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Plant Nutrients
• Plants need at least 17 essential elements: C,
H and O from CO2 and H2O; six others are
called macronutrients (3 primary, 3 secondary),
8 more are micronutrients.
• Night Bulletin: CHOPKNS CaFe
CuMg cuisine mighty-good
(with) ZnMn zinc and manganese
ClMo closed Mondays
Essential Elements
2
Mechanisms of Nutrient Uptake
Prior to absorption, nutrients reach the root by 3
mechanisms:
Mass flow – movement with the water flow.
Most prominent.
Diffusion – movement in response to a
concentration gradient. Slow.
Root interception – root extension. Very
important to find new nutrient sources.
3
Absorption into roots.
Passive Uptake: Some ions such as nitrate, can
move passively through the outer membrane of the
root surface along with water in the transpiration
stream.
Active Uptake: Not well understood, but many
nutrients (e.g., K+ and H2PO4-) must somehow bond
with an ion-specific carrier
Maintaining an Electrical Balance: As cations are
absorbed H+ is excreted or organic anions are
produced. As anions are absorbed HCO3- is excreted
or compensating cations are absorbed.
4
Absorption through leaves
Stomatal Absorption: Rapid absorption of soluble
ions from nutrient enriched water.
Used mostly for the immediate correction of critical
nutrient deficiencies. Most efficient for the
micronutrients. Does not build soil fertility. Danger
of phytotoxic effects if over applied.
5
Soil Nitrogen Gains and Transformations
N is unique in several ways:
• No mineral source (usually); SOM stores nearly all N (~90%).
• Atmosphere is main reserve, but unavailable; must
be fixed.
• Very low soil available pools relative to uptake.
• Volatile phases (NH3, N2O, N2).
• Can be taken up as cation (NH4+) or anion (NO3
-)
form. Assimilating NH4+ costs 2-5% of plant energy,
NO3- costs 15%; forms proteins & amino acids.
• Deposition can be major input in polluted areas.
Nitrogen Cycling in soils: Biologically controlled
NH4+ NO3
- Nitrification
Leaching
Organic N
Mineralization
Immobilization*
Litterfall
Root turnover
Plant N
Uptake Denitrification
N2, N2O N2 fixation
Atmospheric Deposition
Clay-fixed
NH4+
Immobilization*
Uptake
*Includes mostly microbial (biotic) but also some abiotic immobilization
6
• Requires great energy
• Mechanism may be strictly symbiotic, non-symbiotic
(free living), or associative symbiotic.
• 12 g organic carbon per g N.
• Amounts vary enormously: as low as 1-2 kg ha-1 yr-1
for lichens in Douglas-fir canopies to over 300 kg ha-1
yr-1 for alders (not just 170 kg ha-1 yr-1, as in book)
• Non-symbiotic N fixations is usually not important,
except possibly in desert crusts – but no
measurements of the latter are available.
Nitrogen Fixation
Release from SOM by the conversion of organic N into
inorganic N: terminal group aminization, de-
aminization, ammonification
• Highly dependent on C:N ratio (occurs at values <
20-30)
• Approximately 1-5% of the total organic N pool per
year
• Mineralization is critical to N cycling and plant
growth because it is the point at which N is
converted from organic to NH4+ form, the latter of
which is available to plants and nitrifiers
Nitrogen Mineralization
7
Nitrification:
•Conversion of NH4+ to NO3
- + H+ (review)
•Two-stage process
Nitrification of ammonium.
Immobilization:
• Organic tie-up of NH4+ and/or NO3
- by microbes
• Opposite of mineralization
• Highly dependent on C:N ratio (occurs at values
>20-30)
• Can have abiotic immobilization (chemical
reactions between NH4+ and/or NO3
- and soil
organic matter) in some cases
Ammonium Fixation:
• Adsorption and collapse within the crystal lattice
structure (e.g., illites)
Other kinds of fixation
8
Leaching:
• Only the NO3- form is important; NH4
+ adsorbs
strongly
• Nitrification is key to facilitating leaching
• Pollutes water, wastes N, and acidifies soil
• Nitrification inhibitors are sometimes used to
prevent it after fertilization
• Great need to get fertilizer timing and amounts
correctly to minimize this effect.
