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Unit 8: Plant Nutrients Chapter 9

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Unit 8: Plant Nutrients

Chapter 9

Objectives Knowledge of essential elements for plant

growth Mechanisms by which plant roots contact,

absorb nutrients Methods of N cycling and N loss Materials used in N fertilizers Understanding of the behaviors of nutrients

in the soils Fertilizer applications Importance of micronutrients in plant

development

Essential Elements

17 elements known to be essential C, H, O Photosynthesis

Light energy used, H split off of water H combined w/ C & O to make CO2

(diffused through leaf stomata) Results in CHO + other organic

molecules

Essential Elements

Macronutrients N, P, K

Secondary Nutrients Ca, Mg, S

Micronutrients B, Cl, Cu, Fe, Mn, Mo, Ni, Zn

Others as needed

Mechanisms of Nutrient Uptake

Nutrients reach root surfaces by three mechanisms

Mass flow – movement of nutrients in water flowing toward the root

Diffusion – movement down a concentration gradient from high - low

Interception – roots explore new soil areas containing unused soil nutrients

All three in constant operation Root hairs primarily responsible for the

uptake

Mechanisms of Nutrient Uptake

Absorption of Nutrients into Roots Mechanism not well understood

Movement through cell wall easy Movement into cytoplasm much harder Nutrient must go through passageway, or

bond w/ carrier to get through cell membrane

Some actively pulled into cell Electrical balance also involved

Mechanisms of Nutrient Uptake

Absorption through Leaves Leaf stomata

Exchange of H20, O2, & CO2

Some soluble elements can be absorbed in small amounts Mostly micronutrients Macros typically need in too high quantities to

foliar feed

Soil Nitrogen Gains & Transformations

N is the key nutrient in plant growth management Most commonly deficient nutrient,

controlling factor in plant growth Constituent of: proteins, chlorophyll,

nucleic acids Plants w/ sufficient N have thinner cell

walls & are more succulent plants N deficiency = poor plant yields

Soil Nitrogen Gains & Transformations

Much soil N isn’t in a form that can be absorbed Most immobile in organic matter N2 gas in the atmosphere

Must be fixed by soil bacteria first

Unique nutrient Can be absorbed soluble organic form NH4, NO3

Soluble, mobile, easily leached Can be easily denitrified by soil microbes

Soil Nitrogen Gains & Transformations

Deficiency symptoms: poor, spindly, stunted growth

NH4 & NO3 are not necessarily interchangeable NH4 saves the plant energy NO3 is more stable in the soil

Fixation of N Gas Primary source of soil N Taken by soil microbes, converted to NH4

Soil Nitrogen Gains & Transformations

Wide variation in how much N is fixed due to: soil, fertilizers used, crops, etc.

Mineralization of N Release of N from decomposition of

organic materials Mineralization – conversion of organic N

to NH4 form Soil organic matter contains ~5%N Only small % of organic matter decomposes

each yr

Soil Nitrogen Gains & Transformations

Nitrification of Ammonium Nitrification – oxidation of ammonium to

nitrate by bacteria, other organisms Rapid microbial transformation (usually 1-

2d) Most is complete w/in 1-2 wks

Some absorbed, some adsorbed quickly Slowed by anaerobic conditions, dry,

cold, toxic chemicals

Soil Nitrogen Gains & Transformations

Other Fixation Reactions Involving Soil N Immobilization – soluble N held in plant

materials or microbes N not available to plants N can be fixated to clay particles N can be consumed by decomposing

microbes and held until they die

Nitrogen Losses from the System

Leaching of Soil N NO3 – readily leached form of N, toxic to

young mammals Nitrate lost due to negative charge Ammonium held due to positive charge Leaching rates increase as percolation rates

increase, when plant growth rates aren’t quick enough to keep up w/ N production

Losses from crop covered soils usually low

Nitrogen Losses from the System Losses from heavily fertilized, wet soils high

Nitrification Inhibitors Chemicals used to inhibit nitrification

N-Serve, DCD, ATC Inhibit the first step of nitrification, slow the

release of N to the soil N-Serve more volatile & can evaporate slowly DCD, ATC – stable, easy to handle, can be

applied as coatings to granules What is the effect of all this? What options does it allow producers?

