Nutrient Supplying Power of Soil
In the past 150 years, CO2 levels in the atmosphere have increased from 260 to
365 ppm (Follett and McConkey, 2000) and it is expected to rise 1.5 to 2.0 ppm
per year (Wittwer, 1985). This increase
is believed to have increased the average temperature of the earth by 0.5 °C
and thus various reports of global warming as a result of increased evolution
of CO2 into earth's atmosphere
(Perry, 1983). It is possible to
decrease the release of CO2 to the
atmosphere by choosing an alternative energy source. However, total control of the release of CO2 is not easy because there are so many
different sources, including the production of cement, gasoline-driven
automobiles, burning of fuels for home heating, cooking, etc. (Wallace et al.,
1990). There are, however, several
benefits associated with increased atmospheric CO2 including
increased water use efficiency, nitrogen use efficiency and production in many
crops.
If
the expected fossil fuel CO2 released for many years could be stored as soil
organic matter, vastly enhanced productive soil would result. This option requires increased biomass to
produce the needed soil organic matter, but this could be achievable due to
increased CO2 supplies in the
atmosphere (Wallace et al., 1990).
Obstacles to increasing the level of soil organic matter are; 1) needed
organic matter supplies, 2) needed nitrogen to give around a 10:1 carbon:nitrogen
ratio necessary for stable soil organic matter, and 3) efficiency in microbial
activity that can result in more stable soil organic matter, instead of burn
out resulting in return of CO2 to the atmosphere (Wallace et al., 1990).
It
is seldom understood that organic matter contents in soils can be increased via
various management practices. Increased
use of no-till management practices can increase soil organic matter. After ten years of no-tillage with corn, soil
organic carbon in the surface 30 cm was increased by 0.25% (Blevins et al.
1983). Probably the least understood is
increased N rates in continuous crop production on resultant soil organic
matter levels. Various authors have
documented that N rates in excess of that required for maximum yields result in
increased biomass production (decreased harvest index values e.g., unit grain
produced per unit dry matter). This
results in increased amounts of carbon from corn stalks, wheat stems, etc.,
that are incorporated back into soil organic matter pools. Although this effect is well documented, the
deleterious effects of increased fertilizer N rates on potential NO3 leaching and/or NO3 surface runoff should be considered where
appropriate. Use of green manures and
animal wastes have obvious impacts on soil organic matter when used on a
frequent basis.
The
native fertility of forest and grassland soils in
Figure
1.1. Influence of cultivation time on
relative mineralization from soil humus and wheat residue. (From Campbell et
al. (1976)).
When
the reddish prairie soils of
N
removal in the Check (no fertilization)
plot of the Magruder Plots
20 bu/acre * 60 lb/bu * 100 years =
120000lbs
120000 lbs * 2%N in the grain = 2400 lbs
N/acre over 100 years
8000 lbs N in the soil (1892)
2000 lbs N in the soil (1992)
2400 lbs N removed in the grain
=3600 lbs N unaccounted
The
effects that management systems will have on soil organic matter and the
resultant nutrient supplying power of the organic pools are well known. Various management variables and their effect
on soil organic matter are listed;
Organic Matter Management Effect
_________________________________________________________
1) tillage +/- conventional -
zero +
2) soil drainage +/-
3) crop residue placement +/-
4) burning -
5) use of green manures +
6) animal wastes and composts +
7) nutrient management +/- excess N +
_________________________________________________________
The
living component which includes soil microorganisms and fauna make up a
relatively small portion of total soil organic matter (1-8%). It functions however as an important catalyst
for transformations of N and other nutrients (Doran and Smith, 1987). The majority of soil organic matter is
contained in the nonliving component that includes plant, animal and microbial
debris and soil humus.
Common
components of soil organic matter and their relative rates of decay are listed
in Table 1.1. Cellulose generally
accounts for the largest proportion of fresh organic material. It generally decays rather rapidly, however,
the presence of N is needed in order for this to take place. Lignin components decompose much more slowly
and thus, any nutrients bound in lignin forms will not become available for
plant growth. Although lignin is
insoluble in hot water and neutral organic solvents, it can be solubilized in
alkali solutions. Because of this, we
seldom find calcareous soils with extremely high organic matter. All of the polysaccharides decompose rapidly
in soils and thus serve as an immediate source of C for microorganisms. Decomposition of these respective components
is illustrated in Figure 1.2.
Table 1.1. Components of soil organic matter, rate of
decomposition and composition of each fraction.
