Soil Alkalinity and Salinity

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Soil Alkalinity and Salinity
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Alkaline and Saline Soils

• Why do some soils become saline?
• Precipitation is less than potential
  evapotranspiration
• Cations released from mineral weathering
  accumulate
• because there is not enough leaching to wash them
  away

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• Saline soils occur in soils with pHgt8.5
• Ca2, Mg2, K and Na do not produce acid upon
• reacting with water
• The do not produce OH- ions either, but in soils
  with high
• concentrations of carbonate and bicarbonate
  anions,
• pHgt8.5. Hence the association between salinity
  and pH.
• CaCO3 ? Ca2 CO32- or NaCO3 ?
  2Na2 CO32-
• CO32- H2O ? HCO3- OH-
• HCO3- H2O ? H2CO3 OH-
• H2CO3 ? H2O CO2(gas)

• pH rises more for most soluble minerals (eg.
  NaCO3)
• pH rise is limited by the common ion effect

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• Micronutrient deficiencies in saline soils
• Fe and Zn deficiencies are common because their
• solubility is extremely low in alkaline
  conditions
• Addition of inorganic fertilizer may not improve
• this deficiency as they quickly become tied up in
• insoluble forms
• Chelate compounds are often applied to soils (Fe
• associated with organic compounds)
• Under high pH, B tightly adsorbs to clays in an
• irreversible set of reactions. In sandy soils, B
  content is
• generally low under any pH level, especially in
  wet
• environments due to leaching (problematic in wet
  or dry
• environments, but less so in between).

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Effect of soil pH on nutrient content and soil
microorganisms
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• Phosphorus is often deficient in alkaline soils,
  because it is
• tied up in insoluble calcium or magnesium
  phosphates
• eg. (Ca3(PO4)2 and Ca3(PO4)2
• Some plants excrete organic acids in the
  immediate vicinity of
• their roots to deal with low P
• Other notes of interest
• Ammonium volatilization is commonly problematic
  during
• nitrogen fertlization on alkaline soils (changes
  to gas lost
• to atmosphere)
• Molybdenum levels are often toxic in alkaline
  soils of arid
• regions

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• Salinization
• The process by which salts accumulate in the soil
• Soil salinity hinders the growth of crops by
  lowering the osmotic potential of the soil, thus
  limiting the ability of roots to take up water
  (osmotic effect). Plants must accumulate organic
  and inorganic solutes within their cells.
• Specific ion effect Na ions compete with K
• Soil structure breaks down, leading to poor
  oxygenation and
• infiltration percolation rates
• 36 of prairie farmland has 1-15 of its lands
  affected by
• salinization and 2 has more than 15 of its
  lands affected.
• Most prairie farmland (61 in Manitoba, 59 in
  Saskatchewan, and 80 in Alberta) has a low
  chance of increasing salinity under current
  farming practices.

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• Conservation farming practices to control soil
  salinity
• Reducing summerfallow
• Using conservation tillage
• Adding organic matter to the soil
• Planting salt-tolerant crops (eg., canola and
  cabbage)

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• Conditions promoting salinization
• the presence of soluble salts in the soil
• a high water table
• ET gtgt P
• These features are commonplace in
• Prairie depressions and drainage courses
• At the base of hillslopes
• In flat, lowlying areas surrounding sloughs and
  shallow water bodies.
• In areas receiving regional discharge of
  groundwater.