Nitrogen Losses
Denitrification:
• Microbial conversion of NO3- to N2O and N2.
• Anaerobic - may occur in microsites in aerobic soils
• Requires energy (organic matter)
• Usually less important loss than leaching in aerobic soil
Ammonium volatilization:
• Chemical, not biological process
• Occurs only at high pH (8 or above): NH4+ + OH- NH3 +
H2O
• Usually not important in mesic soils because pH is too low;
• Exception may be after urea fertilizer, which creates high
pH and NH4+
Gaseous Losses:
9
Materials Supplying N
10
Anhydrous ammonia
• Highest %N (82%)
• Injected with chisels (ag use only)
• Dangerous
Urea
• Cheapest solid N fertilzer because
highest %N (45-46%)
• Volatilization losses of ammonium can be
high
• In forests, abiotic immobilization can be
high
Ammonium sulfate
• Relatively expensive (21%N)
Ammonium nitrate
• Relatively cheap (33.5%N)
• Explosive (mixed with diesel to make
bombs - Oklahoma City was this)
• Nitrate in it can leach readily
Slow-release fertilizers
• Idea is not to flood soil with NH4+
immediately
11
Soil NH4 + NO3
Normal
fertilier
Slow-release
Time < 1 yr
Soil Phosphorus
• Second most commonly limiting (second on fert
bag)
•Taken up in anion form (H2PO4- or, at higher pH,
HPO42- )
• Both soil mineral (apatite) and organic sources are
important
• Deposition is unimportant over the short-term
except perhaps the Lake Tahoe Basin
• Strongly retained by adsorption in acid soils and by
precipitation with Ca (forming apatite) in alkaline soils
• Forms in plants: ADP, ATP (plant energy currency)
• Most uptake is thought to be by diffusion; root
exploration it therefore critical
12
• Often must add bands or dollops or spikes in P-
fixing soils
• Excess P is retained in soil; does not leach like N
• P fertilization can enhance soil P availability for
decades (unlike N)
• Excess P can fill anion adsorption sites and
cause problems with sulfate retention
• P fertilizers
Materials Supplying Phosphorus
13
Soil Potassium
• Second most in terms of plant use.
• No volatile phases.
• Mineral sources in soils are important.
• Organic sources in soils are not important.
• Taken up as cation (K+ )
• Often limiting and often added; third
number on fertilizer bag
• Roles in plants: stomatal control, cell
division, translocation of sugars, enzymes
• Highly soluble
• Mineral phases (micas and orthoclase feldspar)
are highly insoluble
• Soil total K is often very large
• K can be "fixed" between 2:1 clays; this form
slowly available to plants
• Exchangeable K+ is the major form available to
plants and is only a fraction of the total.
• Smaller than exchangeable Ca2+ and Mg2+
• But much larger than exchangeable NH4+
14
• Book discusses crop K fertilization.
• In forests, K cycling is a major factor allowing
long-term responses
• Also can have long-term increases in soil
exchangeable K+
• K Fertilizers: Table 9-10.
• Added as ionic K+ form with various anions.
• Note also that often expressed as K2O.