____________________________________________________________________________________
Form Formula Decomposition Composition
____________________________________________________________________________________
Cellulose (C6H10O5)n rapid * 15-50%
Hemicellulose 5-35%
glucose C6H12O6 moderate-slow
galactose
mannose
xylose C5H10O5 moderate-slow
Lignin(phenyl-propane) slow 15-35%
Crude Protein RCHNH2COOH** rapid 1-10%
Polysaccharides
Chitin (C6H9O4.NHCOCH3)n rapid
Starch glucose chain rapid
Pectins galacturonic acid rapid
Inulin fructose units
____________________________________________________________________________________
* - decomposition more rapid
in the presence of N
** - amino
acid glycine (one of many building blocks for proteins)
Figure 1.2. Decomposition of Miscanthus sinensis leaf litter.
Table 1.2. Composition of mature cornstalks (Zea mays L.) initially and after 205
days of incubation with a mixed soil microflora, in the presence and absence of
added nutrients (Tenney and Waksman, 1929)
____________________________________________________________________________________
Initial Composition after 205 days (%)
composition No nutrients Nutrients
Constituents or fraction % added added
____________________________________________________________________________________
Ether and alcohol soluble 6 1 <1
Cold water soluble 11 3 4
Hot water soluble 4 4 5
Hemicelluloses 18 15 11
Cellulose 30 13 6
Lignins 11 23 24
Crude protein 2 9 11
Ash 7 19 26
____________________________________________________________________________________
The
composition of mature cornstalks before and after 205 days of incubation with a
mixed soil in the presence and absence of added nutrients is listed in Table
1.2. As decomposition proceeds, the
water soluble fraction (sugars, starch, organic acids, pectins and tannins and
array of nitrogen compounds) is readily utilized by the microflora (Parr and
Papendick, 1978). Ether and
alcohol-soluble fractions (fats, waxes, resins, oils), hemicelluloses and
cellulose decrease with time as they are utilized as carbon and energy
sources. Lignin, tends to persist and
accumulate in the decaying biomass because of its resistance to microbial
decomposition. Decomposition rates of
crop residues are often proportional to their lignin content and some
researchers have suggested that the lignin content may be a more reliable
parameter for predicting residue decomposition rates than the C:N ratio
(Alexander, 1977). Vigil and Kissel (1991)
included the lignin-to-N ratio and total soil N concentration (in g/kg) as
independent variables to predict potential N mineralization in soil. They also noted that the break point between
net N mineralization and net immobilization was calculated to be at a C/N ratio
of 40.
A
simple illustration of the carbon cycle is found in Figure 1.3. The carbon cycle revolves around CO2, its fixation and regeneration. Chlorophyll-containing plants utilize the gas
as their sole carbon source and the carbonaceous matter synthesized serves to
supply the animal world with preformed organic carbon. Upon the death of the plant or animal,
microbial metabolism assumes the dominant role in the cyclic sequence
(Alexander, 1977). Without the microbial
pool, more carbon would be fixed than is released, CO2 concentrations in the atmosphere would
decrease and photosynthesis rates would decrease.
Figure 1.3. Simple illustration of the carbon cycle (from Alexander, 1977). "Higher plants use light to convert water (H2O) and carbon dioxide (CO2) to glucose (C6H12O6) and oxygen (O2)."
C:N Ratios as Related to Organic Matter Decomposition
In
general, the following C:N ratios are considered to be a general rule of thumb
in terms of what is expected for immobilization and mineralization.
_____________________________________________________
C:N Ratio Effect
_____________________________________________________
30:1 immobilization
<20:1 mineralization
20-30:1 immobilization = mineralization
_____________________________________________________
Unfortunately,
C:N ratios say nothing about the availability of carbon or nitrogen to
microorganisms. The reason for this is
because we are not aware of what makes up the carbon (C) component. In tropical soils, significantly higher
proportions of lignin will be present in the organic matter. Even though the percent N within the organic
matter may be the same, it would be present in highly stable forms that were
resistant to decomposition. Therefore,
mineralization rates in organic matter that contain high proportions of lignin
will be much smaller. The C:N ratios
discussed were generally developed from data obtained in temperate
climates. Therefore their applicability
to tropical soils is at best minimal.
Decomposition of Organic Matter (Mineralization)
1. percent organic matter
2. organic matter composition
3. cultivation (crop, tillage, burning)
4. climate (moisture, temperature)
5. soil pH
6. N management (fertilization)
7. soil aeration
During
the initial stages of decomposition of fresh organic material there is a rapid
increase in the number of heterotrophic organisms accompanied by the evolution
of large amounts of carbon dioxide. If
the C:N ratio of the fresh material is wide, there will be a net N
immobilization. As decay proceeds, the
C:N ratio narrows and the energy supply of carbon diminishes.
