Transcript Soil Colloids and Soil Chemistry

Phosphorus
Section 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 _______
5 - 25% as much P as N or K
• Functions:
– Component of amino acids, proteins, DNA, RNA
– Energy transfer reactions ( ATP )
– Cell membranes (phospholipids)
Phosphorus Deficiency
Corn
Tomato
Phosphorus Deficiencies
mobile
• P is a __________
nutrient, so deficiencies
are first seen in ___________
leaves.
old
• Deficiency symptoms:
– stunted plants
– dark green color
– purple streaks or spots on leaves
Nutrient Removal (kg/ha/yr)
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
Source: Plant Nutrient Use in North American Agri., PPI, 2002
Composition of organic fertilizers
Chicken
Cattle
Hog
Horse
Sheep
Municipal solid
waste compost
Sewage sludge
%
Moisture
35
80
72
63
68
N
P
K
N/P Ratio
4.4
1.9
2.1
1.4
3.5
2.1
0.7
0.8
0.4
0.6
2.6
2.0
1.2
1.0
1.0
2.1
2.7
2.6
3.5
5.8
40
80
1.2
4.5
0.3
2.0
0.4
0.3
4.0
2.25
(% of dry weight)
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-, H2PO42-)
– 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
Plant uptake
Mineralization
Solid Inorganic
P compounds
Dissolved
Inorganic P
Organic P
Microbial
immobilization
Adsorbed P
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-, H2PO42-) 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.
Phosphorus availability and pH
Brady and Weil, Figure 13.10
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
7.0
9.0
6.0
5.0
4.5
8.0
5.5
6.5
7.5
8.5
Mole fraction of total P
1.0 -
HPO42-
H2PO4-
H3PO4
PO43-
0.8 0.6 0.4 0.2 -
0.0
0
2
4
6
8
pH
10
12
14
Phosphorus Availability in Soils
• Only H2PO4- and HPO42- 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
Inorganic P
H2PO4HPO4=
Calcareous
soils
Ca8H2 (PO4)6
octocalcium
phosphate
Sodic
soils
Na2HPO4
sodium
phosphate
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
Form added in fertilizer
DCP
TCP
OCP
Least soluble
Apatite
Chemical transformation
with time in a calcareous
soil
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-weathered
clay soil
Consumption of N, P2O5, and K2O in the
U.S.
14
Consumption, million tons
12
N
P2O5
K2O
10
8
6
4
2
Current P consumption is similar to the late 1960s
0
1960
1965
1970
1975
1980
1985
1990
1995
2000
Year
From PPI
U.S. phosphate fertilizer consumption
by crop in 2001
Total P2O5 consumption
4.3 million short tons
Sorghum, 2.5%
Other crops
17.6%
Potatoes, 2.5%
Corn grain
38.4%
Cotton, 3.6%
Corn silage, 3.7%
Alfalfa
7.5%
Soybeans
7.7%
USDA-ERS, USDA-NASS, AAPFCO, PPI
Wheat
16.5%
P2O5, lb/A
Average P use on corn and soybeans
relative to crop removal
50
45
40
Gap is
Use
growing
35
30
25
Removal
20
15
10
1960 1965 1970 1975 1980 1985 1990 1995 2000
Potash and Phosphate Institute, 2001
Potash and Phosphate Institute, 2001
Ratio of P removal by crops to fertilizer applied.
BC
AB
MB
SK
ON
PQ
PEI
WA
NB
ME
MT
ND
NS
MN
OR
VT
ID
MI
MA
CT
WY
IA
PA
NE
NV
IL
IN
OH
DE
WV
CO
CA
NJ
MD
UT
VA
MO
R/F
NH
NY
WI
SD
RI
0.00-0.49
0.50-0.89
0.90-1.09
1.10-1.49
>1.50
KS
KY
NC
AZ
TN
OK
NM
AR
SC
MS
TX
AL
GA
LA
FL
Potash and Phosphate Institute, 2001
Ratio of P removal by crops to fertilizer
applied plus recoverable manure.
BC
AB
MB
SK
ON
PQ
PEI
WA
NB
ME
MT
ND
NS
MN
OR
ID
NH
NY
WI
SD
MI
MA
CT
WY
IA
PA
NE
NV
IL
IN
>1.50
MD
DE
WV
CO
RI
0.00-0.49
0.50-0.89
0.90-1.09
1.10-1.49
NJ
OH
UT
CA
R/(F+M)
VT
VA
MO
KS
KY
NC
AZ
TN
OK
NM
AR
SC
MS
TX
AL
GA
LA
FL
Potash and Phosphate Institute, 2001
Increasing concerns about P from fertilizers and
animal manures entering surface water.
Unfertilized lake
Canadian lake fertilized with P
P Availability
• P Availability is most likely to be limited in:
–
–
–
–
–
Fe oxide content binds P ions
Weathered soils: high
___________________________
Fe oxide content binds P ions
Acid soils: high
________________________________
precipitates with Ca, lower solubility
Alkaline soils: P
______________________________
P ions move slowly
Cold soils: ________________________________
ions bind to Fe oxides
Soils high in Fe oxides: P
______________________
P sorbed (%)
100
80
25ppm
60
1000ppm
40
20
0
0
Sanchez 1980
100
200
Time (hr)
300
400
1000
P sorbed (ppm)
900
800
700
600
500
400
300
200
100
0
0
Sanchez 1982
10
20
30
40
Equilibrium P (ppm)
50
60
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)
• mono ammonium phosphate (11-52-0)
• di ammonium phosphate (18-46-0)
• ammonium polyphosphate (10-34-0)
• Phosphoric Acid (0-52-0)
– Organic: manures contain 0.5 to 2.0% P
• P analysis in commercial fertilizers is
expressed as %P2O5
solid
solid
solid
liquid
liquid
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??
PS-P (ppm) for maximum yield
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
8
10
12
Soil Temperature
14
16
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 (SO42-), and Cl (Cl-) are
considered mobile in soils
• Most other elements are less mobile in soils.
Nutrient Mobility in Soil
Soil volume exploited
for mobile nutrients:
N, S, Cl
Soil volume exploited
for immobile nutrients:
Most others
Apply
immobile
nutrients here
(close to
roots)
Because P is immobile, we cannot rely
on movement of irrigation water to transport P.
Take-Home Message for P Management
•
•
•
•
P is less exciting, but no less important than N.
5-25
Plants take up ______%
as much P as N
50
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.
Yield (Mg/ha)
50
45
40
35
30
25
20
15
10
5
0
Band
Broadcast
0
100
200
P rate (kg/ha)
300
Sanchez, Swanson, and Porter 1990
400
Response of Celery to P
Rate and Placement
P rate
(kg/ha)
0
50
100
150
200
P placement
Broadcast
Band
Marketable yield
(Mg/ha)
21.6
40.1
36.7
40.8
39.9
L**Q**
35.3
32.6
NS
Espinoza, Sanchez, and Schueneman, 1993
Response of lettuce to Preplant and Sidedress NPK
70
60
No SD
SD 7 DAT
Yield (Mg/ha)
50
40
SD 14 DAT
SD 21DAT
SD 28 DAT
30
20
10
0
0
25
50
NPK Fertilizer Recommendation (%)
100