Active fraction: lends structural
stability, enhanced infiltration,
resistance to erosion and ease
of tillage (relatively high C/N)
Soil Nutrient Concentrations vs. Successional
Stage (Tambito, Cauca, Colombia)
1.2
15000
40
NITROGEN
POTASSIUM
PHOSPHORUS
30
0.8
10000
1st/late 2nd.
Early 2nd.
Deforested
m.eq.
p.p.m.
p.p.m.
20
0.4
5000
10
0.0
0
0
0
40
80
0
Depth (cm)
8
40
Depth (cm)
0
60
20
40
60
Depth (cm)
5
12
CALCIUM
m.eq.
20
BORON
ALUMINIUM
4
m.eq.
p.p.m.
8
3
4
2
4
1
0
0
0
20
Depth (cm)
40
60
0
0
20
40
Depth (cm)
60
0
10
20
30
Depth (cm)
40
50
Alkaline and Saline Soils
•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 pH>8.5,
there are higher concentrations of carbonate and bicarbonate
anions (due to dissolution of certain minerals)
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
Nutrient deficiencies in saline soils
•Fe deficiency common because its 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 acid soils).
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)
•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., rapeseed 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
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
Nitrogen Forms
Reduced
NH4+
N2
(Ammonia)
(molecular N)
N2O
NO
(nitrous oxide) (nitric oxide)
Oxidized
2NO2-
NO2-
NO3-
(nitrite)
(nitrogen
dioxide)
(nitrate)
Nitrogen
•An essential component of amino acids, and therefore all
proteins.
•An essential component of nucleic acids, and therefore needed
for all cell division and reproduction.
•Enzymes are specialized proteins, and serve to lower energy
requirements to perform many tasks inside plants.
Nitrogen is contained in all enzymes essential for all plant
functions.
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).
Nitrification
•Ammonia can be taken up directly by plants, but most of the
ammonia produced by decay is converted into nitrates.
•Nitrifying bacteria
•Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites
(NO2−).
•Bacteria of the genus Nitrobacter oxidize the nitrites to
nitrates (NO3−).
•Many legumes, in addition to fixing atmospheric nitrogen, also
perform nitrification — converting some of their organic nitrogen to
nitrites and nitrates.
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.
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
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.
Nitrogen
• Most nutrient problems in plants are caused by
only three elements: N, P and K
• More time and money are spent on the
management of Nitrogen than any other element.\
• Nitrogen is an essential component of protein and
due to its relative scarcity is sought after by most
mammals.
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 3% of the N stored is used on an
annual basis.
• Over long time frames N is stable as the
losses don’t tend to exceed the additions.
Nitrogen Storage in Soils
• Soils with high levels of OM usually contain the
highest levels of N.
• Requires conditions where the accumulation of
plant residue is very high, while limiting plant
decay.
• With the exception of swampy areas, these
conditions are found in the wet regions and in
relatively wet semi-arid
regions (eg. in some Mollisols)
Phosphorous and Potassium
• In order of importance N => P => K.
• So phosphorous comes next.
• Why is phosphorus so important?
– Nothing grows without it (plants or animals.).
– Essential component of ATP (adnosine
triphosphate).
NPK Fertilizer Changes.
Figure 14.1
Phosphorous
• ATP is synthesized through respiration and
photosynthesis.
• Drives most energy-requiring biochemical
reactions.
• Aside from photosynthesis, phosphorus is
essential for nitrogen fixation, flowering,
fruiting (seed production), maturation, root
growth, and structural tissue.
•
•
•
Phosphorus in Soil Fertility
Major limiting factor.
Leads of a variety of problems.
3 major issues
1. Due to relative importance, levels are usually low.
2. Most phosphorous found in soils is unavailable.
3. When it’s added, it often gets fixed.
Phosphorous Fixation
• Only a small proportion ever gets used (1015%).
• Rest gets fixed by the solid fraction of the
soil or is lost.
• Not a lot is found in plant material.
• Careful management is required as losses
are environmentally detrimental.
Figure 14.22
Figure 14.14
Potassium
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.
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.