PhosphorusSection J

Soil Fertility and Plant Nutrition

Phosphorus as a Plant Nutrient

• After N, it is the nutrient most likely to be deficient to plant growth.

• Plants use about _______ as much P as N or K• Functions:

– Component of amino acids, proteins, DNA, RNA– Energy transfer reactions ( ATP )– Cell membranes (phospholipids)

5 - 25%

Phosphorus Deficiency

Corn Tomato

Phosphorus Deficiencies

• P is a __________ nutrient, so deficiencies are first seen in ___________ leaves.

• Deficiency symptoms:– stunted plants– dark green color– purple streaks or spots on leaves

mobile

old

Nutrient Removal (kg/ha/yr)

Source: Plant Nutrient Use in North American Agri., PPI, 2002

N P K N/P Ratio

Broccoli (100 lb yield) 0.44 0.07 0.35 6.3

Celery (100 lb yield) 0.19 0.05 0.42 3.8

Corn (bushel of grain – 56 lb) 0.75 0.19 0.24 4.0

Alfalfa (ton) 56 6.6 50 8.5

Oranges (ton) 8.8 0.8 9.1 11.0

Composition of organic fertilizers

N P K N/P Ratio

% Moisture (% of dry weight)

Chicken 35 4.4 2.1 2.6 2.1 Cattle 80 1.9 0.7 2.0 2.7 Hog 72 2.1 0.8 1.2 2.6 Horse 63 1.4 0.4 1.0 3.5 Sheep 68 3.5 0.6 1.0 5.8 Municipal solid waste compost 40 1.2 0.3 0.4

4.0

Sewage sludge 80 4.5 2.0 0.3 2.25

Phosphorus vs. Nitrogen

• Similarities:– Mineralization and immobilization of both N

and P can be important for supplying available P to plants

– Both occur as oxyanions: nitrate (NO3-) and

phosphate and (HPO4-, H2PO4

2-)

– Both can contribute to pollution

Phosphorus vs. Nitrogen

• Differences:– Most (>95%) of soil N is organic in nature,

usually 50% or less of soil P is organic– Plants use about 4-10X more N than P– Phosphate does not leach through soils as

readily as nitrate– No gaseous forms of P; therefore no gaseous

losses– There is no P counterpart to N fixation

The Phosphorus Cycle

Adsorbed P

Solid Inorganic P compounds

Dissolved Inorganic P

Organic P

Plant uptake

Microbial immobilization

Mineralization

Phosphorus in Soils

• Soils may contain from 0.1 to 0.02% P• N:P ratio in soils is about 8:1• There is little relationship between total soil P

and available P; only a tiny fraction of total P is available to plants

• Forms of soil P:– Organic - various P forms associated with humus– Inorganic - mineral P, adsorbed P– P in soil solution (ionic forms)

Mineralization-Immobilization of P

Organic P Inorganic P

Immobilization and Mineralization of soil P are similar to that of N:

If added organic materials have a C:P ratio of >300, there will be net immobilization P

if <200 there will be net mineralization of P

Mineral Forms of P• In neutral to alkaline soils, most mineral P will

be as Ca-phosphates. Most of these are quite insoluble.

• In acid soils, most mineral P will be as Fe and Al-phosphates. Most of these are quite insoluble.

• The insolubility of most P minerals is one important reason that P availability to plants is usually low.

Adsorbed P

• Phosphate ions (HPO4-, H2PO4

2-) are strongly adsorbed to the surfaces of:– Iron oxides, especially in acid soils– CaCO3, especially in alkaline soils

– Adsorption is at a minimum in neutral (6-7) pH

• Adsorption reactions are another reason that P availability in soils is limited.

