3/21/2016
UNDERSTANDING PHOSPHORUS
IN PLANTS AND SOILS
Agronomy Department
Iowa State University
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TOPIC OUTLINE
Importance and Abundance
Deficiency symptoms
Phosphorus cycle in soils
Organic soil P
Inorganic soil P
Factors affecting P availability
Importance of P
• Essential element for all life forms
• Source of energy – e.g. N2 fixation
16ATP + N2
rhizobia
16ADP + 16Pi + energy + 2NH3
•Adequate P enhances physiological processes:
– Photosynthesis
– Increased root growth
– Flowering
– Fruiting
– Grain production
– Earlier crop maturity
– Greater stalk strength
– Improve overall quality
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Abundance of P
• In lithosphere is about 0.12%
• Present in soils, rocks, water, and plant and
animal remains (No gaseous form)
• Inorganic or organic forms
• Total P is 1000-2000 lbs/ac in furrow slice
•Available soil P concentration usually less than
0.1 mg/L in soil solution (0.2 to 0.4 mg/L needed
to maximize plant growth)
Deficiency Symptoms
• Stunting of the plant during early growth
• Shriveled grain
• Abnormal discoloration such as in corn. The
plants are usually dark bluish-green in color
with leaves and stem becoming purplish
• Reduced growth and yield
• Delayed maturity
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Corn Deficiency Symptoms
Deficiency Symptoms
The Phosphorus cycle
The concentration of P in solution is determined by three
factors
1. adsorption/desorption reactions
2. mineralization/immobilization
3. precipitation/dissolution reactions
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The Phosphorus cycle
The Phosphorus Cycle
• Uptake: by plant roots
• Adsorption: removal of ionic P (H2PO4-, HPO42-) from solution by reaction
with solid phase of soil.
• Desorption: occurs when the orthophosphates, H2PO4- and HPO42- release
into the soil solution
• Dissolution: dissolve and increase the concentration of soil solution P
• Precipitation: as secondary minerals Ca, Fe, Al – P
• Immobilization: available P is taken up by soil microbes
• Mineralization: conversion of organic P to inorganic P
Soil Organic P
• 15-80% of total P
– Not directly available, must first be mineralized
• Inositol hexaphosphate (phytic acid) 2-50%
• Nucleic acids, phospholipids, sugar phosphates 1-5%
• Carbon to P ratio in organic matter is 100 to 200
– Determines net mineralization or immobilization
• Nature of more than 50% of organic P is unknown
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Abundant Organic P Forms in Soils
R―O―P―OH
OH
Monoester-P
O
O
R―O―P―OH
OR1
Diester-P
Inositol Hexaphosphate or Phytic acid
Dynamics of P in Soil
Nonlabile Soil P
slow
Labile Soil P
fast
Solution P
< 0.01 mg P L-1 in infertile soils
1 mg P L-1 in well-fertilized soils
7-8 mg P L-1 in recently-amended soils
Inorganic P in Soils
• High pH – P complexes with Ca and Mg
• pH below 7.5 – P complexes with hydrous
oxides of Al, Fe, and Mn
• Insoluble Al, Fe, and Mn – P form below pH
5.5
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Inorganic P in Soils
• Apatites (compounds of very low solubility that tend
to be present more in nonacid soils than in acid
soils):
• Hydroxyapatite Ca10(PO4)6OH2
• Fluorapatite Ca10(PO4)6F2
• Calcium phosphates (Forms of P that tend to form
when P fertilizers are added to nonacid soils.):
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Monocalcium phosphate Ca(H2PO4)2 . H2O
Dicalcium Phosphate Ca(HPO4) . H2O
Tricalcium Phosphate Ca3(PO4)2
Octacalcium phosphate Ca8H2(PO4)6 5. H2O
Inorganic P in Soils
• Aluminum phosphates (Forms of P that tend to form
when P fertilizers are added to acid soils that have
Al.)
