Soil Quality: The view through the prism of Soils 101

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Soil Quality The view through the prism of
Soils 101

• D.W. Johnson
• Natural Resources and Environmental Science
• University of Nevada, Reno

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Soil Quality Who or What defines it?

• Soil Scientists?
• Plants?
• Water quality?
• Lawyers?
• Farmers?
• Conservationists?
• Will one definition fit all?
• Not likely
• Will the various definitions conflict?
• Almost certainly

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Factors of soil formation

• You cannot modify parent material, climate, or
  time
• You can modify biota (vegetation easily, microbes
  less easily) and, with some effort, topography

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SOIL ORDERS (12 major units of classification
according to the US 10th Approximation) Alfisols
Clay migration, moderately high BS Andisols
Volcanic parent material, high P
fixation AridisolsArid soils, high in salts and
pH Entisols Not well-developed even after long
periods (can occur anywhere) Gelisols
Permafrost Histosols Soils formed from organic
matter (peats and mucks) InceptisolsStill
forming, water is available for soil formation
MollisolsOrganic-rich A horizons, BS usually gt
50 Oxisols Highly-weathered (e.g., tropical
rainforest) Spodosols Fe, Al, and organic
matter transport, whitish E Horizon (e.g., boreal
forest) UltisolsClay transport like Alfisols,
but much more acidic higher temperature often
highly weathered (e.g., Southeastern U.S.)
Vertisols Mixed soils Swelling clays, frost,
etc cause lower horizons to mix with upper
horizons Often characterized by cracks
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Alfisols Relatively high base saturation not
organic rich evidence of clay transport
The central concept of Alfisols is that of soils
that have an argillic, a kandic, or a natric
horizon and a base saturation of 35 or greater.
They typically have an ochric epipedon, but may
have an umbric epipedon. They may also have a
petrocalcic horizon, a fragipan or a
duripan http//soils.usda.gov/technical/classifica
tion/orders/alfisols.html.
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Andisols Soils derived major properties from
volcanic parent material. High P fixation. Many
soils locally derived from andesite are andic
The central concept of Andisols is that of soils
dominated by short-range-order minerals. They
include weakly weathered soils with much volcanic
glass as well as more strongly weathered soils.
Hence the content of volcanic glass is one of the
characteristics used in defining andic soil
properties. Materials with andic soil properties
comprise 60 percent or more of the thickness
between the mineral soil surface or the top of an
organic layer with andic soil properties and a
depth of 60 cm or a root limiting layer if
shallower. http//soils.usda.gov/technical/classif
ication/orders/andisols.html.
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AridisolsArid soils Low in organic matter high
in salts and pH
The central concept of Aridisols is that of soils
that are too dry for mesophytic plants to grow.
They have either (1) an aridic moisture regime
and an ochric or anthropic epipedon and one or
more of the following with an upper boundry
within 100 cm of the soil surface a calcic,
cambic, gypsic, natric, petrocalcic petrogypsic,
or a salic horizon or a duripan or an argillic
horizon, or (2)A salic horizon and saturation
with water within 100 cm of the soil surface for
one month or more in normal years. An aridic
moisture regime is one that in normal years has
no water available for plants for more than half
the cumulative time that the soil temperature at
50 cm below the surface is gt5 C. and has no
period as long as 90 consecutive days when there
is water available for plants while the soil
temperature at 50 cm is continuously gt8
C http//soils.usda.gov/technical/classification/o
rders/aridisols.html.
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Entisols Leftovers Not well-developed even
after long periods (can occur anywhere)
 The central concept of Entisols is that of soils
that have little or no evidence of development of
pedogenic horizons. Many Entisols have an ochric
epipedon and a few have an anthropic epipedon.
Many are sandy or very shallow http//soils.usda.g
ov/technical/classification/orders/aridisols.html.