The
addition of materials that contain more than 1.5 to 1.7% N would ordinarily
need no supplemental fertilizer N or soil N to meet the demands of the
microorganisms during decomposition (Parr and Papendick, 1978). Note that the 'demands of the microorganisms'
is what is discussed first, with no regard as to what plant N needs might
be. The addition of large amounts of
oxidizable carbon from residues with less than 1.5% N creates a microbiological
demand for N which can immobilize residue N and available inorganic soil N for
extended periods. The addition of
supplemental inorganic fertilizer N to low N residues can accelerate their rate
of decomposition (Parr and Papendick, 1978).
In
the thousands of years prior to the time cultivation was initiated, C and N had
built up in native prairie soils.
However, the C:N ratio was wide, reflecting conditions for
immobilization of N. The combined influence
of tillage and the application of additional organic materials (easily
decomposable wheat straw and/or corn stalks) is illustrated in Figure 1.4. Cultivation alone unleashed a radical
decomposition of the 4% organic matter in
Figure 1.4.
Effect of cultivation and addition of straw materials on immobilization
and mineralization of N and associated evolution of CO2
Figure
1.5. Changes in the nitrogen content of
decomposing barley straw (From Alexander, 1977).
Figure 1.6. Changes in soil mineral N as a function of time, and addition of manure and straw.
Table 1.2. Example calculations
of total N in organic matter fractions in soils and expected amounts of N
mineralized on a yearly basis.
____________________________________________________________________________________
min max min max
____________________________________________________________________________________
Organic Matter, % 1 2 4 12
1 ha (0-15cm), kg 2241653 2241653 2241653 2241653 (Pb = 1.47)
Organic, matter, kg 22416 44833 89666 268998
%
N in
(5%)
kg N in
% N mineralized/yr 0.03 0.03 0.03 0.03
(3%)
TOTAL (kg N/ha/yr) 33.6 67.2 134.4 ? 403.5 ?
____________________________________________________________________________________
DB= Mass of dry soil/volume of solids and voids
2000000 pounds/afs
ft3*0.02832 = m3
0.4535 lb/kg
1 ha = 2.471ac
1 ha = 10000m2
1 ac = 4047m2
2000000 lb = 907184.74 kg = 907.184 Mg
43560 ft2 * 0.5 ft = 21780 ft3 = 616.80m3
907.184Mg/616.80m3 = DB 1.4707
10000m2 * 0.15m = 1500 m3
2241653 kg /1000 = 2241.6 Mg
2241.6/1500 = DB 1.49 (g/cm3 = Mg/m3)
What will happen if
a) bulk density is changed?
b) % N in organic matter?
c) % N mineralized per year?
Organic Matter = 0.35 + 1.80 * (organic carbon) Ranney (1969)
The
most important function of the microbial flora is usually considered to be the
breakdown of organic materials, a process by which the limited supply of CO2 available for photosynthesis is replenished
(Alexander, 1977).
Five
major groups of microorganisms in the soil are:
1. Bacteria
2. Actinomycetes
3. Fungi
4. Algae
5. Protozoa
Soil
Bacteria: 108 to 1010 / g of soil
Heterotroph:
(chemoorganotrophic) require preformed organic nutrients to serve as sources of
energy and carbon.
1. Fungi
2. Protozoa
3. Most Bacteria
Autotroph:
(lithotrophic) obtain their energy from sunlight or by the oxidation of inorganic
compounds and their carbon by the assimilation
of CO2.
Photoautotroph:
energy derived from sunlight
1. Algae (blue-green, cyanobacteria)
2. Higher Plants
3. Some Bacteria
Chemoautotroph:
energy for growth obtained by the oxidation of inorganic materials.
1. Few Bacterial species (agronomic
importance)
a. nitrobacter,
nitrosomonas and thiobacillus
Arnon's Criteria of Essentiality
1.
Element required
to complete life cycle.
2.
Deficiency can
only be corrected by the ion in question.
3.
Element needs to
be directly involved in the nutrition of the plant and not indirectly via the
need of another organism.
Any mineral element that functions in plant
metabolism, whether or not its action is specific (Tisdale et al., 1985).