• Variscite AlPO4. 2H2O
• Taranakite H6K3Al5(PO4)8 .18H2O
• Wavellite Al8(OH3(PO4))2
• Iron phosphates (forms of P that tend to form when
P fertilizers are added to acid soils that have Fe.)
• Vivanite Fe3(PO4)2 .8H2O (reduced Fe)
• Strengite
FePO4.H2O (oxidized Fe)
Factors Affecting P Availability
• Soil pH
– Highest near pH 6.5
– Most important reason for liming in acid soils
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Factors Affecting P Availability
• Type of Clay
– Oxide clays: contain a lot of Al3+ and Fe3+ that can absorb P
ions. Low availability because P held very strongly
– Kaolinite (1:1) clays, have lots of OH- ions that can be
replaced with P ions ---- low availability
– Montmorillonite, Illite, etc (2:1) clays: have few OH- ions
and they are mostly inaccessible, low anion exchange
capacity and adsorb little P
Factors Affecting P Availability
• At pH of 7.21, H2PO4- = HPO42- (predominant P absorbed by plants)
Mole Fraction of Total P
[Total P] = [H3PO4°] + [H2PO4-] + [HPO4-2] + [PO4-3]
1.0
0.8
0.6
H3PO40
0.4
HPO4-2
H2PO4-1
PO4-3
0.2
0.0
0
2
4
6
pH
8
10
12
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Factors Affecting P Availability
• Temperature
– The lower the temperature the slower the P used
by plants
– Iowa soils are somewhat cold in the spring, so
higher P levels are needed than warmer areas
• Organic matter decomposition
– Release organic P
– Organic acids formed can dissolve Ca-P
– Complexes Fe and Al
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Factors Affecting P Availability
• Soil moisture
– The amount of P present is proportional to the amount of
water present in the soil
– Too much water restricts root growth, reduces the amount
of P that roots can reach
– Low P is more damaging in dry and poorly drained soils
than in good air-water relations
• Time of reaction
– Fertilizer P is slowly converted into less soluble form
– Low solubility of P causes slow reaction
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TOPIC OUTLINE (NEXT CLASS)
Phosphorus fertilization
Fertilizer nomenclature
P fertilizer
Placement of P fertilizer
Environmental implications
Phosphorus Fertilization
• Soil Test
– Based on chemical principles that relate to
inorganic P minerals
– When the solution P decreases with plant uptake,
P minerals can dissolve and be released to
resupply soil solution P
– The process simulated by chemical extractant
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Soil P determination
• Mild Acid extractant (Bray P-1)
– Used in most eastern and midwestern States (acid soils)
– 0.025M HCl + 0.03M NH4F
– Not recommended for soils with pH > 7.4
• Stronger acids (Bray P-2)
– 0.1MHCl + 0.03M NH4F
– Is not suitable for Iowa soils, too acid especially in high lime soils
(Harps)
– Give high readings on these soils
• Mehlich III
– Used in various regions
– 0.0015M NH4F + 0.2M CH3COOH + 0.25M NH4NO3 + 0.013M
HNO3+EDTA
– The same manner as Bray P-1
Extracting Solution
• Distilled water
– Does not extract much – low extraction
• Alkaline extractant (Olsen test)
– 0.5M NaHCO3 at pH 8.5
– Used in neutral to calcareous soils
– Used in most western states where most soils are
alkaline
Soil test Classifications
% of maximum yield
Soil tests classifications indicate whether or not adding a
nutrient is likely to result in a yield increase.
100
50
Soil test:
Very low
low medium/optimum high
Fertilizer response likely.
very high
Response to fertilizer not likely.
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Recommendations
Recommendations for P are based on soil tests:
Determine how much needs to be added to reach optimum
soil test levels: the fertilizer uptake efficiency.