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Gelisols permafrost
The central concept of Gelisols is that of soils
that have permafrost within 100 cm of the soil
surface and/or have gelic materials within 100 cm
of the soil surface and have permafrost within
200 cm. Gelic materials are mineral or organic
soil materials that have evidence of
cryoturbation (frost churning) and/or ice
segeration in the active layer (seasonal thaw
layer) and/or the upper part of the
permafrost http//soils.usda.gov/technical/classif
ication/orders/gelisols.html.
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Histosols Soils formed from organic matter
(peats and mucks)
The central concept of Histosols is that of soils
that are dominantly organic. They are mostly
soils that are commonly called bogs, moors, or
peats and mucks. A soil is classified as
Histosols if it does not have permafrost and is
dominated by organic soil materials. http//soils.
usda.gov/technical/classification/orders/histosols
.html.
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InceptisolsStill forming Water is available for
soil formation (e.g., glaciated soils). Common in
the Sierra Nevada
The central concept of Inceptisols is that of
soils of humid and subhumid regions that have
altered horizons that have lost bases or iron and
aluminum but retain some weatherable minerals.
They do not have an illuvial horizon enriched
with either silicate clay or with an amorphous
mixture of aluminum and organic carbon. The
Inceptisols may have many kinds of diagnostic
horizons, but argillic, natric kandic, spodic and
oxic horizons are excluded. http//soils.usda.gov/
technical/classification/orders/histosols.html.
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MollisolsBrown-black surface horizons High in
organic matter, vermiculite or smectite clays
Base saturation usually gt 50 (e.g., Iowa farm
soils) Most extensive in the US (25), present in
the Great Basin at higher elevations
The central concept of Mollisols is that of soils
that have a dark colored surface horizon and are
base rich. Nearly all have a mollic epipedon.
Many also have an argillic or natric horizon or a
calcic horizon. A few have an albic horizon. Some
also have a duripan or a petrocalic horizon.
http//soils.usda.gov/technical/classification/or
ders/mollisols.html.
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Oxisols Highly-weathered Only quartz,
kaolinite, and Fe and Al oxides left (e.g.,
tropical rainforest)
The central concept of Oxisols is that of soils
of the tropical and subtropical regions. They
have gentle slopes on surfaces of great age. They
are mixtures of quartz, kaolin, free oxides, and
organic matter. For the most part they are nearly
featureless soils without clearly marked
horizons. Differences in properties with depth
are so gradual that horizon boundaries are
generally arbitrary. . http//soils.usda.gov/tech
nical/classification/orders/oxisols.html.
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Spodosols Evidence of Fe, Al, and organic matter
transport Often a whitish E Horizon (e.g.,
boreal forest)
The central concept of Spodosols is that of soils
in which amorphous mixtures of organic matter and
aluminum, with or without iron, have accumulated.
In undisturbed soils there is normally an
overlying eluvial horizon, generally gray to
light gray in color, that has the color of more
or less uncoated quartz. Most Spodosols have
little silicate clay. The particle-size class is
mostly sandy, sandy-skeletal, coarse-loamy,
loamy, loamy- skeletal, or coarse-silty http//soi
ls.usda.gov/technical/classification/orders/spodos
ols.html.
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UltisolsClay transport like Alfisols, but much
more acidic. Higher temperature Often highly
weathered (e.g., Southeastern U.S.)
The central concept of Ultisols is that of soils
that have a horizon that contains an appreciable
amount of translocated silicate clay (an argillic
or kandic horizon) and few bases (base saturation
less than 35 percent). Base saturation in most
Ultisols decreases with depth. http//soils.usda.g
ov/technical/classification/orders/ultisols.html.
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Vertisols Mixed soils Swelling clays, frost,
etc cause lower horizons to mix with upper
horizons Often characterized by cracks. Present
locally, San Rafael park
The central concept of Vertisols is that of soils
that have a high content of expending clay and
that have at some time of the year deep wide
cracks. They shrink when drying and swell when
they become wetter. http//soils.usda.gov/technic
al/classification/orders/vertisols.html.
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Basic Soil Physical Properties

