Transcript P (Phyllis) behavior in soils

Phosphorus Behavior in
Soils
Plant and Soil Sciences
Department
Oklahoma State University
G.V. Johnson
Understanding phosphorus
behavior in soils
Start with “stripped” down, bare P.
 “P” stands for Norwegian Goddess
Phyllis.
 Like elemental phosphorus, we
remember the ancient story of Phyllis,
and how violent she reacted when she
swam to shore and, as she stepped
onto the beach, found herself naked!
 P

Understanding phosphorus
behavior in soils

Phyllis (P) ran to the nearest beachfront store and quickly bought a twopiece bathing suit (O2) and matching
beach sandals (O2).

She was much more comfortable.

P + O2 + O2 = PO43-
Understanding phosphorus
behavior in soils

But, Phyllis was chilled when it got
cloudy and the wind blew, so she
went back to the store and bought a
“beach –wear” cover (H+)and
matching shoes (2 H+).


Now she was comfortable
anywhere along the beach.
PO43- + H+ +2 H+ = H3PO4
Understanding phosphorus behavior in soils
Even a Norwegian Goddess can’t spend
all her time at the beach. Phyllis needed
to move about the land.
– She was conspicuous in her beachwear. It rained a lot. People were
always staring at her.
 She went shopping again.


She bought a jump-suit (Al3+) and got rid
of the beach-wear.
H3PO4 + Al3+ = AlPO4 + 3 H+

Now she was comfortable anywhere on
land along the beach
Understanding phosphorus
behavior in soils

In time Phyllis became lonely, she
missed her family, especially her twin
sister Phylline.
 Phylline lived in dry, southern California,
but not too far from the beach.
 She was quite stylish, but unstable, and
wore OSU sweats (Ca2+) and OU
CaHPO4
+
running shoes (H ).
Understanding phosphorus
behavior in soils

Phyllis went to visit her sister Phylline.

At first they weren't comfortable
together. Their styles clashed.
CaHPO4 + AlPO4
Understanding phosphorus
behavior in soils

With time the sisters learned to
overcome their differences.
 Phyllis finally found a sweat-suit like
Philline’s (Ca2+), and a two-for-one sale
for matching (OSU) shoes (Ca2+)

The sisters became inseparable and
went everywhere together.
CaHPO4 + AlPO4 + 2 Ca2+  Ca3(PO4)2
Understanding phosphorus
behavior in soils


Review
In nature, P always exists
in combination with
oxygen (O) in the form of
phosphates.
 PO43 Three
separate sites
for reaction with a
single charge cation
(H+, K+, NH4+).
O
ll
- O - P - Ol
O-
Understanding phosphorus
behavior in soils
PO43- will react with:
 whatever cation is in greatest abundance
and,
 whatever cation is held with the strongest
bond.
 In nature, there is plenty of water around:

H2O === H+ + OHConcentration of H+ = OH- = 10-7
Understanding phosphorus
behavior in soils


Review
When the charges on phosphate are all
satisfied by H+, in the laboratory, the
compound phosphoric acid is formed.
 H3PO4
O
ll
HO - P - OH
l
OH
Understanding phosphorus
behavior in soils

The H+ leave (dissociate from) phosphoric
acid in a stepwise manner when the acid is
reacted with base, like sodium hydroxide
(NaOH).
 H3PO4 
H+ + H2PO4 H2PO4- H+ + HPO42 HPO42- H+ + PO43
One or more of the phosphate forms will be
present in solution, depending upon the
solution pH.
Understanding phosphorus
behavior in soils
One or more of the phosphate forms will be
present in solution, depending upon the
solution pH.
Mole fraction of phosphate ionic species with pH.
H3PO4
1.00
Mole fracton P

H2PO4
HPO4
PO4
0.80
0.60
0.40
0.20
0.00
0
1
2
3
4
5
6
7
8
Solution pH
9
10
11
12
13
14
Understanding phosphorus
behavior in soils

In the normal pH range of soils the
concentration of H+ in the soil solution is too
weak to effectively compete for reaction with
phosphate anions.
 At pH 6, the H+ concentration is 1 x 10-6
mole/liter.
 = 0.000001 g/liter.
 = 0.001 ppm.
 = 0.001 milliequivalent weights.
 At pH 5 the H+ concentration is 10 times
greater and at pH 7 it is 10 less than at pH 6.
Understanding phosphorus
behavior in soils
Since the H+ concentration is so low in the
normal pH range of soils, the concentration of
H2PO4- and HPO42- might also be expected to
be low.
 If there is not enough H+ to react with PO43- in
the soil, what else can PO43- react with?
 Consider the lyotropic series and the
relationship of soil pH and percent base
saturation.