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Signs of Salinization A. Irregular crop growth
on a solonetz
Source Agriculture and Agri-food Canada
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Whitish crust of salts exposed at the surface
(B,C)
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Aerial photo of saline deposits at Power, Montana
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D. Presence of salt streaks within soils
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E. Presence of salt-tolerant native plants, such
as Red Sapphire
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Human activities can lead to harmful effects of
salinization, even in soils of humid regions
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Effect of road salt on Maple leaves
(b)
(a)
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Calcium carbonate accumulation in the lower B
horizon
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The white, rounded "caps" of the columns are
comprised of soil dispersed because of the high
sodium saturation
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Salinization in response to conversion of
natural prairie to agriculture
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Measuring the electrical conductivity (EC) of a
soil sample in a field of wheatgrass to determine
the level of salinity.
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A portable electromagnetic (EM) soil conductivity
sensor used to estimate the electrical
conductivity in the soil profile
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Effect of salinity on soybean seedlings
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Influence of irrigation technique on
salt movement and plant growth in saline soils
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Soil Nutrients
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The Importance of Soil Nitrogen
Amino acids
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Enzymes
Proteins that catalyze chemical reactions in
living organisms
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Nucleic Acids
A nucleic acid is a complex, high-molecular-weight
, biochemical macromolecule composed of
nucleotide chains that convey genetic
information.
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• Nitrogen Deficiency
• Pale, yellowish-green colour due to low
  chlorophyll content
• Older leaves turn yellow first and may senesce
  prematurely
• Spindly stems or few stems
• Low protein, but high sugar content (not enough N
  to combine
• with carbon chains to produce proteins)
• Low shootroot ratio
• Rapid maturity

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Nitrogen Deficiency Chlorosis Yellowing of
older foliage Restricted growth Few stems or
spindly stems
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• Nitrogen oversupply
• Lodging with wind or heavy precipitation due to
• excessive growth
• Delayed maturity
• Susceptibility to fungal diseases
• Reduced flower production
• Poor fruit flavour
• Low vitamin and sugar content of fruits and
  vegetables

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Nitrogen Forms
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• Nitrogen Fixation
• The nitrogen molecule (N2) is very inert. Energy
  is required to
• break it apart to be combined with other
  elements/molecules.
• Three natural processes liberate nitrogen atoms
  from its atmospheric form
• Atmospheric fixation by lightning
• Biological fixation by certain microbes alone
  or in a symbiotic relationship with plants
• Industrial fixation

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• Atmospheric fixation by lightning
• Energy of lightning breaks nitrogen molecules.
• N atoms combine with oxygen in the air forming
  nitrogen oxides.
• Nitrates form in rain (NO3-) and are carried to
  the earth.
• 5 8 of the total nitrogen fixed in this way
  (depends on site)
• Industrial Fixation
• Under high pressure and a temperature of 600C,
  and with
• the use of a catalyst, atmospheric nitrogen and
  hydrogen
• (usually derived from natural gas or petroleum)
  is combined
• to form ammonia (NH3).
• Ammonia can be used directly as fertilizer, or
  further processed
• to urea and ammonium nitrate (NH4NO3).

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• Biological Fixation
• Performed mainly by bacteria living in a
  symbiotic relationship with plants of the legume
  family (e.g., soybeans, alfalfa), although some
  nitrogen-fixing bacteria live free in the soil.
• Biological nitrogen fixation requires a complex
  set of enzymes and a huge expenditure of ATP.
• Although the first stable product of the process
  is ammonia, this is quickly incorporated into
  protein and other organic nitrogen compounds.

Carried out by Rhizobium bacteria in a
SYMBIOTIC relationship. Host provides carbohydrat
es for energy Rhizobium supplies plant
with fixed nitrogen.
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Nitrogen mineralization
95-99 of N is in organic compounds, unavailable
to higher plants, but protected from loss 1.
Soil microbes attack these organic molecules,
(proteins, nucleic acids, amino sugars, urea),
forming amino compounds 2. The amine groups are
hydrolyzed, with N released as NH4 (ammonium
ions See pg. 548) 3. Oxidation of NH4 to NO2-
and NO3- The reverse process (incorporation of
NO3- or NH4 into soil micro-organisms) is called
immobilization
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• Nitrification
• Bacteria of the genus Nitrosomonas oxidize NH3 to
  nitrites (NO2-).
• Bacteria of the genus Nitrobacter oxidize the
  nitrites to nitrates (NO3-).