• Texture particle size distribution (sand, silt,
  clay)
• Structure arrangement of particles (blocky,
  single grained, massive, platy)
• Coarse fragments/rocks gt 2mm
• Bulk density soil weight in g cm-3
• Porosity inversely proportional to bulk density
• Water properties field capacity, permanent
  wilting percentage, available water capacity,
  hydraulic conductivity

• You cannot modify texture or coarse fragments
• You can modify structure, bulk density, porosity,
  water properties

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Textural Triangle Terms like sandy loam
actually mean something specific relative to the
fine earth fraction (that is, the fraction of
soil that passes through a 2 mm sieve).
Image from http//en.wikipedia.org/wiki/Soil_text
ure
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Soil structure
Image Courtesy of NASA, Soil Science Education
Home Page http//ltpwww.gsfc.nasa.gov/globe/pvg/pr
op1.htm, accessed 14 Feb 2008 .
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Basic Soil Chemical Properties

• Total C organic matter carbonates
• Total N mostly organic
• CN Ratio major factor affecting N availability
• Cation exchange capacity (CEC) permanent charge
  (clays) and pH-dependent (organic matter)
• Base saturation (Ca2 Mg2 K Na)/CEC x
  100
• Adsorbed ortho-P and SO42- related to
  sesquioxide concentrations and organic coatings
• Micronutrients varying factors affect them

• You cannot modify permanent charge CEC or
  sesquioxides
• You can modify organic matter, N, CN ratio, base
  saturation, adsorbed ortho-P and SO42-

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• Cation Exchange Capacity (CEC)
• Sources
• Ionizeable H
• Organic matter, clay edges
• pH-dependent, just as in the case of a weak acid.
• Isomorphous substitution in clays
• Substitution of Al3 for Si4 in the tetrahedral
  layer of clays
• Substitution of Mg2 for Al3 in the octahedral
  layer of clay
• This type of CEC is often referred to as
  permanent charge CEC because it is not affected
  by pH.

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A simple example Ca2 exchange displaces
exchangeable Na
- - - - - -
..Na
Ca2
Na
..Ca2
..Na
Na
Dissolved in soil solution
Negatively-charged clay
2XNa Ca2 ? XCa2 2Na
X exchangeable
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• Cation Exchange
• Strength of cation adsorption (lyotropic series)
• Na lt K NH4 lt Mg2 Ca2 lt Aln lt H
• Adsorption depends on charge density
  (charge/vol), so increases with valence and
  decreases with size.
• Not all exchangeable ions are Aln and H because
  mass action allows the others to be present but
  at equal soil solution conc's, this will be the
  order.

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• Mass action
• Displacement of one adsorbed/exchangeable cation
  by another by competition for sites when the
  second has a high number of ions in solution
  (high concentration)
• Works even when trying to drive off most strongly
  absorbed cations like H and Al3
• This is why fertilization with K, Mg2 and
  liming (Ca2) work

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• Soil organic matter as a source of CEC
• Temporary (will ultimately decompose)
• Nearly insoluble in water, but soluble in base
  (high pH)
• Contains 30 each of proteins, lignin, complex
  sugars
• 50 C and O, 5 N
• Very high CEC on a weight basis
• Develops a net negative charge due to the
  dissociation of H from
• enolic (-OH), carboxyl (-COOH), and phenolic (
  -OH) groups as pH increases (solution H
  concentration decreases)

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pH-dependent CEC on Organic Matter

• No charge CEC and exch. K (could be any
  cation)
• R-OH0 OH- --------gt R-O- K H2O
• (R stands for some organic molecule)
• This leaves a net negative charge on the organic
  colloid (R-O-) which attracts cations just as the
  net negative charge on an isomorphously-substitute
  d clay does.
• Organic matter is the most important source of
  pH-dependent CEC in soils.

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• NRES 322 Soils
• Measurement of Cation Exchange Capacity (CEC)
  and Base Saturation (BS)
• CEC is measured by applying concentrated ammonium
  chloride (NH4Cl) or ammonium acetate (NH4OAc) to
  the sample to exchange all exchangeable cations
  with NH4 by mass action
• The extractant solution is analyzed for Ca2,
  Mg2, K, Na, and in some cases Al to
  determine what was on the exchanger.
• At that point, one measure of CEC can be made.
  Then the NH4 is displaced by another cation
  (typically Na or K ) by mass action, and NH4
  is then measured to obtain another estimate of
  CEC.

