1. Section J Soil Fertility and Plant Nutrition
Phosphorus
Section J
Soil Fertility and Plant Nutrition

2. 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%

3. Phosphorus Deficiency
Corn
Tomato

4. 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

5. Nutrient Removal (kg/ha/yr)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
0.24
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

6. Composition of organic fertilizers
Here are compositions of some commonly used organic fertilizer materials.

7. 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

8. 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

9. The Phosphorus Cycle Plant uptake Mineralization Solid Inorganic
P compounds
Dissolved
Inorganic P
Organic P
Microbial
immobilization
Adsorbed P

10. 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)

11. 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

12. 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.

13. 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.

14. Phosphorus availability and pH
Brady and Weil, Figure 13.10

15. from Foth and Ellis, 1997

16. P Reactions with Soil Minerals

17. Many tropical soils are depleted of P
without phosphate, even weeds barely grow
Courtesy Potash and Phosphate Institute

18. 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

19. Soil pH and Phosphorus Availability
6.5
4.5
5.0
5.5
7.5
8.5
9.0
8.0
7.0
6.0

20. Mole fraction of total P
H3PO4
H2PO4-
HPO42-
PO43-
1.0 -
0.8 -
0.6 -
0.4 -
0.2 -
0.0
Mole fraction of total P pH

21. Phosphorus Availability in Soils
Only H2PO4- and HPO42- in solution can be utilized by plants

22. 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

23. 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

24. Soil Likely to “fix” P

25. 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

26. Phosphorus Reactions in Desert Soils
Calcareous
soils
H2PO4-
HPO4=
Ca8H2 (PO4)6
Inorganic P
octocalcium
phosphate
Sodic
soils
sodium
phosphate
Na2HPO4

27. 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

28. Ca Phosphates Most soluble MCP DCP TCP OCP Least soluble Apatite
Form added in fertilizer
Chemical transformation
with time in a calcareous
soil

29. 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)

30. Griffin is a
highly-weathered
clay soil

31. Consumption of N, P2O5, and K2O in the U.S.
N P2O K2O
Current P consumption is similar to the late 1960s
US and Canadian commercial fertilizer consumption of N+P2O5+K2O products.
From PPI

32. U.S. phosphate fertilizer consumption by crop in 2001
Total P2O5 consumption
4.3 million short tons
Corn grain
38.4%
Other crops
17.6%
Alfalfa
7.5%
Soybeans
7.7%
Wheat
16.5%
Corn silage, 3.7%
Cotton, 3.6%
Potatoes, 2.5%
Sorghum, 2.5%
USDA-ERS, USDA-NASS, AAPFCO, PPI

33. Average P use on corn and soybeans relative to crop removal
Gap is
growing
Use
Removal
Potash and Phosphate Institute, 2001

34. Potash and Phosphate Institute, 2001

35. Ratio of P removal by crops to fertilizer applied.DE ND SK MB ON BC AB WA OR MT ID SD MN PQ NY PA OH IN IL IA WI MI WY UT NV CA AZ NM NB NS
PEI ME NH VT MA CT RI NE KS MO KY WV VA MD NJ NC TN AR OK TX LA MS AL GA SC FL CO
>1.50
R/F
Potash and Phosphate Institute, 2001

36. Ratio of P removal by crops to fertilizer applied plus recoverable manure.ND SK MB ON BC AB WA OR MT ID SD MN PQ NY PA OH IN IL IA WI MI WY UT NV CA AZ NM NB NS
PEI ME NH VT MA CT RI NE KS MO KY WV VA MD DE NJ NC TN AR OK TX LA MS AL GA SC FL CO
>1.50
R/(F+M)
Potash and Phosphate Institute, 2001

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

38. Unfertilized lake
Canadian lake fertilized with P

39. P Availability P availability to plants is limited because:
soils often contain low amounts of P
mineral forms of P are insoluble
adsorption of ionic P
P does not move to the roots by mass flow because it is so insoluble
P must move to roots by diffusion

40. 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 ions
high Fe oxide content binds P ions
P precipitates with Ca, lower solubility
P ions move slowly
P ions bind to Fe oxides

41. Sanchez 1980

42. Sanchez 1982

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

44. 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

46. Sample of P Soil Test Guidelines

50. 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 ( ) solid
di ammonium phosphate ( ) solid
ammonium polyphosphate ( ) 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

51. 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??

53. 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.

54. Nutrient Mobility in Soil
Soil volume exploited
for mobile nutrients:
N, S, Cl
Soil volume exploited
for immobile nutrients:
Most others

55. Apply immobile nutrients here
(close to roots)
Because P is immobile, we cannot rely
on movement of irrigation water to transport P.

56. 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

57. Band
Broadcast
Sanchez, Swanson, and Porter 1990

58. Response of Celery to P Rate and Placement
Espinoza, Sanchez, and Schueneman, 1993