C, H, O, N, P, and S (constituent of proteins)
Ca, Mg, K, Fe, Mn, Mo, Cu, B, Zn, Cl, Na, Co, V, Si
(essential to one or more plants)
'CHOPKNS CaFe MgB Mn Cl CuZn Mo'
Mobile Nutrients
A. Plant
1.
deficiency symptoms appear in the lower older leaves
B. Soil
1. can
be taken up from a large volume of soil
Immobile Nutrients
A. Plant
1. deficiency
symptoms appear in the upper younger leaves
B. Soil
1.
taken up from a small volume of soil
Deficiency Symptom Element Mobility Mobility Form taken up
Soil Plant by Plants
____________________________________________________________________________________
overall
chlorosis seen N Nitrogen Yes Yes NO3-,NO2-,NH4+
first on
lower leaves
purple
leaf margins P
Phosphorus No Yes HPO4=,H2PO4-,H3PO4
chlorotic
leaf margins K Potassium No Yes K+
uniform
chlorosis, stunting
(younger
leaves) S Sulfur Yes Yes(no) SO4=,SO2*
N*S
interaction
stunting -
no root
elongation Ca Calcium No No Ca++
interveinal
chlorosis,
veins
remain green Fe Iron No (ls) No Fe+++,Fe++
interveinal
chlorosis Mg Magnesium No (ls) Yes/No Mg++
reduced
terminal
growth =
chlorotic tips B Boron (NM) Yes No H3BO3°
interveinal
chlorosis Mn Manganese No No Mn++, Mn+++
wilting,
chlorosis, reduced
root
growth Cl
Chlorine Yes Yes Cl -
young
leaves, yellow &
stunted Cu Copper No (ls) No Cu++
interveinal
chlorosis in
young
leaves Zn Zinc No (ls) No Zn++
interveinal
chlorosis,
stunting Mo
Molybdenum Yes/No(ls) No MoO4=
dark green
color Na Sodium No(ls) Yes Na+
C Carbon CO2
H Hydrogen H2O
O Oxygen H2O
____________________________________________________________________________________
* absorbed
through plant leaves
(NM) Non
Metal
(ls) Low
Solubility
Mo
availability increases with soil pH, other micronutrients show the opposite of
this.
Immobile nutrients
in plant; symptoms of deficiency show up in the younger leaves.
Stage of
growth when deficiency symptom is apparent = later stage
NITROGEN:
·
Key building
block of the protein molecule upon which all life is based
·
Indispensable
component of the protoplasm of plants animals and microorganisms
·
One of the few
soil nutrients lost by volatilization and leaching, thus requiring continued
conservation and maintenance
·
Most frequently
deficient nutrient in crop production
Nitrogen Ion/Molecule Oxidation States
Nitrogen ions and molecules that
are of interest in soil fertility and plant nutrition cover a range of N
apparent oxidation states from -3 to +5.
It is most convenient to illustrate these oxidation states using common
combinations of N with H and O, because H can be assumed in the +1 oxidation
state (H+1) and O in the -2 oxidation state (O=). The apparent N oxidation state, and the
electron configurations involved may be depicted as follows.
Hydrogen:
The electron configuration in the
ground state is 1s1 (the first electron shell has only one electron in it), as
found in H2 gas. Since the s
shell can hold only two electrons, the atom would be most stable by either
gaining another electron or losing the existing one. Gaining an electron by sharing occurs in H2,
where each H atom gains an electron from the other resulting in a pair of
electrons being shared. The electron
configuration about the atom, where: represent a pair of electrons, may be
shown as
H:H
and the bond may be shown as H-H
Hydrogen most commonly exists in
ionic form and in combination with other elements where it has lost its single
electron. Thus it is present as the H+
ion or brings a + charge to the molecule formed by combining with other elements.
Oxygen:
The ground state of O, having a
total of eight electrons is 1s2, 2s2, 2p4.
Both s orbitals are filled, each with two electrons. The 2p outer or valence orbital capable of
holding six electrons, has only four electrons, leaving opportunity to gain
two. The common gain of two electrons
from some other element results in a valence of -2 for O (O=). The gain of two electrons also occurs in O2
gas, where two pairs of electrons are shared as
O::O
and the double bond may be shown as O=O
Nitrogen:
The ground state of N is 1s2, 2s2,
2p3. It is very similar to that for
oxygen, except there is one less electron in the valence 2p orbital. Hence, the 2p orbital contains three
electrons but, has room to accept three electrons to fill the shell. Under normal conditions, electron loss to
form N+, N2+ or N3+ or electron gain to form N-,
N2-, or N3- should not be expected. Instead, N will normally fill its 2p orbital
by sharing electrons with other elements to which it is chemically (covalent)
bound. Nitrogen can fill the 2p orbital
by forming three covalent bonds with itself as in the very stable gas N2.