Brady and Weil, Figure 13.10

Phosphorus availability and pH

from Foth and Ellis, 1997

P Reactions with Soil Minerals

Many tropical soils are depleted of P

without phosphate, even weeds barely grow

Courtesy Potash and Phosphate Institute

P Availability

• Governed by:– Mineralization-Immobilization of humus P– But primarily by:

Adsorption-desorption reactions of ionic P with Al and Fe oxides or

CaCO3 and

Solubility of various P minerals - Fe and Al phosphates in acid soils, and

Ca phosphates in alkaline soils

Soil pH and Phosphorus Availability

6.5

4.5

5.0

5.5

7.58.5

9.08.07.0

6.0

H3PO4 H2PO4- HPO4

2-

PO43-

1.0 -

0.8 -

0.6 -

0.4 -

0.2 -

0.0

Mol

e fr

actio

n of

tot

al P

0 2 4 6 8 10 12 14

pH

Phosphorus Availability in Soils

• Only H2PO4- and HPO4

2- in solution can be utilized by plants

Phosphorus “Fixation”

• Like N, much of the P applied in fertilizers is not recovered by plants in the first year. The reason is different:– P reversion is the process wherein available,

soluble P forms applied in fertilizers naturally transform back into less soluble forms over time.

– This is a non-biological process

Phosphorus Fixation

• Phosphorus “fixation” (sometimes called “reversion”) refers to reactions of P in soils that cause P added in fertilizer to become less available with time:– Reactions with Ca in calcareous soils– Reactions with Al/Fe in acid soils

Soil Likely to “fix” P

Factors Causing P fixation in Neutral or Calcareous Soils

• P forms relatively insoluble Ca phosphates in neutral to alkaline soils– hydroxyapatite– octacalcium phosphate

• Phosphate ions may be adsorbed to CaCO3 particles and on Ca-saturated clays

Phosphorus Reactions in Desert Soils

H2PO4-

HPO4=

Ca8H2 (PO4)6

octocalcium phosphate

Na2HPO4

sodiumphosphate

Calcareoussoils

Sodicsoils

Inorganic P

Phosphorus Reversion

• Alkaline soils– MCP (fertilizer) over time transforms:

– MCP → DCP → TCP → OCP → Apatite

– A similar process happens (with different forms) in acid soils

– This lowers the availability of P– The reversion process usually takes several

months to years to be complete

Ca Phosphates• Most soluble MCP

DCP

TCP

OCP

Least soluble Apatite

Form added in fertilizer

Chemical transformationwith time in a calcareoussoil

Factors Causing P Fixation in Acid Soils

• Precipitation from soil solution with Al or Fe:• vivianite

– Fe3(PO4)2.8H2O

• strengite– FePO4

.H2O• variscite

– AlPO4.2H2O

– Adsorbed on surface of Fe and Al oxides– Adsorbed on clay particles (i.e. kaolinite)

Griffin is a highly-weatheredclay soil

Consumption of N, P2O5, and K2O in the U.S.

0

2

4

6

8

10

12

14

1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

Co

nsu

mp

tio

n,

mil

lio

n t

on

s

N P2O5 K2O

Current P consumption is similar to the late 1960s

From PPI

U.S. phosphate fertilizer consumption by crop in 2001

USDA-ERS, USDA-NASS, AAPFCO, PPI

Corn grain38.4%

Other crops17.6%

Alfalfa7.5%

Soybeans7.7%

Wheat16.5%

Corn silage, 3.7%

Cotton, 3.6%

Potatoes, 2.5%

Sorghum, 2.5%

Total P2O5 consumption4.3 million short tons

101520

25303540

4550

1960 1965 1970 1975 1980 1985 1990 1995 2000

P2O

5, lb

/AAverage P use on corn and soybeans

relative to crop removal

Use

Removal

Gap isgrowing

Potash and Phosphate Institute, 2001

Potash and Phosphate Institute, 2001

DE

ND

SKMB

ON

BCAB

WA

OR

MT

ID

SD

MN

PQ

NY

PAOHINIL

IA

WIMI

WY

UT

NV

CA

AZ NM

NB

NS

PEI

ME

NH

VT

MACT

RI

NE

KSMO

KY

WVVA

MDNJ

NCTN

AROK

TX LA

MS AL GA

SC

FL

CO

0.00-0.49

0.50-0.89

0.90-1.09

1.10-1.49

>1.50

R/F

Ratio of P removal by crops to fertilizer applied.

Potash and Phosphate Institute, 2001

Ratio of P removal by crops to fertilizer applied plus recoverable manure.