Nitrogen Losses from the System

Gaseous Losses of Soil N Denitrification – change by bacteria of

NH4 to N gas Biological process Can be most extensive gaseous N loss

Especially w/ poorly aerated/wet soils Rapid process

Substantial N loss can occur in <1d ~10-20% normal ~40-60% in extreme conditions, 100% in

wetlands

Nitrogen Losses from the System Three reasons large amounts of N lost:

Lack of adequate free O in the soil Energy source of organic matter for the

bacteria Warm, slightly acidic soils

Ammonia Volatilization Occur when ammonium is in alkaline

environment Chemical process Losses occur from surface applications of

ammonium/urea Can be ~30%, normally <10%

Nitrogen Losses from the System

Most extensive under following conditions: High pH, calcareous soils Fertilizer left on soil surface High temps Low CEC soils

Materials Supplying Nitrogen

Ammonia & Aqueous N Anhydrous Ammonia (NH3)

Most common N fertilizer >90% of all N fertilizers made up of some form

of ammonia 82% N Manufactured from atmospheric N using

natural gas to supply H (Haber Process) First usable fertilizer product of this process Other N fertilizers require more processing

Materials Supplying Nitrogen Applied w/ chisels to ~5” Pressured liquid in the tank, gas at

atmospheric pressure Least expensive N fertilizer (per unit N) Very dangerous to handle

Burns Blindness Inhalation risks Safety precautions

Wear proper safety equipment (gloves, goggles)

Materials Supplying Nitrogen Keep away from flames Keep away from ammonia clouds Have water available Store in proper tanks, don’t overfill Paint tanks white to reflect heat Inspect tanks regularly for leaks/problems

Solid Fertilizers Urea

Synthetic, organic fertilizer Cheaper per lb than any other solid N fertilizer 45% N Must be converted in the soil to NH4

Materials Supplying Nitrogen Readily soluble & leachable Stabilized & can be stored when converted in

the soil to NH4 Popular

Cheapest solid N source Soluble in water Convenient for application in sprinkler,

spray, solution Ammonium Sulfate

21% N High cost Less popular

Materials Supplying Nitrogen Commonly used in rice

Ammonium is all available to plant Sulfate keeps it from being denitrified

quickly Why is this so important for them?

Strongest acidic N fertilizer UAN

Urea-Ammonium Nitrate solution 28% N or 32% N

Materials Supplying Nitrogen Organic Wastes

Considered controlled-release fertilizers Nutrient concentration is low Depends on decomposition rates May carry undesirables

Weed seed, diseases, soluble salts, etc.

Materials Supplying Nitrogen

Controlled-Release N Fertilizers Standard N fertilizer crop use rates ~40-

70% Rest is leached, denitrified, etc.

Slow-release N fertilizers used to control proportion of fertilizers available at one time

More efficient use of N means more cost savings & less pollution

Materials Supplying Nitrogen Slow-release N products most commonly used in

turf grass Urea-Formaldehydes

Varying rates of urea & formaldehyde Greater urea, more available N

Environmental conditions must favor microbe activity to release N Losses may be ~20%

Polymer-coated N Soluble form of N (urea) diffuses through

polymer membrane Reliable, consistent control of N release

Soil Phosphorus

Traditionally, second-most prescribed nutrient in the soil K has now passed in use

Essential part of nucleoproteins in cell nuclei Carry DNA

Main component of cell energy currency (ATP)

Soil Phosphorus

Roles in: Cell division Root growth Plant maturation Energy transformation w/in cells Fruit/seed production Animal/human nutrition

Growth of bones & teeth

Soil Phosphorus

Young plants absorb soil P readily Most critical for plants to have available

P sources early in development Wheat from tillering to flowering Corn ~3 wks into growing season

The P Problem

Soil forms of P very low solubilityP applied through fertilizer often

combines w/ substances to reduce solubility

Most P supplied to plants by diffusion in the soil

Diffusion rates extremely slow (.02 - .1 mm/hr)

Major problem to keep P soluble & available to the plants in soils

Soil P doesn’t leach

The P Problem Mineral P

Available P critical Supply of P in soils is low Phosphates in soils not readily available

