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
•Saline soils occur in soils with pH>8.5
•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,
pH>8.5. Hence the association between salinity and pH.
CaCO3  Ca2+ + CO32or
CO32- + H2O  HCO3- + OHHCO3- + H2O  H2CO3 + OHH2CO3  H2O + CO2(gas)
NaCO3  2Na2+ + CO32-
•pH rises more for most soluble minerals (eg. NaCO3)
•pH rise is limited by the common ion effect
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).
Effect of soil pH
on nutrient content
and soil
microorganisms
•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
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.
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)
Conditions promoting salinization:
•the presence of soluble salts in the soil
•a high water table
•ET >> 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.
Signs of Salinization
A. Irregular crop growth on a solonetz
Source: Agriculture and Agri-food Canada
Whitish crust of
salts exposed at
the surface (B,C)
Aerial photo of saline deposits at Power, Montana
D. Presence of salt streaks within soils
E. Presence of salt-tolerant native plants, such as
Red Sapphire
Human activities can lead to
harmful effects of salinization,
even in soils of humid regions
Effect of road salt on Maple leaves
(a)
(b)
Calcium carbonate accumulation
in the lower B horizon
The white, rounded "caps" of the columns
are comprised of soil dispersed because
of the high sodium saturation
Salinization in
response to
conversion of
natural prairie
to agriculture
Measuring the electrical conductivity (EC) of a soil sample
in a field of wheatgrass to determine the level of salinity.
A portable electromagnetic (EM) soil conductivity sensor
used to estimate the electrical conductivity in the soil profile
Effect of salinity on
soybean seedlings
Influence of irrigation
technique on salt
movement and plant
growth in saline soils
The Importance of Soil Nitrogen
Amino acids
Enzymes
Proteins that
catalyze
chemical
reactions
in living
organisms
Nucleic Acids
A nucleic acid is a complex, high-molecularweight, biochemical macromolecule
composed of nucleotide chains that convey
genetic information.
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 shoot:root ratio
•Rapid maturity
Nitrogen Deficiency
“Chlorosis”
Yellowing of older
foliage
Restricted growth
Few stems or spindly
stems
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
Nitrogen Forms
Reduced
NH4+
N2
N 2O
NO
ammonia
molecular N
nitrous oxide
nitric oxide
Oxidized
NO2-
NO2-
NO3-
nitrite
nitrogen dioxide
nitrate
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
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 600°C, 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).
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
carbohydrates for energy;
Rhizobium supplies plant with
fixed nitrogen.
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 NO3The reverse process (incorporation of NO3- or NH4+
into soil micro-organisms) is called immobilization
Nitrification
•Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites
(NO2−).
•Bacteria of the genus Nitrobacter oxidize the nitrites to
nitrates (NO3−).
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
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
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.
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)
Soil Phosphorous and Potassium
Why is phosphorus so important?
1. 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
2. Incorporated into nucleic acids
DNA and RNA
Genetic instructions for
the development and
functioning of all living
organisms
Sugar-phosphate backbone
3. Phospholipid bilayer
Cell membranes,
composed of a
phospholipid bilayer,
control what goes
into and out of a cell
Phospholipid
Active transport across
the cell membrane
requires ATP
Phospholipid bilayer
The Importance of Phosphorus
P is involved in:
P deficiency:
Photosynthesis
Nitrogen fixation
Flowering
Fruiting & fruit quality
Maturation
Root growth
Tissue strength
Stunting
Thin stems
Bluish-green leaves
Delayed maturity
Sparse flowering
Poor seed quality
•Similarly to nitrogen deficiency, the older
leaves are often first affected
•P deficiency is often difficult to diagnose
as visual changes are subtle
The Phosphorus Problem in Soil Fertility
1. The total P content of soils is low.
200-2000 kg/ha in uppermost 15 cm (topsoil)
2. Phosphorus compounds found in soils are often
highly insoluble
3. 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)
N, P and K Fertilizer Use in USA
Figure 14.1
Impact of Phosphorus on Environmental Quality
1. P deficiency: Land degradation
Little P is lost in natural ecosystems as P cycles between
living biomass and soils
Once cleared for agriculture:
(i) Soil erosion loss
(ii) 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)
Combined P & N deficiency limits biomass and promotes
further erosion
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”
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
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
Leaching of P after saturation of fixed pool
Figure 14.22
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
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 2:1 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
plant K is typically removed in product). Excess in plants
can cause a dietary imbalance in ruminants
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.
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.
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.
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 nitrogenfixing bacteria in legumes. Responses of legumes to Molybdenum
application are mainly due to the need by these symbiotic
bacteria.