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• NRES 322 Soils
• Measurement of CEC and BS
• The usual assumption is that NH4 constitutes a
  negligible proportion of CEC.
• Exchangeable NH4 is often measured separately
  using concentrated KCl extractant.
• H (pH) is not measured on this extractant,
  either exchangeable H is measured another way.
• Some soil scientists argue that there is no
  exchangeable H on mineral soils all H that
  becomes absorbed onto clay minerals quickly
  enters the lattice structure and causes clay
  decomposition to hydrous oxides.

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• There are three ways to measure CEC (two from
  one method and one from another method)
• 1. Sum of cations Method
• The sum of Ca2, Mg2, K, Na, and Al after
  extraction with 1M NH4Cl (a neutral salt which
  does not buffer pH).
• CEC by sum of cations, CECsum, and is measured in
  the first extractant in Figure 1.
• In a pure clay system (no organic matter Fe, Al
  hydrous oxides, of allophane i.e., no
  pH-dependent CEC) this represents CEC and cations
  on the clay minerals (permanent charge CEC).

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Three ways to measure (cont.)

• 2. Effective CEC (CECeff) at existing soil pH.
• This includes the permanent charge CEC plus that
  portion of pH-dependent CEC that is in effect at
  existing soil pH.
• It is determined from the second extractant in
  Figure 1, After the 1M NH4Cl extraction, the soil
  is washed with ethanol to remove soluble NH4 ,
  and then extracted with 1M NaCl to displace the
  exchangeable NH4.
• The extractant is analyzed for NH4 .

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Three ways to measure (cont.)

• 3. Ammonium acetate CEC (CECOAc).
• This includes permanent charge CEC all
  pH-dependent CEC. Is is measured by extracting
  the soil with either ammonium acetate (NH4OAc,
  buffers pH at 7.0). (Figure 2).
• Then the same produre is followed as for the
  neutral salt CEC.
• Note exchangeable Al should be measured
  separately because Al precipitates as Al(OH)3 at
  high pH

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Base Saturation Base Cation Saturation
Percentage (BCSP) (often stated as simply base
saturation) BCSPis defined as the sum of
exchangeable base cations (Ca2, Mg2, K, and
Na) divided by CEC. It is usually expressed as
a percentage of CEC thus BS () Ca Mg
K Na CEC x100
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• Base Saturation
• Since CEC can be measured in different ways, BCSP
  will vary with the method used, and must be
  specified.
• For a soil with a given amount of exchangeable
  bases, Base saturation calculated from CECsum
  will be greater than that calculated from CECeff
  which will be greater than that calculated from
  CECtot because more of the potential acidity on
  the pH-dependen CEC is counted as CEC (i.e.,
  CECsum lt CECeff lt CECtot).
• The example in Figure 4 shows how this might
  occur. In each case, the base cations are the
  same (6 cmolc kg-1) only the measure of CEC (the
  denominator) changes.

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• Anion adsorption and retention on soils
• Negatively-charged ions adsorbed on
  positively-charges sites.
• In general, anion adsorption is associated with
  allophane and the hydrous oxides of Fe and Al in
  soils.
• H2PO4- gtgt SO4-2- gtgt NO3- gt Cl- (the latter being
  nil in all but the most sequoixide-rich soils)
• Anion adsorption on these surfaces is highly
  dependent upon pH.
• Usually much lower than CEC in temperate,
  non-volcanic ash soils.

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• Soil pH
• pH is the negative log of the H activity -log
  (H) therefore,
• 10-pH (H) (in moles L-1)
• Soil reaction, or pH is taken in a paste of water
  or 0.01 CaCl2.
• The latter gives a lower pH than the former, in
  most cases, because the Ca2 displaces
  exchangeable H and Al3 by mass action.