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Soil Organic Nitrogen Organic (as opposed to
mineralized) nitrogen has variable structure
(still poorly understood) Most SON uptake occurs
after mineralization of SON to NO3- or
NH4 Plants may also take up SON directly, or
the N can be assimilated by mychorrizal
associations Ammonium fixation by clay
minerals Occurs more in the subsoil than in the
topsoil Ammonium may become fixed, or
entrapped within Cavities of the crystal
structure of vermiculites, micas and smectites
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Ammonia volatilization NH4 OH- H2O
NH3(gas)
Occurs more in soils with high pH, especially
when drying and when temperatures are high Soil
colloids (clay and humus) inhibit ammonia
volatilization through adsorption
Nitrate leaching Negatively-charged nitrate ions
are not adsorbed by colloids, so they move freely
with drainage water Result (i) impoverishment
of soil N (ii) environmental problems
(eutrophication) especially in heavily
irrigated zones with N-fertilizer application
or manure
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• Denitrification
• Denitrification reduces nitrates to nitrogen gas,
  thus replenishing the atmosphere.
• Performed by bacteria in anaerobic conditions.
  They use nitrates as an alternative to oxygen for
  the final electron acceptor in the respiration
  process.

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Nitrogen Storage in Soils

• Current levels of nitrogen in soils reflect the
  accumulation of N in the organic fraction over
  long periods of time.
• Only about 1.5-3 of the N stored is used on an
  annual basis.
• Over long time frames N is stable in natural
  ecosystems (dynamic equilibrium established
• between losses and additions)

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Soil Phosphorous and Potassium

• Why is phosphorus so important?
• Essential component of ATP
• (adenosine triphosphate)

Molecular currency of intercellular energy
transfer Used as energy source during
photosynthesis and cellular respiration Consumed
by many enzymes in metabolic reactions and during
cell division.
Notice the 5 N and 3 P in the ATP molecule
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• Incorporated into nucleic acids

DNA and RNA
Genetic instructions for the development and
functioning of all living organisms
Sugar-phosphate backbone
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• Phospholipid bilayer
• Cell membranes,
• composed of a
• phospholipid bilayer,
• control what goes
• into and out of a cell
• Active transport across
• the cell membrane
• requires ATP

Phospholipid
Phospholipid bilayer
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The Importance of Phosphorus P is involved
in P deficiency Photosynthesis Stunting Nitr
ogen fixation Thin stems Flowering Bluish-green
leaves Fruiting fruit quality Delayed
maturity Maturation Sparse flowering Root
growth Poor seed quality Tissue strength

• Similarly to nitrogen deficiency, the older
• leaves are often first affected
• P deficiency is often difficult to diagnose
• as visual changes are subtle

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• The Phosphorus Problem in Soil Fertility
• The total P content of soils is low.
• 200-2000 kg/ha in uppermost 15 cm (topsoil)
• Phosphorus compounds found in soils are often
• highly insoluble
• When soluble sources are added (fertilizers and
  manure)
• they often become fixed into insoluble compounds
• 10-15 of P added is taken up by crop in year of
  application
• Overfertilization for decades has led to
  saturation of the
• P-fixation capacity (large P reserves in N.
  American soils)
• In contrast, P deficiency is a serious problem in
  sub-Saharan
• Africa (removal repeatedly has exceeded addition)

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N, P and K Fertilizer Use in USA
Figure 14.1
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• Impact of Phosphorus on Environmental Quality
• P deficiency Land degradation
• Little P is lost in natural ecosystems as P
  cycles between
• living biomass and soils
• Once cleared for agriculture
• Soil erosion loss
• Biomass removal
• P-supplying capacity decreases, even if total P
  is sufficient
• Nodulation is affected by P-deficiency, thereby
  promoting
• N-deficiency
• Most problematic in most highly weathered soils
• Warm, moist environments of the tropics
• Oxisols Andisols
• Low availability of P when in association with Fe
  Al
• Lots of P needs to be applied to Andisols (Fig
  14.20)

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Combined P N deficiency limits biomass and
promotes further erosion
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Water Quality Degradation due to Excess P (and
N) Point sources Sewage outflows (phosphates in
soaps) Industries Non-point sources Runoff
water Eroded sediment from soils in affected
watershed
Too much of a good thing
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PHOSPHORUS CYCLE
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• The Phosphorus Cycle
• Phosphorus in the soil solution
• Very low concentrations (0.001 to 1 mg/L)
• Roots absorb phosphate ions, HPO42- (alkaline
  soils)
• and H2PO4- (acid soils)
• Uptake by Roots
• Slow diffusion of phosphate ions to root surfaces
• Mychorrizal hyphae extend outward several cm from
  root surface
• P can then be incorporated into plant tissues
  (Fig 14.9)
• Soil P replenished by plant residues, leaf
  litter, and animal waste
• Soil microorganisms can temporarily incorporate P
  into their cells
• Some soil P gets tied up in organic matter
  (storage
• future release)
• Available P seldom exceeds 0.01 of
• total soil phosphorus