If there is not enough H+ to react with
PO43- in the soil, what else can PO43- react
with?

The lyotropic series is the order of adsorption
strength of cations adsorbed on soil colloids:
 Al3+ ~ H+ > Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+
 Soil pH and % base saturation are directly
related:
7
Soil pH
6
5
4
0
20 40
60 80
100
Percent Base Saturation
If there is not enough H+ to react with PO43in the soil, what else can PO43- react with?

The lyotropic series also relates to acidic and
basic cations:
 Al3+ ~ H+ > Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+
 Strongly acid======== Strongly alkaline:
7
Soil pH
6
5
4
0
20 40
60 80
100
Percent Base Saturation
If there is not enough H+ to react with PO43in the soil, what else can PO43- react with?

Above soil pH 5.5 there is an abundance of:
 Ca2+
~ Mg2+ > K+ ~ NH4+ > Na+
 Below soil pH 5.5 there are increasing amounts
of Al3+ ~ H+.
7
Soil pH
6
5
4
0
20 40
60 80
100
Percent Base Saturation
Below soil pH 5.5 there are increasing
amounts of Al3+ ~ H+.
Al3+ + H3PO4 == AlPO4
Al3+ + (NH4)2HPO4 ==  AlPO4 + 2 NH4+
Al3+ + NH4H2PO4 ==  AlPO4 + NH4+

DAP
MAP


 AlPO4
is very insoluble.
 The reaction rate depends on the concentrations
of Al3+ and H3PO4 in the soil solution.
lbs Al3+ will react with about 71 lbs P2O5 to
form AlPO4.
 27
Below soil pH 5.5 there are increasing
amounts of Al3+ ~ H+.
lbs Al3+ will react with about 71 lbs
P2O5 to form AlPO4.
 When P2O5 is banded with seed,
the P2O5 concentration is several
times greater than the Al3+
concentration in the band zone.
 The concentrations of H2PO4- and
HPO42- are very low in acid soils
because of their reaction with Al3+.
 27
Above soil pH 5.5 there is an abundance of:
Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+
Ca2+ + H3PO4  Ca(H2PO4)2 (0-46-0)
DAP Ca2+ + (NH4)2HPO4  CaHPO4 + 2 NH4+
MAP Ca2+ + NH4H2PO4  Ca(H2PO4)2 + NH4+
 Ca(H2PO4)2 and CaHPO4 are “highly” water soluble.
7
Soil pH
6
5
4
0
20 40
60 80
100
Percent Base Saturation
Above soil pH 5.5 there is an abundance of:
Ca2+ ~ Mg2+ > K+ ~ NH4+ > Na+

Ca(H2PO4)2 and CaHPO4 are highly water soluble.

These phosphates revert to apatite (rock-phosphate).
Chemical
formula
Ca/P
Solubility
g/100 mL
Ca(H2PO4)2 . H2O
0.5
1.8
3+ weeks
CaHPO4
1.0
0.03
Octacalcium
phosphate
2 to 5 mo.
Ca4H(PO4)3 . 2.5
H2O
1.33
Tricalcium
phosphate
8 to 10
mo.
Ca3(PO4)2
1.5
Hydroxyapatite
1 to 2 yr.
Ca5(PO4)3OH
1.66
Mineral
Reversion
time
Monocalcium
phosphate
Dicalcium
phosphate
0.002
(4 ppm)
Understanding phosphorus
behavior in soils: Fertilizers

Phosphate fertilizers dissolve slowly, but react
quickly in soils to form solid calcium and aluminum
phosphates, which, in time become very insoluble.
 DAP (solubility = 23g/100mL)
100 H2O + 23 (NH4)2HPO4 (dissolves slowly) = HPO42+ 2 NH4+
(solid)
(solution)
>
Ca2+ + HPO42-
<==
=(precipitates quickly)CaHPO4
Equilibrium P concentration is ~ 60 ppm
Equilibrium Ca2+ concentration is ~ 70 ppm.
Most soil Ca2+ concentrations will be > > 400 ppm
Understanding phosphorus
behavior in soils: Fertilizers
Phosphate fertilizers dissolve slowly, but react
quickly in soils to form solid calcium and
aluminum phosphates, which, in time become
very insoluble.
140
% Relative Yield

120
100
80
60
40
20
0
0.000
0.025
0.050
Soil solution P (ppm)
0.075
0.100