging or reducing the soil's ability to absorb water. Even without
compaction, ploughed soils have a porosity equal to a density of 1.4, whereas
no-till or natural soil has density 1.0 near the surface. Deeper than plough
depth, they become 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 to twice that depth.
eluviation of nutrients:
Excessive irrigation can cause loss of nutrients.
pan-forming: Hard pans
of iron oxide can form between B and C horizons when the soil is unusually
acid, such as under pine forests and other forests producing resinous leaf
litter. The resins decompose to acids that leach minerals down into the
B horizon. Here they react with newly weathered soil to form a hard pan,
a thin layer, which is impenetrable to water and roots. (note that this
happens mainly in cold pine forests). Acids may originate from fertiliser,
acid rain and lack of calcium. Poor drainage is also a factor.
salinisation: When land
is irrigated with water from lakes, water that ran off other land first,
cropland can become salty from the salts dissolved in the water. Normally,
salt (NaCl) is not held by the soil (but enough is retained in its organisms),
and it is leached downward through the soil profile and groundwater. Salinisation
thus happens in arid climates with high evaporation and too little rain
to wash excessive salt away. It can also be caused by a drainage problem.
application: Herbicides (plant killers), fungicides (mould killers)
and pesticides (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 entire functioning of the soil.
Manufacturers' claims of chemicals 'neutralising' in the soil, must be
treated with suspicion. How do they neutralise? By killing soil biota?
Persistent biocides may cause long-term damage to soil biota and thus soil
Grasslands can be sustainable but many are not. Because most meadowland
has no trees, it is sensitive to erosion (see above) but other dangers
won't leave enough organic matter for the soil organisms, causing soils
to become less fertile. Overgrazing does not leave enough leaf coverage
so that rain drops cause erosion. Lack of leaf cover also unnecessarily
dries the land out. Lost nutrients are not replaced, causing soils to degrade.
The slope of the land worsens all overgrazing effects. Overgrazing also
reduces plant biomass because the small part left above ground cannot maintain
a large part underground.
Cattle left to range over the land freely, compact the soil by their small
hooves, which concentrate their weight over a small surface. The soil recovers
slowly but now that the grassland has been made far more productive than
natural prairie or steppe, many more hooves tread it per acre. Pugged areas
can become waterlogged.
pollution: Farms do contribute to atmospheric pollution and global
fossil fuel use: Modern
farming uses some 7 Gcal/ha = 830 l/ha fuel each year.
CO2: Carbondioxide is
produced by farm machinery.
NH3: Ammonia is produced
by soil bacteria when converting wastes and fertilisers.
N2O: Laughing gas and
other nitrous oxides are produced when nitrogen is in excess, 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, being burnt 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 matter degradation and
timescales Soil is perhaps the only
resource that is not directly consumed. Water and air are principally inexhaustible
renewable resources, but they are used. Water is used. The used water does
not return directly, but recycles through the atmosphere at high rates.
So does air (carbon dioxide). But soil forms a noticeable exception. 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 go through
many thousands of years of succession until finally a stable community
with its soil profile is formed. Soil without a ground cover of vegetation
will never form a profile, since erosion exceeds soil formation. Soil formation:
1mm per 10 years. 1m = 10,000 years. New Zealand soils developed under
very slow metabolising forests and therefore took much longer to form.
Erosion rates 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 still there
and plants and micro organisms have to adjust to its new place. Some forests
take a long time to recover because trees over 300 years old may need to
soil degradation through careful
farming: 50-200 years, depending on the slope. Much farming has been a
hit-and-miss affair by trial and error. Modern scientific farming aims
to change this time scale.
soil degradation through careless
farming: 5-50 years. Depending on slope, bad soil can be lost very 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: ?
rock and soil chemistry
Acidic - intermediate
- Basic = grouping according to the ratio of metal to oxygen atoms.
Basic= high ratio (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 not
unique 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 that
silicate rocks allow for substitution makes them easy to take apart through
Bowen Series: the
Bowen Series orders igneous minerals by how soon they condensate out of
a magma melt, as it cools. First 'ultra-basic' minerals are formed. These
have a high content of heavy elements and are correspondingly low in silica
content. Likewise, the last minerals to condensate are 'acidic', having
high 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:
series = the relative capacity for cations to replace one another if
present in equivalent 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
relative replacement. 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,
hold and eschange cations Ca++, K+, Mg++, NH4+, H+ and Na+. The more negatively
charged sites a clay contains, the more cations it can hold and the higher
its CEC. A high CEC increases the soil's buffering capacity (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