While there is lots of P in the soil, minutes fractions actually available

Original natural source of P – apatite (rock phosphate) Along with others, these can be used as

low-quality fertilizer sources

The P Problem Soluble phosphate often reacts w/ other

soil substances to form insoluble compounds Also readily adsorbs to other molecules Explains soil P buildup

Soil P most available at pH ~6.5 Phosphates in Anaerobic Soils

Phosphates more soluble than in aerated soils

Iron phosphates are soluble in flooded soils, less tie-up for P

The P Problem Works out good for rice growers

Organic Soil P Phosphatases used by plant roots &

some microbes to split P from organic residues – making it available for absorption

P in organic residues tends to more soluble, therefore, more useful to plants May comprise >50% of soluble soil P

The P Problem The more favorable conditions are for

microbe decomposition, > available soil P

Managing Soil P

Major pollutant of surface waters Not really discussed in depth in this unit

Mycorrihzae helps plants access soil P Fumigated soils, non-healthy microbe

population soils - < access to soil P - <growth

Soil pH influences Changes solubilities of Fe, Ca, Al, &

affects soil bacterial growth

The P Problem ~6.5 pH optimal for P availability

Phosphate fertilizer effectiveness Most efficient use when banded

What does this mean? What problems does this cause? Want to place ~2” away from root zone on

either side Only 10-30% of soil applied P is actually

used

The P Problem Excess P retained in the soil

Can cause Zn deficiency P pollution from runoff

Maximizing P efficiency Maintain soil pH 6-7 Promote healthy soil organic matter content Band P fertilizer for row crops, broadcast &

incorporate for non-row crops What’s wrong w/ this?

Materials Supplying P

U.S. is world’s largest producer Estimated our supply may run out in 20 yrs

at current usage Most comes from FL or western U.S.

Western Africa has 6x more supply than U.S.

Phosphate Ores & Deposits Rock phosphate mined, ground

Materials Supplying P Mixed w/ sulfuric acid to form

superphosphate 8-9% P, 48% gypsum

Mixed w/ phosphoric acid to form triple superphosphate 20-22% P (40-45% phosphate)

Mixed N-P Fertilizers Monoammonium & Diammonium

Phosphate fertilizers Apply N & P w/ same product These will be covered more in a later unit

Soil K

Ranks 2nd to N in plant use & fertilizer applied

Chemical compounds of K very soluble, but mineral form is not

Can see considerable soil amounts of K, but much of may not be available

Decomposition of plant residues provides much soluble K

Soil K

Roles of K Cell division Formation of CHO’s Movement of sugars Enzyme actions

>60 enzymes known to need K for activation Disease resistance Cell permeability Important for water balance

Soil K Forms of Soil K

Most K used by plants in exchangeable or soluble form Exchangeable K forms as micas & feldspars

weather, or as plant residues release Exchangeable soil K in root zone may be

small amount Often must supply 150-180 lb/ac

K Losses & Gains K may be taken up in excess amounts by

plants – Luxury Consumption

Soil K May be expensive waste of K fertilizer May inhibit Mg absorption May not increase crop yield

Soluble K losses Immobilized by microbes Leached Trapped in soil clay layers Eroded

Soil K K gains

Mineralization of organic matter K Can be used ~ as fast as water moves through

soils Held on cation sites in the soil

Soil K is relatively stable & not volatile w/ temp changes

Soil K Supplying K to Plants

K fertilizers usually very soluble May not be very mobile in the soil

Is held on cation sites, or will replace other ions on those sites

Needs to be supplied in the root zone to be most effective

Acidic soils often result in K deficiencies Abundance of soil Ca, Mg, or K may antagonize

uptake of one of the others Competition for plant absorption

Soil K KCl – cheapest K fertilizer source

Can choose sulfate or nitrate forms to add additional nutrients…but more costly

Managing Soil K Hay harvesting removes much K from

the soil each year Highest K requirement during vegetative

growth

Soil K K Management keys

Maximize efficient use of added K Minimize luxury consumption

Split applications – especially in sandy soils Maximize use of natural K (organic matter

sources) Maintain soil pH 6-6.5 – reduces leaching

losses

Soil K Materials Supplying K

Potash Most potash imported from Canada Muriate of potash (KCl) – principle source