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• Forms of Soil Phosphorus
• Organic phosphorus
• Calcium-bound phosphorus (alkaline soils)
• Iron-bound phosphorus (acid soils)
• Aluminium-bound phosphorus (acid soils)
• Low solubility not readily available for plant
  uptake
• P is slowly released from each of these types of
  compounds
• Leaching loss is low, but can play a role in
  eutrophication
• Unlike N, P is not generally lost in a gaseous
  form
• Gains and Losses
• Losses from plant removal, erosion of
  P-containing soil
• particles and dissolved P in surface runoff water
• Gains from atmospheric dust are very limited, but
  a balance is
• established in most natural ecosystems

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Leaching of P after saturation of fixed pool
Figure 14.22
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• Potassium
• The nutrient third most-likely to limit
  productivity
• Present in soils as K ion (not in structures of
  organic compounds)
• Soil cation exchange and mineral weathering
  dominate its
• exchange and availability (as opposed to
  microbiological
• processes)
• Causes no off-site environmental problems
• Igneous rocks are a good source alkaline soils
  keep it.
• Activates certain enzymes.
• Regulates stomatal opening
• Helps achieve a balance between negatively and
  positively charged ions within plant cells.
• Regulates turgor pressure, which helps protect
  plant cells from disease invasion.
• Promotes winter-hardiness and drought-tolerance

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• Potassium deficiency
• Leaves yellow at tip (chlorosis) and then die
  (necrosis)
• The leaves, therefore, appear burnt at the edges
  and may
• tear, leaving a ragged edge
• White, necrotic spots may appear near leaf edges
• Oldest leaves are most affected
• The Potassium Cycle
• High concentrations in micas and feldspars
• K between 21 crystal layers becomes available
• Returned to soil through leaching from leaves and
  from
• plant residue decomposition
• Some loss by eroded soil particles and leaching
• Replenishment required in most agroecosystems
  (1/5 of

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• Calcium
• Vast reserves in calcareous (chalk) soil.
• Calcium is a part of cell walls and regulates
  cell wall construction.
• Cell walls give plant cells their structural
  strength.
• Enhances uptake of negatively charged ions such
  as nitrate, sulfate, borate and molybdate.
• Balances charge from organic anions produced
  through metabolism by the plant.
• Some enzyme regulation functions.

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Magnesium Reserves in magnesium limestone.
Magnesium is the central element within the
chlorophyll molecule. It is an important cofactor
the production of ATP, the compound which is the
energy transfer tool for the plant. Sulphur Found
in rocks and organic material. Sulphur is a
part of certain amino acids and all proteins. It
acts as an enzyme activator and coenzyme
(compound which is not part of all enzyme, but is
needed in close coordination with the enzyme for
certain specialized functions to operate
correctly). It is a part of the flavour
compounds in mustard and onion family plants.
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Boron Boron is important in sugar transport
within the plant. It has a role in cell division,
and is required for the production of certain
amino acids, although it is not a part of any
amino acid. Manganese Manganese is a cofactor in
many plant reactions. It is essential for
chloroplast production. Copper Synthesis of some
enzymes important in photosynthesis Copper is a
component of enzymes involved with
photosynthesis. Iron Iron is a component of the
many enzymes and light energy transferring
compounds involved in photosynthesis. Zinc Zinc
is a component of many enzymes. It is essential
for plant hormone balance.
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Molybdenum Molybdenum is needed for the
reduction of absorbed nitrates into ammonia prior
to incorporation into an amino acid. It performs
this function as a part of the enzyme nitrate
reductase. Molybdenum is also essential for
nitrogen fixation by nitrogen-fixing bacteria in
legumes. Responses of legumes to Molybdenum
application are mainly due to the need by these
symbiotic bacteria.