• This is 39.1*2 + 16 = 94.2; %K in this is 83%
K Management
15
Soil Calcium
Ca2+ and Mg2
+ have many similarities in soils:
Both divalent
Primary minerals are the major source for
both
Abundant in most soils (except acid soils)
Mass flow dominates plant uptake
Difference:
Ca2+ is immobile in plants (pectates, etc)
Mg2+ is mobile (chlorophyll)
16
• Present in many primary minerals and very
abundant in soils
• Forms secondary minerals (calcite, CaCO3;
gypsum, CaSO4)
• Dominates the exchanger in non-acidic soils
• Ca2+ dominates exchange sites and soil solution
in non-acidic, non-serpentine soils
• Ca2+ also usually dominates the base cation
component in acidic soils (where H+ and Al3+
dominate the exchange sites)
• Ca is a component of cell wall material in plants;
very immobile in plants
• Mass flow is usually adequate for transport to
roots
• Very rarely limiting in nature and when so often
indistinguishable from Al toxicity
• Very large variation in plant demand for Ca
• In forests we have high Ca uptake trees (oaks,
hickories, aspen, cedars)
17
• Rare, because soils low in Ca are usually
extremely acidic and have Al3+ or H+ toxicity first
• Large quantities of Ca2+ are added in lime
• Can also add gypsum or CaCl2
Calcium Deficiencies
• Present in many primary minerals
• Mg2+ form
• Adsorbed to exchange sites about equal to
Ca2+
• Usually second most abundant in non-acid
soils and soil solutions
• Forms less soluble carbonates and sulfates
than calcium does
• Mass flow dominates uptake mechanism
Soil Magnesium
18
• Role in chlorophyll in plants; mobile in
plants; less variable in plant uptake than Ca
• Deficiencies common in acidic soils
• Fertilizers: dolomitic lime, Mg,K2 -SO4,
MgSO4 (epsom salts)
Soil Sulfur
Many similarities to N:
Atmosphere a major source
Role in three plant proteins
Mobile in plants (translocates)
Gaseous phases
Few mineral phases (sulfides)
C:S and N:S ratio can control
mineralization
Oxidized and reduced forms
19
Major differences from N:
• Inorganic form (SO42-) can be a major
form in plants and soils
• Some soil mineral sources
• Inorganic soil SO42- pools can be quite
large
• Not limiting as often (air pollution)
• Required by plants in much lower
quantities
20
• Excess S in plants present as SO42- (not in
book)
• Some foliar SO42- appears to be necessary
• Foliar SO42- is a good diagnostic for S
deficiency in trees
• Anionic form in aerobic soils (SO42-)
• "…easily leached" according to book; not
always true
• Maybe very immobile in acid soils
• Adsorbed to Fe, Al hydrous oxides
• Some acid forest soils retain 50-80% of
atmospherically-deposited S
• Elemental S or sulfide S (FeS, PbS) is oxidized
by Thiobacillus to sulfuric acid in aerobic soils
• Sulfate is reduced to sulfide (S2-) by S-reducing
bacteria in anaerobic soils
• This further forms either H2S (hydrogen sulfide
gas, rotten egg smell in swamps) or FeS
21
•S is supplied from:
• Natural sources
• Soil organic matter (major reservoir in non-
acid soils)
• Atmospheric deposition
Minor except in volcanic and polluted areas
But may be major source over the very long
term - no S fixation
• Some soluble minerals (FeS in acid soils,
CaSO4 in arid soils)
S Management
S is supplied from:
•Anthropogenic sources:
• Air pollution - atmospheric deposition
(acid rain, SO2)
• P Fertilizers
• Pesticides
22
• S deficiencies are becoming more common
in crops because of: reduced S emissions,
reduced use of S in P fertilizer and pesticides
• S deficiencies in forests are very rare
(unpolluted areas like Australia, PNW US)
• S fertilizers: ammonium sulfate, potassium
sulfate, ammonium thiosulfate [(NH4)2S2O3],
gypsum, elemental S
23
The Micronutrients
• B, Fe, Mn, Zn, Cu, Cl, Mo
• All essential for plant growth, but taken up
in very small quantities
• Except for Cl, the dominant role of micros
is enzyme activation
• Role of B not well understood; seems
related to shoot growth and water
• Most come from soil primary minerals (but
not usually Cl)
• Essential for growth of new cells