60% potash Potassium sulfate – 2nd most used K fertilizer Potassium-magnesium sulfate – provides 3

nutrients Potassium nitrate – adds N w/ K

Soil Ca

Occurs in many minerals, more plentiful in soils than any other plant nutrient

Ca deficiency is rare due to wide range of Ca sources in soils

Mobility of Ca Taken up as Ca Strongly adsorbed to cations

Large amounts may be leached simply due to large supply in the soil

Soil Ca Mass flow usually supplies enough Ca to

root zone Only absorbed through root tips

Plant Need for Ca Dividing cells – forms Ca pectate which

cements cells together Physical integrity & normal cell function Deficiencies

Deformation of new leaves/necrotic appearance

Death of buds

Soil Ca Used more than Mg, less than K Needs to be supplemented in

greenhouses Ca deficiency common due to not enough

fertilization w/ higher Ca sources Ca Fertilizers

Limestone Usually only used on soils if they’ve

become acidic Can use gypsum if pH raise not needed

Soil Mg Mobility of Mg

Most soluble/exchangeable forms supplied in the soil

Reacts similar to Ca Lower total leaching loss, less present

Plant Need for Mg Most supplied to the roots by mass flow 1/5 of Mg used by plants for chlorophyll Stabilizes ribosome structure Enzyme activator

Soil Mg Readily mobile in the soil Deficiency symptoms

Interveinal chlorosis of older leaves Hypomagnesia (grass tetany)

Can occur in livestock grazing soils low in Mg

Mg can be tied up by heavy applications of K and/or ammonium fertilizers

Soil Mg

Mg Fertilizers Dolomitic limestone

Ca w/ Mg Can also use Mg salts

Soil S

Constituent in 2 of the 20 amino acidsEssential part of proteinsAlso found in vitamins, oilsMuch overlookedFactors increasing need for S fertilizers

Lower amounts of sulfate added incidentally w/ other nutrients

Lower pollution from sulfur oxides into air Higher plant yields, greater demands on

soils

Soil S

Sources of S Availability of soil S hard to determine –

major portions come from organic matter Depends on decomposition, climate, temp,

etc. Rainfall

Can be toxic to fish, if S is too high S also supplied as part of other fertilizers

Rare need to supply S separately, but the need has been observed

Soil S Characteristics of Soil S

Decomposition can release much S Exists in many chemical forms,

depending on the soil Easily leached

Why? Waterlogged soils can cause soil S

sources to convert to sulfide – toxic gas to plants

Acidifies the soil

Soil S Managing Soil S

Reduced air pollution, purer fertilizers, better understanding = reduced incidental S additions

Some increased reports of S deficiencies Sulfur Fertilizers or Amendments

Select ammonium sulfate or potassium sulfate fertilizers

Gypsum Others can be recommended

Soil B

Essential for: Cell wall formation, sugar movement,

pollination

Deficiencies: Terminal bud death Reduced flowering, retention of flowers Reduced pollen germination Less fruiting

Soil B Soil Chemistry of B

Forms a weak acid Deficiencies common in high rainfall

areas Various borates (forms) may exist in

different soils Sources for B

Primary rocks & minerals Combined in soil organic matter Adsorbed in soil clays Boric acid

Soil B Boron Deficiency & Amendments

Deficiency in grapes greatly reduces yield

Cost to supplement relatively inexpensive If over-supplemented can be toxic

Fine line between adequate & excess amounts Supplemental B supplied by borax

Very soluble 11% B

Soil Cl

Found in soil as Cl- Very soluble, mobile Not very reactive in the soil Will it be held in the soil?

Osmotic role – maintains/equalizes cell charges

Unique Features of Cl Cycles easily Supplied by manures, KCl, rainfall, etc.