ND

SKMB

ON

BCAB

WA

OR

MT

ID

SD

MN

PQ

NY

PAOHINIL

IA

WIMI

WY

UT

NV

CA

AZ NM

NB

NS

PEI

ME

NH

VT

MA

CTRI

NE

KSMO

KY

WV

VA

MD

DE

NJ

NCTN

AROK

TX LA

MS AL GA

SC

FL

CO

1.10-1.49

0.50-0.89

0.00-0.49

0.90-1.09

>1.50

R/(F+M)

Potash and Phosphate Institute, 2001

Increasing concerns about P from fertilizers and animal manures entering surface water.

Canadian lake fertilized with P

Unfertilized lake

P Availability

• P Availability is most likely to be limited in:– Weathered soils: ___________________________– Acid soils: ________________________________– Alkaline soils: ______________________________– Cold soils: ________________________________– Soils high in Fe oxides: ______________________

high Fe oxide content binds P ionshigh Fe oxide content binds P ions

P precipitates with Ca, lower solubility

P ions move slowly

P ions bind to Fe oxides

0

20

40

60

80

100

0 100 200 300 400

Time (hr)

P so

rbed

(%)

25ppm

1000ppm

Sanchez 1980

0

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40 50 60

Equilibrium P (ppm)

P s

orb

ed (

pp

m)

Sanchez 1982

Improving P Availability

• Soil and tissue testing• Control soil pH if possible• Use organic sources, i.e. manure• Placement - critical!!

Measuring P Availability

• Soil tests– Neutral to alkaline soils - extraction of soil with 0.5 M

NaHCO3, measure P in the extract

– Acid soils - extraction of soil with HCl and NH4F, measure P in the extract

• Tissue tests– Not as many P tissue tests as for N, fewer standards

exist

Sample of P Soil Test Guidelines

Phosphorus Fertilizers• Manufactured from mined apatite minerals

– Apatite is treated with H2SO4 or H3PO4 to form various inorganic P fertilizers:• superphosphate (0-20-0)

solid• triple superphosphate (0-45-0) solid• mono ammonium phosphate (11-52-0) solid• di ammonium phosphate (18-46-0) solid• ammonium polyphosphate (10-34-0) liquid• Phosphoric Acid (0-52-0) liquid

– Organic: manures contain 0.5 to 2.0% P• P analysis in commercial fertilizers is

expressed as %P2O5

Managing Soil P

• Managing soil P for maximum availability– If possible, assure an optimum pH (6-7)– Keep in mind that P is especially unavailable in

cold soils.– Apply P in bands in soil– Use soil testing before planting each season, use

appropriate guidelines.– Band-apply NH4

+ and P together--this usually increases P availability, particularly in alkaline soils. Why??

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

8 10 12 14 16

Soil Temperature

PS

-P (

pp

m)

for

max

imu

m y

ield

Nutrient Mobility in Soils

• Mobility in soils refers to the relative rate of movement of soluble nutrient forms in soils.

• Mobility is a function of soil texture and mineralogy (generally slower in clay soils)

• Usually, N (NO3-), S (SO4

2-), and Cl (Cl-) are considered mobile in soils

• Most other elements are less mobile in soils.

Nutrient Mobility in Soil

Soil volume exploitedfor mobile nutrients:N, S, Cl

Soil volume exploitedfor immobile nutrients:Most others

Because P is immobile, we cannot relyon movement of irrigation water to transport P.

Apply immobile

nutrients here(close to

roots)

Take-Home Message for P Management

• P is less exciting, but no less important than N.• Plants take up ______% as much P as N• Manures contain about ____% as much P as N.• P is less subject to losses in soils compared to N,

is usually immobile in soils.• Timing of P applications to crops is less critical

than for N.

5-25

50

05

101520253035404550

0 100 200 300 400

P rate (kg/ha)

Yie

ld (

Mg

/ha)

Band

Broadcast

Sanchez, Swanson, and Porter 1990

Response of Celery to P Rate and Placement

P rate Marketable yield

(kg/ha) (Mg/ha)0 21.650 40.1100 36.7150 40.8200 39.9

L**Q**P placementBroadcast 35.3Band 32.6

NS

Espinoza, Sanchez, and Schueneman, 1993

Response of lettuce to Preplant and Sidedress NPK

0

10

20

30

40

50

60

70

0 25 50 100

NPK Fertilizer Recommendation (%)

Yie

ld (

Mg

/ha)

No SD

SD 7 DAT

SD 14 DAT

SD 21DAT

SD 28 DAT