• Not mobile in the plant
• Forms a weak acid in soils (boric acid, H3BO3)
• At pH>8.5, forms borate [B(OH)4-]; thus mostly
in un-dissociated form and easily leached in
soils
• H3BO3 + H2O B(OH)4- + H+
Soil Boron
24
•Non-metal; sources include:
• Primary minerals (as trace element);
SOM; Adsorbed to Fe, Al hydrous oxides
• Exists as H3BO4 or B(OH4)- in soil solution;
Commonly deficient in forests of PNW,
Australia, Scandinavia
• Fertilizer: Borax (sodium tetraborate,
Na2BO4O7 . 5H2O; 14% B)
• Problems with leaching away too fast;
Must use great care not to over-fertilize:
toxic (we have this at Steamboat area)
Soil Chloride
• Exists as Cl-
• Highly soluble at all pH's
• Cycles rapidly
• Poorly adsorbed to soils; often used as a
tracer for water
• Believed to have a role in osmosis
• About the same concentrations as S (0.2%)
• Salt tolerant plants may contain 10%
• No known deficiencies, but may be
disease related
25
Deficiencies:
• Organic amendments and organic soils
• Sandy soils (low total Cu contents)
• Calcareous soils
• Common in forest nurseries
•Can be toxic (boat bottoms)
• Add very little - 1.2-2.5 kg ha-1 can supply
plants for many years
• Types of fertilizer
Soil Copper
• Plant enzymes
• Exists at cupric (Cu2+) and less as cuprous
(Cu+) ions
• Plants absorb Cu2+, but CuOH+ is common in
less acid soils
• Cu(OH)2 at neutral to alkaline pH's
• Most common copper mineral is CuFeS2
(chalcopyrite)
• Strongly bonds with OM
26
In soils:
• Least soluble in high pH and aerobic soils
(mainly Fe3+, as FeOH3)
• For that reason, liming often causes iron
chlorosis
• Most heavily weathered soils are rich in
Fe(OH)3
• Problem is not supply of iron, but how to
keep it soluble for plant uptake
• In anaerobic soils, Fe2+ is more soluble and
can leach
Soil Iron
27
Chelates and solubility
• Soil solution pH is important for Fe solubility
•Fe is soluble enough for plant needs at pH 3;
however, this is too low for most plants and
many other nutrients
• Solubility decreases 1000 X per unit pH rise
• Chelation is what keeps iron available: organic
"claw" around Fe atom, keeping it in solution
• Ligand: the organic molecule
• Chelate: Fe + organic
• Some plants can produce these chelates
when Fe deficient (siderophores, iron chelate
reductases)
• Some plants produce too much
siderophores (mutant pea) and creates Fe
toxicity
• Chelation potential:
Fe3+>Al3+>Cu2+>Co2+>Zn2+>Fe2+>Mn2+>
Ca2++Mg2+
28
Deficiencies, Fe fertilization
• High pH: calcareous soils, arid soils
• Rare in forests except nurseries
• Some plants have high Fe demand
• Visual: general extreme yellowing, even
whitening (iron chlorosis)
• Fertilize with chelates commonly; also
foliar sprays (but these do not last long - 3-4
times/yr)
29
• Many enzyme reactions in plants and
electron transport
• Occurs as Mn2+ in soil solution
• In aerobic soils, precipitates as MnO2
(Mn4+)
• OM decomposition furnished electrons to
reduce Mn4+ to Mn2+
Deficiencies:
• Sandy soils, organic soils, high pH soils
• Interveinal chlorosis of younger leaves
• Mn toxicity can be a problem in acidic
soils; liming can control this
Soil Manganese
30
• Occurs as molybdate anion (MoO4-)
• Many of the same reactions as phosphate - ads
to Fe and Al hydrous oxides
• More available as pH increases because of this
• May be toxic to grazing animals
• Very low amounts
•Very low amounts needed: 0.04 to 0.4 kg ha-1
Soil Molybdenum
31
• Essential for enzyme systems
• No oxidation reduction reactions as for Fe
• Occurs as Zn2+, and above pH 7.7, as
Zn(OH)+
• Does not precipitate to Zn(OH)2 until pH 9.1
• ZnS is only major insoluble form in soils
• Less soluble in anaerobic than in aerobic
soils
Soil Zinc
Deficiencies:
• Basic soils (limed, naturally high pH)
• High Zn plants (corn, onions, fruit trees)
• Commonly deficient in forest nurseries and
occurs in plantation forests
• Interveinal chlorosis in both younger and
older leaves
• Solubility increases 100 X with each unit
pH decrease
• Zn fertilizers
• Foliar sprays also used, but not efficient
32