The Nitrogen cycle is not well
understood, largely because of how it is communicated. Similar to the way we communicate the
differences between normal, saline, sodic and saline-sodic soils, we should do
the same for response variables in the Nitrogen cycle. In addition to temperature and pH included
below, we could add reduction/oxidation, tillage (zero vs. conventional), C:N
ratios, fertilizer source and a number of other variables. These mechanistic models would ultimately
lead to many 'if-then' statements/decisions that could be used within a
management strategy.
>50F
|
denitrification |
volatilization |
|
leaching |
leaching |
<50F 7.0
Soil
pH
Assuming that we could speed up
the nitrogen cycle what would you change?
1.
Aerated environment (need for O2)
2.
Supply of ammonium
3.
Moisture
4.
Temperature (30-35C or 86-95F) <10C or 50F
5.
Soil pH
6.
Addition of low C:N ratio materials (low lignin)
Is oxygen required for
nitrification?
Does nitrification proceed during
the growing cycle? (low C:N ratio)
N Oxidation States:
oxidized: loses electrons, takes
on a positive charge
reduced: gains electrons, takes on
a negative charge
Ion/molecule Name
NH3 ammonia -3
NH4+ ammonium -3
N2 diatomic
N 0
N2O nitrous
oxide +1
NO nitric oxide +2
-
+ -
NO2- nitrite +3
-
NO3- nitrate +5
_____________________________________
H2S hydrogen
sulfide -2
SO4= sulfate +6
_____________________________________
N:
5 electrons in the outer shell
·
loses 5 electrons
(+5 oxidation state NO3)
·
gains 3 electrons
(-3 oxidation state NH3)
O: 6 electrons in the outer shell
·
is always being
reduced (gains 2 electrons to fill the outer shell)
H:
1 electron in the outer shell
N
is losing electrons to O because O is more electronegative
N
gains electrons from H because H wants to give up electrons
_______________________________________________
N recommendations
1.
Yield goal (2lb N/bu)
a. Applies fertilization risk on the
farmer
b. Removes our inability to predict
'environment' (rainfall)
2.
Soil test
a. For every 1 ppm NO3, N recommendation reduced by 2lbN/ac
Nitrite accumulation?
1.
high pH
2.
high NH4 levels (NH4 inhibits nitrobacter)
NITROGEN
CYCLE
Inorganic
nitrogen buffering is defined as the ability of the soil-plant system to
control the amount of inorganic N accumulation in the rooting profile when N
fertilization rates exceed that required for maximum yield.
If
N rates required to detect soil profile NO3 accumulation
always exceeded that required for maximum yields, what biological mechanisms
are present that cause excess N applied to be lost via other pathways prior to
leaching?
Nitrogen Buffering Mechanisms
1. Increased Applied N results in increased plant N
loss (NH3)
2. Higher rates of applied N - increased
volatilization losses
3. Higher rates of applied N - increased
denitrification
4. Higher rates of applied N - increased organic C, --
increased N in organic pools
5. Increased applied N - increased grain protein
6. Increased applied N - increased forage N
7. Increased applied N - increased straw N
· Urease
activity ·
Air Exchange
·
Temperature ·
N Source and Rate
· CEC
(less when high) ·
Application method
· H
buffering capacity of the soil ·
Crop Residues
· Soil
Water Content
NH4+ « NH3 + H+
If pH and temperature can be kept
low, little potential exists for NH3
volatilization. At pH 7.5, less than 7%
of the ammoniacal N is actually in the form of NH3 over the range of temperatures likely for
field conditions.
A+B « AB
Kf = AB/A x B
AB « A+B
Kd =
A x B/AB
Kf = 1/Kd (relationship between formation and dissociation
constants)
Formation constant (Log K°)
relating two species is numerically equal to the pH at which the reacting
species have equal activities (dilute solutions).
pKa and Log K° are sometimes synonymous
Henderson-Hasselbalch
pH = pKa + log [(base)/(acid)]
when (base) = (acid), pH = pKa
· Urea is the most important solid fertilizer in the world today.
· In the early 1960's, ammonium sulfate was the primary N product in world trade (Bock and Kissel, 1988).
·
The majority of all urea production in the
· Since 1968, direct application of anhydrous ammonia has ranged from 37 to 40% of total N use (Bock and Kissel, 1988).
· Urea: high analysis, safety, economy of production, transport and distribution make it a leader in world N trade.
· In 1978, developed countries accounted for 44% of the world N market (Bock and Kissel, 1988).
· By 1987, developed countries accounted for less than 33%.