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• Soil pH
• pH decreases as base saturation decreases (recall
  that you must keep the methods constant, that is
  by sum, eff, or Oac the soil in Figure 4 has
  only one pH although base saturation value
  differs by method).
• pH has a strong effect on plant growth and
  nutrient availability It not only changes the
  solubility of many nutrients, but may also cause
  direct toxicity (Al, usually) to plant roots.

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http//www.terragis.bees.unsw.edu.au/terraGIS_soil
/sp_soil_reaction_ph.html
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Basic Soil Biological Properties

• Difficult to generalize what is a good soil
  relative to microorganisms
• CN Ratio major factor affecting decomposition
  and N availability
• pH low pH disfavors bacteria and favors fungi
• Aeration/flooding anaerobes vs aerobes

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• Soil Micro-organisms
• Many ways to classify
• Based on how they get energy
• Autotrophic Use sunlight of inorganic chemical
  reactions for energy
• Heterotrophic Use organic compounds for energy
• Based on oxygen requirements
• Aerobic
• Anaerobic
• Facultative

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• Some Important Heterotrophs
• Fungi
• Decomposers (tolerate low pH)
• Mycorrhizae (vital for growth of many plants)
• Bacteria/Actinomycetes
• Decomposers (do not tolerate low pH)
• Denitrifying bacteria (anaerobic)
• Sulfur reducing bacteria (anaerobic)
• Nitrogen fixers
• Rhizobium legumes
• Frankia actinomycetes alders, snowbrush

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Decomposition and N-Mineralization and
Immobilization in Soils Critical Processes for
Plant Nutrition!!!

• Most N taken up by plants in forest ecosystems is
  derived from decomposed organic matter (recycled)
• Decomposition is microbially mediated, yet
  microbial biomass is lt 3 of soil OM
• Microbes have higher concentrations of nutrients
  than the substrates they consume
• For example, bacteria and fungi have CN ratios
  of around 6 to 1 (4 to 8 N), whereas substrates
  they consume have CN ratios of 25 to 200 (3.0 to
  0.05 N)

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• Nitrogen Cycling in Soil
• Microbes concentrate nutrients in their bodies
• This is termed immobilization

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• Nitrogen Cycling in Soil
• When microbes concentrate nutrients in their
  bodies
• This is termed immobilization
• When microbes release N from during decomposition
• This is termed mineralization

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Nitrogen Cycling in Soils Importance of the CN
Ratio
Material C/N ratio Soil Microbes Bacteria
61 Actinomycetes 61 Fungi
121 Litter Types Alfalfa
131 Clover 201 Straw
801 Deciduous litter 401
to 801 Coniferous litter 601 to
1301 Woody litter 2501 to
6001 Soil Organic Matter 101
to 501
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CN Ratio

• In order for soil microbes to decompose most
  litter types, they must initially incorporate N
  from the soil
• Thus, inputs of high C/N ratio organic matter,
  such as sawdust or wood chips, can cause N
  deficiency to plants unless accompanied by
  fertilization
• As C is lost at CO2 gas, the C/N ratio of the
  litter decreases to a value ranging from 201 to
  301, at which point N is released from
  decomposing litter

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• Nitrogen Cycling in Soil
• Microbes concentrate nitrogen in their bodies
• This is termed immobilization
• When microbes release N from during decomposition
• This is termed mineralization

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• Other Important heterotrophs
• Mycorrhizae Essential for nutrient and water
  uptake in most plants
• Denitrifying bacteria N loss to gas in anaerobic
  conditions
• Nitrogen fixers
• Convert N2 gas in the atmosphere to ammonium
  (NH4)
• Very important source of N for soils and
  vegetation, especially in unpolluted areas
  soils have no mineral N source!
• The atmosphere is 78 N2 gas but plants cannot
  utilize it because of the strong triple bond
• NN
• Nitrogen fixers take energy from host plants
  (symbiotic) or associate (non-symbiotic) and
  convert this N to usable form using nitrogenase
  enzyme