Soil Cl Can accumulate to toxic amounts

Especially in soils high in soluble salts Some diseases linked to Cl deficiencies

(stripe rust, take-all root rot, leaf rust) Cl Amendments

Deficiencies rarely seen in the field Cl typically supplied incidentally w/ other

fertilizers

Soil Cu

Essential for many enzymesVery low solubility

Solubility related to soil pH

Strongly adsorbed to soil clays Problem Soils & Susceptible Plants

Deficiencies: Common in organic soils

Bonds strongly to organic substances & won’t become soluble

Soil Cu Sandy soils Calcareous soil – pH 8-8.4 High competition w/ other metals

Less common than other micro deficiencies

Symptoms of deficiency Yellowing of younger leaves Off-color (bluish/green) Small dead spots Leaf curling

Soil Cu Sensitive plants:

Alfalfa Rice Wheat Oats, etc.

Cu Amendments & Their Use Successful, when applied Often only need supplement few ppm/ac CuSO4

Can be applied as foliar treatment

Soil Fe

Important part of energy-providing reactions

Much Fe association w/ chloroplasts

Very low solubility Difficult to keep Fe soluble for plants to

absorb

Very low amounts needed for plants

Soil Fe Fe in Soil Solution – Chelates &

Availability pH has dominant effect of iron solubility

Very soluble at pH – 3 Solubility decreases by factor of 1000/pH

unit rise At normal pH – soluble iron very low

Fe needs mostly provided by soil organic matter, stays bonded to something else to keep it soluble

Soil Fe Some supplied in chelate form

Keep metals in a mobile/soluble form Move to plant roots by diffusion or mass

action

Problem Soils, Susceptible Plants, & Fe Amendments Deficiencies common in calcareous soils High P levels also antagonize Fe

Soil Fe Fe deficiency symptoms:

Interveinal chlorosis Soluble chelate supplementation will

often correct deficiencies Foliar sprays

May need to be repeated Soil applications have longer residual, but

much slower acting Keep organic matter high

Soil Mn

Involved in enzyme systemsSolubility increases w/ pH increasesOrganic matter decomposition aids Mn

solubility Toxicity, Problem Soils, & Deficiency

Symptoms Toxic concentrations more common than any

other micro Soils may naturally have high Mn Conditions can cause Mn toxicity easily

Soil Mn High Mn soils may show toxicities at pH

just below 6, excessive water, or even at high pH’s

Somewhat common in Hawaii Treatment w/ lime & gypsum

Deficiency symptoms – chlorosis of younger leaves

Soil Mo

Exists & needed in minute amountsImportant for enzyme function & N

fixationStrongly adsorbed, yet soluble Problem Soils & Susceptible Plants

Deficiencies common in acid/sandy soils Susceptible crops:

Soybeans, alfalfa, corn, tomatoes, etc.

Soil Mo Toxicities usually only show up in grazing

animals Known to happen on soils w/ high organic

matter & neutral/alkaline pH Problem related to imbalances of Cu & Mo Stunted growth, bone deformation

Feed, inject Cu will often correct

Mo Amendments Foliar sprays Lime acidic soils

Soil Zn

Essential for enzyme systems Zn in the Soil Solution

Quite immobile in the soil (+ charge) Can become deficient in flooded soils

Problem Soils & Susceptible Plants Deficiencies:

Occur in basic soils, limed soils, cropping w/ high Zn demand crops (corn, fruits, etc.)

Most expected at high soil pH

Soil Zn Cotton responds to Zn supplementation

in SW US Symptoms:

Interveinal chlorosis in young & old leaves Reduced stem elongation Bunched leaves Small, thick leaves Early defoliation

Soil Zn

Zn Amendments ZnSO4 most commonly used to cure

deficiencies Foliar application for treatment Soil application if problem is anticipated

Ni & Other Beneficial Elements May not be essential for all plants, but

may be essential for one plant Co

Essential for microbes involved w/ N fixation Can be deficient in high Ca soils, sandy,

leached soils Si

Very abundant in the environment Can be deficient in very weathered soils Appears to strengthen cell walls

Ni & Other Beneficial Elements Na

Essential for desert species to maintain turgor

Growers usually reluctant to add Why?

V (Vanadium) Essential for algae, microbes May substitute for Mo in enzyme activation

Ni & Other Beneficial Elements Ni

Raised to essential status in 1983 Scientists still argue over its roles Suspected roles in plant metabolism

Enzyme activator No fertilizer w/ Ni currently available Soybeans have demonstrated a positive

response to Ni treatment

Assignment