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