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Some Important Chemautotrophs Nitrifying
bacteria One of the most important autorophic
bacteria are nitrifying bacteria, who convert
ammonium (NH4) to nitrite (NO2-) and nitrate
(NO3-) 2NH4 3O2 ? 2NO2- 4H
2H2O Nitrosomonas 2NO2- O2 _?
2NO3- Nitrobacter 2NH4 4O2 ? 4H 2H2O
2NO3- Note that nitrification is acidifying,
and therefore self-limiting (in theory) because
bacteria do now tolerate low pH. However,
nitrification has been observed many times in
very acid soils.
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Some Important Chemautotrophs
Sulfur oxidizing bacteria Another important
chemautotroph is Genus Thiobacillus most
important of chemautotroph mineral oxidizers
(elemental sulfur and sulfide minerals). For
elemental S 2S 3O2 2H2O -------gt 4H
2SO42- Thiobacillus thiooxidans
Another important reaction carried out by these
bacteria is the oxidation of pyrite, FeS2, which
occurs commonly in mine spoils by Thiobacillus
thiooxidans and Thiobaccillus ferroxidans 4FeS2
1502 2H2O ? 2Fe2(SO4)3 4H 2SO42-
Both reactions produce strong acid
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So given that brief background, what is a good
quality soil?

• I am guessing
• Relatively rich in organic matter
• Good N status (meaning low CN ratio)
• Circumneutral pH
• Good supplies of P, K, Ca, Mg, S and
  micronutrients
• Good texture or structure (water holding
  capacity)
• Bulk density near 1.0 (good infiltration and
  aeration, not compacted)
• But what does all this imply for
• Plants with varying nutritional needs
• Invasive species
• Water quality
• Soil water characteristics
• ?

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Case Study 1 Compaction

• Bob Powers LTSP sites included experimental
  compaction
• Gomez et al (2002) reported some results
• Many thanks to Bob for providing the following
  slides for my class!

Gomez, A., R.F. Powers, M.J. Singer, and W.R.
Horwath. 2002. Soil compaction effects on growth
of young ponderosa pine following litter removal
in Californias Sierra Nevada. Soil S ci. Soc.
Amer. J. 66 1334-1343.
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INFLUENCE OF COMPACTION ON SOIL PORES BY
FUNCTIONAL GROUPINGS
Soil Pore Diam.
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Measured Total Soil Porosity
Soil Texture Total Soil Porosity (cm3/cm3) Total Soil Porosity (cm3/cm3) Total Porosity Decrease (cm3/cm3) Total Porosity Decrease (cm3/cm3)
Soil Texture Non-Compacted Treatments Compacted Treatments Abs Rel
Loam 0.64 0.60 0.04 6
Sandy loam 0.55 0.51 0.04 7
Clay loam 0.55 0.56 0.01 2
Loam (volcanic ash) 0.63 0.62 0.01 2
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Decrease in Soil Pores gt30µm as a Result of Soil
Compaction
Soil Texture Soil Macro Pores (cm3/cm3) Soil Macro Pores (cm3/cm3) Decrease in Soil Macro Pores Decrease in Soil Macro Pores
Soil Texture Non-Compacted Treatments Compacted Treatments Abs Rel
Loam 0.29 0.18 0.11 38
Sandy loam 0.29 0.19 0.10 34
Clay loam 0.23 0.18 0.05 22
Loam (volcanic ash) 0.29 0.22 0.07 24
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WHAT DOES COMPACTION DO TO SITE PRODUCTIVITY?
PRODUCTIVITY INCREASED ON SANDY TEXTURES
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Gomez, A., R.F. Powers, M.J. Singer, and W.R.
Horwath. 2002. Soil compaction effects on growth
of young ponderosa pine following litter removal
in Californias Sierra Nevada. Soil S ci. Soc.
Amer. J. 66 1334-1343.
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Lesson from Case Study 1

• Compaction can either increase or decrease soil
  quality as defined by
• Soil water characteristics
• Tree growth