Share of world N consumption by product group
1970 1986
Ammonium nitrate 27 15
Urea 9 37
Ammonium phosphates 1 5
Other N products (NH3) 36 29
Other complex N
products 16 8
increase pH (less H+ ions in soil solution)
CO(NH2)2 + H+ + 2H2O --------> 2NH4+ + HCO3-
pH 6.5 to 8
HCO3- + H+ --->
CO2 + H2O
(added H lost from soil solution)
CO(NH2)2 + 2H+ + 2H2O --------> 2NH4+ + H2CO3 (carbonic acid)
pH <6.3
Potential for gaseous loss from applied urea, both broadcast and incorporated.
During hydrolysis, soil pH can
increase to >7 because the reaction requires H+ from the soil
system.
(How many moles of H+ are consumed for each mole of urea hydrolyzed?) 2
In alkaline soils less H+ is initially needed to drive urea hydrolysis on a
soil already having low H+.
In an alkaline soil, removing more
H+(from a soil solution already low in H+),
can increase pH even higher
NH4+ + OH- ---> NH4OH ---->NH3 + H2O
pH = pKa + log [(base)/(acid)]
At a pH of 9.3 (pKa 9.3) 50% NH4
and 50% NH3
pH Base (NH3) Acid (NH4)
7.3 1 99
8.3 10 90
9.3 50 50
10.3 90 10
11.3 99 1
Equilibrium relationship for ammoniacal N and resultant amount of NH3 and NH4 as affected by pH for a dilute solution.
As the pH increases from urea
hydrolysis, negative charges become available for NH4+ adsorption because of the release of H+ (Koelliker and Kissel)
Decrease NH3 loss with increasing CEC (Fenn and Kissel, 1976)
assuming increase pH = increase
CEC, what is happening?
In acid soils, the exchange of NH4+ is for H+ on the exchange complex (release of H here,
resists change in pH, e.g. going up)
In alkaline soils with high CEC, NH4 exchanges for Ca,
precipitation of CaCO3 (CO3= from HCO3- above) and one H+ released which helps resist the increase in pH
However, pH was already high.
Soil surface pH and cumulative NH3 loss as influenced by pH buffering capacity (from Ferguson et al., 1984).
Ernst and Massey (1960) found
increased NH3 volatilization when liming a silt loam soil. The effective CEC would have been increased
by liming but the rise in soil pH decreased the soils ability to supply H+
Rapid urea hydrolysis: greater potential for NH3 loss. Why?
management: dry soil surface,
incorporate, localized placement- slows urea hydrolysis
H
ion buffering capacity of the soil:
Ferguson et al., 1984
(soils total acidity, comprised of
exchangeable acidity + nonexchangeable titratable acidity)
A large component of a soils total
acidity is that associated with the layer silicate sesquioxide complex (Al and
Fe hydrous oxides). These sesquioxides
carry a net positive charge and can hydrolyze to form H+ which resist an increase in pH upon an addition of a
base.
H+ ion supply
comes from:
1.
2.
hydrolysis of water
3.
Al and Fe hydrous oxides
4.
high clay content
A soil with an increased H+ buffering capacity will also show less NH3 loss when urea is applied without
incorporation.
1. hydroxy Al-polymers added (carrying a net positive
charge) to increase H+ buffering capacity.
2. strong acid cation exchange resins added (buffering
capacity changed without affecting CEC, e.g. resin was saturated with H+).
resin: amorphous organic substances (plant
secretions), soluble in organic solvents but not in water (used in plastics,
inks)
Consider the following
1. H+ is required for urea hydrolysis.
2. Ability of a
soil to supply H+ is related to amount of NH3 loss.
3. H+ is produced via nitrification (after urea is
applied): acidity generated is not beneficial.
4. What could
we apply with the urea to reduce NH3 loss?
an acid; strong electrolyte;
dissociates to produce H+;increased H+ buffering; decrease pH
reduce NH3 loss by maintaining a low pH in the vicinity of the
fertilizer granule (e.g. H3PO4)
Factors Affecting Soil Acidity
Acid: substance
that tends to give up protons (H+) to some other
substance
Base: accepts
protons
Anion: negatively
charged ion
Cation: positively
charged ion
Base cation: ? (this has been taught in the past but
is not correct)
Electrolyte: nonmetallic electric conductor in which
current is carried by the movement of ions
H2SO4 (strong electrolyte)
CH3COOH (weak electrolyte)
H2O
HA --------------> H+ + A-
potential active
acidity acidity
1. Nitrogen
Fertilization
A.
ammoniacal sources of N
2. Decomposition
of organic matter
CO2 + H2O --------> H2CO3 (carbonic acid)
H2CO3 ------> H+ + HCO3-
(bicarbonate)
humus
contains reactive carboxylic, phenolic groups that behave as weak acids which
dissociate and release H+
3. Leaching of
exchangeable bases/Removal
Ca, Mg, K
and Na (out of the effective root zone)
-problem in
sandy soils with low CEC
a. Replaced first by H and subsequently by Al (Al
is one of the most abundant elements in soils.
