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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

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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.

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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.

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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.

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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

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• 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

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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

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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:

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Materials Supplying N

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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

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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

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• 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

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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+

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• 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

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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)

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• 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)

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• 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

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• 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

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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

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• 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

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•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

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• 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

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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

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•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

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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

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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

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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+

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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)

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• 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

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• 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

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• 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

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