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So given that brief background, what is a good
quality soil?
Good N status (low CN ratio) and circumneutral
pH is fertile ground for nitrifying bacteria.
What does this imply for nitrate pollution of
ground and surface waters? What does good N
status imply for N-loving invasive species like
cheatgrass? What does it imply for native
N-fixers like alder and snowbrush? What does
circumneutral pH imply for the competitive
advantage of native acid-tolerant species on
naturally acidic soils?
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Case Study 2
Here are some soil data from two forested
ecosystems with different vegetation cover for
the last 50 years. Which soil is of better
quality?
Depth (cm) Soil pH BS C (mg g-1) N (mg g-1) CN Bray P (mg kg-1) Db (g cm-3)
0-15 1 5.30.2 137 4311 1.60.4 27 8239 0.960.11
0-15 2 4.50.2 85 9526 4.81.5 29 2114 0.850.18
15-30 1 5.30.2 128 2910 1.20.3 24 4329 1.030.12
15-30 2 4.80.2 74 5917 3.20.7 18 106 0.860.15
30-45 1 5.30.1 95 268 1.20.2 22 3019 1.130.13
30-45 2 4.90.2 98 5824 3.11.1 19 83 1.010.21
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Case Study 2
Here are some soil data from two forested
ecosystems with different vegetation cover for
the last 50 years. Which soil is of better
quality?
Depth pH pH BS BS C (mg g-1) C (mg g-1) N (mg g-1) N (mg g-1)
cm Soil 1 Soil 2 Soil 1 Soil 2 Soil 1 Soil 2 Soil 1 Soil 2
0-15 5.30.2 4.50.2 137 85 4311 9526 1.60.4 4.81.5
15-30 5.30.2 4.80.2 128 74 2910 5917 1.20.3 3.20.7
30-45 5.30.1 4.90.2 95 98 268 5824 1.20.2 3.11.1
Depth CN CN Bray P (mg kg-1) Bray P (mg kg-1) Db (g cm-3) Db (g cm-3)
cm Soil 1 Soil 2 Soil 1 Soil 2 Soil 1 Soil 2
0-15 27 29 8239 2114 0.960.11 0.850.18
15-30 24 18 4329 106 1.030.12 0.860.15
30-45 22 19 3019 83 1.130.13 1.010.21
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Case Study 2
Soil 1 is from a Douglas-fir stand, soil 2 is
from an adjacent red alder stand. Same soils
before vegetation changed (Van Miegroet and Cole,
1984). So how do these soil properties affect
tree growth?
Depth pH pH BS BS C (mg g-1) C (mg g-1) N (mg g-1) N (mg g-1)
cm Soil 1 Soil 2 Soil 1 Soil 2 Soil 1 Soil 2 Soil 1 Soil 2
0-15 5.30.2 4.50.2 137 85 4311 9526 1.60.4 4.81.5
15-30 5.30.2 4.80.2 128 74 2910 5917 1.20.3 3.20.7
30-45 5.30.1 4.90.2 95 98 268 5824 1.20.2 3.11.1
Depth CN CN Bray P (mg kg-1) Bray P (mg kg-1) Db (g cm-3) Db (g cm-3)
cm Soil 1 Soil 2 Soil 1 Soil 2 Soil 1 Soil 2
0-15 27 29 8239 2114 0.960.11 0.850.18
15-30 24 18 4329 106 1.030.12 0.860.15
30-45 22 19 3019 83 1.130.13 1.010.21
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• Planting red alder on former red alder soil
  resulted in slower growth than planting red alder
  on former Douglas-fir soil.
• At this stage, no response in Douglas-fir, but
  past experience has shown that it will grow much
  better on former red alder soil.
• (The stand was destroyed in a wind storm after
  these measurements were taken).