7.1% by weight of earth's crust)
b. Al displaced from clay minerals, hydrolyzed to
hydroxy aluminum complexes
c. Hydrolysis of monomeric forms liberate H+
d. Al(H2O)6+3 + H2O ----->
Al(OH)(H2O)++ + H2O+
monomeric: a
chemical compound that can undergo polymerization
polymerization:
a chemical reaction in which two or more small
molecules combine to form larger molecules that contain repeating
structural units of the original molecules
4. Aluminosilicate
clays
Presence of
exchangeable Al
Al+3
+ H2O -----> AlOH= + H+
5. Acid Rain
Acidification from N Fertilizers (R.L. Westerman)
1. Assume that
the absorbing complex of the soil can be represented by CaX
2. Ca represents
various exchangeable bases with which the insoluble anions X are combined in an
exchangeable form and that X can only combine with one Ca
3. H2X refers to dibasic acid (e.g., H2SO4)
(NH4)2SO4 -----> NH4+ to the exchange complex, SO4= combines with the base on the exchange complex
replaced by NH4+
Thought: Volatilization losses of N as NH3 preclude the development of H+ ions produced via nitrification and would
theoretically reduce the total potential development of acidity.
Losses
of N via denitrification leave an alkaline residue (
Table X. Reaction of N
fertilizers when applied to soil.
______________________________________________________________________
1. Ammonium sulfate
a. (NH4)2SO4 + CaX
----> CaSO4 + (NH4)2X
b. (NH4)2X + 4O2 nitrification
>2HNO3 + H2X + 2H2O
c. 2HNO3 + CaX
----> Ca(NO3)2 + H2X
Resultant
acidity = 4H+ /mole of (NH4)2SO4
2. Ammonium nitrate
a. 2NH4NO3 + CaX ----> Ca(NO3)2 + (NH4)2X
b. (NH4)2X + 4O2 nitrification
>2HNO3 + H2X + 2H2O
c. 2HNO3 + CaX
----> Ca(NO3)2 + H2X
Resultant
acidity = 2H+ /mole of NH4NO3
3. Urea
a. CO(NH2)2 + 2H2O ---->
(NH4)2CO3
b. (NH4)2CO3 + CaX ---->
(NH4)2X + CaCO3
c. (NH4)2X + 4O2 nitrification
>2HNO3 + H2X +2H2O
d. 2HNO3 +CaX
----> Ca(NO3)2 + H2X
e. H2X + CaCO3 neutralization >CaX + H2O + CO2
Resultant
acidity = 2H+ /mole of CO(NH2)2
4. Anhydrous Ammonia
a. 2NH3 +2H2O ----> 2NH4OH
b. 2NH4OH + CaX
----> Ca(OH)2 + (NH4)2X
c. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2O
d. 2HNO3 + CaX
----> Ca(NO3)2 + H2X
e. H2X +
Ca(OH)2 neutralization
> CaX + 2H2O
Resultant
acidity = 1H+/mole of NH3
5. Aqua Ammonia
a. 2NH4ON +
CaX ----> Ca(OH)2 + (NH4)2X
b. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2O
c. 2HNO3 +CaX
----> Ca(NO3)2 + H2X
d. H2X +
Ca(OH)2 neutralization
> CaX +2H2O
Resultant
acidity = 1H+/mole of NH4OH
6. Ammonium Phosphate
a. 2NH4H2PO4 + CaX
----> Ca(H2PO4)2 + (NH4)2X
b. (NH4)2X + 4O2 nitrification
>2HNO3 + H2X +2H2O
c. 2HNO3 +CaX
----> Ca(NO3)2 + H2X
Resultant
acidity = 2H+/mole of NH4H2PO4
______________________________________________________________________
In
grain production systems, N use efficiency seldom exceeds 50 percent. Variables which influence N use efficiency
include
a. Variety
b. N
source
c. N
application method
d. Time
of N application
e. Tillage
f. N
rate (generally decreases with increasing N applied)
g. Production
system
1. Forage
2. Grain
Olson
and Swallow, 1984 (27-33% of the applied N fertilizer was removed by the grain
following 5 years)
h. Plant
N loss
i. Soil
type (organic matter)
Calculating N Use Efficiency using The Difference Method
______________________________________________________________________
Applied N Grain
Yield N content N uptake Fertilizer Recovery
kg/ha kg/ha % kg/ha %
______________________________________________________________________
0 1000 2.0 20 -
50 1300 2.1 27.3 (27.3-20)/50=14.6
100 2000 2.2 44 (44-20)/100=24
150 2000 2.3 46 (46-20)/150=17
______________________________________________________________________
Estimated N use efficiency for grain production
systems ranges between 20 and 50%. The
example above does not include straw, therefore, recovery levels are
lower. However, further analysis of forage
production systems (Altom et al., 1996) demonstrates that N use efficiency can
be as high as 60-70%. This is largely
because the plant is harvested prior to flowering thus minimizing the potential
for plant N loss. Plant N loss is known
to be greater when the plant is at flowering and approaching maturity. It is important to observe that estimated N
use efficiencies in forage production systems do not decrease with increasing N
applied as is normally found in grain production systems. This is suggestive of 'buffering' whereby
increased N is lost at higher rates of applied N in grain production systems,
but which cannot take place in forage production systems.