Van Miegroet et al., 1989
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Interplantings of red alder and conifers
(Binkley, 2003)

• Initially, red alder inhibits D. fir growth in
  the poorer (Wind River) site
• Over time, D. fir takes the site, grows faster
  because of more N in soil at Wind River
• At N-rich Cascade Head site, alder continues to
  inhibit D. Fir

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What about water quality considerations? The red
alder soil produced high rates of nitrate
leaching and no doubt contributed to soil
acidification
Van Miegroet et al., 1984
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What about water quality considerations? Subseque
nt studies by Compton et al (2003) showed that
nitrate in streamwater from Oregon Coast
Watersheds was related to the presence of red
alder.
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What about water quality considerations? Harvesti
ng in red alder caused large reductions in
nitrate leaching Water quality problems were
related to excessive N-fixation, not directly to
soil properties in this case
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• Summary of Case Study 2
• Red alder improves C and N status of soils, which
  is good for Douglas-fir. But this comes at a
  price in water quality.
• Red alder acidifies soils which is (apparently)
  not good for red alder but does not bother
  Douglas-fir
• Therefore, what is good soil quality for
  Douglas-fir is not good quality for red alder.

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Case Study 3, Site 1 (cont)
Soil data from adjacent sites which have had
different vegetation cover for 100 years.
Vegetation 1 Vegetation 2
Vegetation 1 Vegetation 2
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Depth cm Soil 1 Soil 2
0-7 1.340.04 1.150.04
7-20 1.430.05 1.290.03
20-40 1.420.05 1.320.02
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Site 1 Upper Little Valley, Nevada. Mature,
adjacent snowbrush and 100 year old jeffrey pine
stands.
Veg 1 Ceanothus velutinus
Veg 2 Pinus jeffreyii
Comparisons of soils in beneath 5 paired
adjacent, mature stands of Ceanothus velutinus
and Pinus jeffreyii in Little Valley, Nevada
(Johnson, 1995).
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Site 2 In and near 1981 wildfire in lower Little
Valley, Nevada. 20-year-old snowbrush and nearby
mature jeffrey pine stands.

• Immediate post-fire
• Foliage and forest floor are totally combusted
• Soil organic matter losses unknown

• 20 years post-fire
• 80 N-fixing Ceanothus velutinus
• 20 non-fixing shrubs

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Soil solutions from beneath snowbrush in upper
Little Valley had extremely low (lt 3 umolc L-1)
nitrate concentrations (Johnson, 1995)
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Soil solutions from snowbrush-dominated former
fire site in Little Valley had only slightly
elevated nitrate concentrations (Stein, 2006)
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Summary of Case Study 3 Snowbrush studies in
Little Valley

• Both studies showed that snowbrush improves soil
  quality.
• Furthermore, soil solution data from both sites
  showed that snowbrush, unlike red alder, produce
  only very slight increases in nitrate leaching
  and does not acidify soils, but in fact,
  increases base saturation.
• So why not manage for snowbrush if we want to
  improve soil quality?
• Because snowbrush competes with regenerating
  forest vegetation for water if measures are not
  taken to control it, former forests will revert
  to chaparral for 50-100 years after wildfire!

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The Nitrogen Problem

• Nitrogen has some features that are unique among
  major nutrient that make it most frequently
  limiting and problematic
• No significant primary mineral source
• Will not accumulate in ionic form in soils in any
  substantial amounts for long
• N present in excess of biological demand nearly
  always nitrifies (if not already in NO3- form)
  and leaches away as NO3-, causing water pollution
  and soil acidification

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The Nitrogen Problem
Nitrogen is the most frequently limiting nutrient
and a high quality soil must have adequate N.
However, it is very difficult to manage
nitrogen at an optimal level for plant growth
while at the same time maintaining water quality
and not causing negative effects on other soil
nutrients (and causing deteriorating soil
quality)
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Summary and Conclusions

• A single soil quality standard does not make any
  sense
• Plants vary in nutritional needs
• Invasive species tend to love high quality soil
• We tend to like like oligotrophic (nutrient poor)
  surface water
• Soil quality, or soil fertility as it was
  previously termed, should be viewed from the
  perspective of management objectives and
  priorities
• Increase production
• Grow specific species (some love acid, some
  cannot tolerate it)
• Control invasive species
• Preserve water quality

It is probable that not all of these objectives
can be met at once with the same soil quality
standard!