Work by Moll et al. (1982) suggested the presence of
two primary components of N use efficiency: (1) the efficiency of absorption or
uptake (Nt/Ns), and (2) the efficiency with which the N absorbed is utilized to
produce grain (Gw/Nt) where Nt is the total N in the plant at maturity (grain +
stover), Ns is the nitrogen supply or rate of fertilizer N and Gw is the grain
weight, all expressed in the same units.
Other parameters defined in their work and modifications (in italics)
are reported in Table 4.2.
Recent
understanding of plant N loss has required consideration of additional
parameters not discussed in Moll et al. (1982).
Harper et al. (1987) documented that N was lost as volatile NH3
from wheat plants after fertilizer application and during flowering. Maximum N accumulation has been found to
occur at or near flowering in wheat and corn and not at harvest. In order to estimate plant N loss without the
use of labeled N forms, the stage of growth where maximum N accumulation is
known to occur needs to be identified.
The amount of N remaining in the grain + straw or stover, is subtracted
from the amount at maximum N accumulation to estimate potential plant N loss
(difference method). However, even the
use of difference methods for estimating plant N loss are flawed since
continued uptake is known to take place beyond flowering or the point of
maximum N accumulation.
Figure 4.1 Total N uptake in winter wheat with time and estimated loss following flowering.
Francis et al. (1993) recently documented that plant N
losses could account for as much as 73% of the unaccounted-for N in 15N
balance calculations. They further noted
that gaseous plant N losses could be greater when N supply was increased. Similar to work by Kanampiu et al. (1997)
with winter wheat, Francis et al. (1993) found that maximum N accumulation in
corn occurred soon after flowering (R3 stage of growth). In addition, Francis et al. (1993)
highlighted the importance of plant N loss on the development and
interpretations of strategies to improve N fertilizer use efficiencies.
Consistent with work by Kanampiu et al. (1997), and
Daigger et al. (1976), Figure 4.1 illustrates winter wheat N accumulation over
time. Estimates of plant N loss are
reported in Table 4.1. Harper et al. (1987) reported that 21% of the applied N
fertilizer was lost as volatile NH3 in wheat, of which 11.4% was
from both the soil and plants soon after fertilization and 9.8% from the leaves
of wheat between anthesis and physiological maturity. Francis et al. (1993) summarized that failure
to include direct plant N losses when calculating an N budget leads to
overestimation of N loss from the soil by denitrification, leaching and ammonia
volatilization.
NO3- + 2e (nitrate reductase) NO2-
+ 6e (nitrite reductase) NH4+
Reduction of NO3- to NO2-
is the rate-limiting step in the transformation of N into amino forms.
Does the plant wake up in the morning and turn on the
TV to check the weather forecast, to see if it should assimilate NO3
and attempt to form amino acids?
Could we look at the forecast and attempt to
communicate with the plant, letting it know that weather conditions will be
good (or bad), thus proceeding with increased NO3 uptake?
Major pathways for assimilation of NH3
1. Incorporation
into glutamic acid to form glutamine, a reaction catalyzed by glutamine synthetase
(Olson and Kurtz, 1982)
2. Reaction of
NH3 and CO2 to form carbamyl phosphate, which in turn is
converted to the amino acid arginine.
3. Biosynthesis
of amides by combination of NH3 with an amino acid. In this way aspartic acid is converted to the
amide, asparagine.
Table 4.1. Means over N rate and variety for protein, NUE components and estimated plant N loss