P Recommendations for Various Crops
Soil Test
Category
Corn
(grain)
Soybean Wheat Corn/Sorghum Alfalfa
(silage)
Very Low
100
80
--------------P2O5 to apply (lb acre-1) --------------60
120
110
Low
75
60
50
100
80
High
0
0
0
0
0
Optimum 58
40
30
80
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Adapted from ISU Extension PM 1688 Rev Oct 2013
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Phosphorus Fertilization
• P Fertility Evaluation
– Plant test
• Deficiency symptoms (stunting, purple color,
deformation of grain – shriveled grain)
– Sap test
• Can be used to verify deficiency suspicions: adequate
vs. inadequate
– Tissue analysis
• Optimum for P approximately 0.3% for corn and many
other crops
Phosphorus Content of Fertilizer
• Usually expressed as %P2O5 (old chemist nomenclature
from Liebig)
• P2O5 = phosphorus oxide
Amount of P in fertilizers
Rate of P to apply in recommendations
Lb P2O5/acre
• H2PO4 , HPO42 = ionic forms of P that plants use
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Mined Phosphorus
• 90% of the world’s
estimated P
reserves are found
in five countries:
Morocco, China,
South Africa,
Jordan and the
United States
• Reserves
remaining 40 years
or 400 years?
• Peak P
Nature 2009
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Cordell et al. 2009_ The story of phosphorus: Global food security and food for thought
TED Talk – “A simple solution to the coming phosphorus crisis” by Mohamed Hijri
Manufacturing process for common solid and liquid P fertilizers
from rock phosphate
Symbol Compound
P Fertilizers
Formula
%
11-52-0
TSP
Triple superphosphate
Ca(H2PO4)2
MAP
Monoammonium phosphate
NH4H2PO4
TPP1
Triammonium pyrophosphate (NH4)3HP2O7
DAP
1Main
Diammonium phosphate
in liquid form
(NH4)2HPO4
0-46-0
10-34-0
18-46-0
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Phosphorus Content of Fertilizer
• % P = 0.44 x % P2O5
(2 x Atomic. wt. P)/(Mol. wt. P2O5)=
(2 x 31)/([2 x 31]+ [5 x16]) = 0.4366
• % P2O5 = % P x 2.29
(Mol. wt. P2O5)/(2 x Atomic wt. P) =
([2 x 31]+ [5 x 16])/(2 x31)=2.29
Fertilizers are designed to have a known "grade" or
concentration of nutrients. For example, the fertilizer
shown below has a 20-5-10 grade, meaning it is 20% N, 5%
P205, and 10% K2O by weight.
The concentration of nutrients in
the fertilizer is multiplied by the
amount of the fertilizer material
applied per acre to find the actual
amount of N, P205, and K2O
applied per acre.
For example, if 100 lbs of the fertilizer above were
applied evenly over an acre, you would apply:
20% N x 100 lbs fertilizer/acre = 20 lbs N/acre
5% P205 100 lbs fertilizer/acre = 5 lbs P205/acre
10% K2O x 100 lbs fertilizer/acre = 10 lbs K2O/acre
On the other hand, if you wanted to satisfy a crop P
fertilizer guideline of 15 lbs P205/acre with that same
fertilizer, you would....
(15 lbs P205/acre) / (0.05) = 300 lbs/acre of the fertilizer
would need to be applied
In this case, the crop P205 guideline is divided by the
concentration of P205 in the fertilizer and the amount of
fertilizer material to be applied per acre is calculated!
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Phosphorus Content of Fertilizer
Triple superphosphate (0-46-0) costs $280 per ton.
What is the cost per pound of P2O5?
First, calculate the pounds of P2O5 in the fertilizer:
2,000 lbs fertilizer x 0.46 = 920 lbs.
Next, calculate the cost per pound of P2O5:
$280 / 920 lbs = $.30/lb P2O5.
Classification of P Fertilizer
Based on Solubility
• Water soluble
– Usually small amount, available quickly and may react
quickly to unavailable forms
• Citrate soluble
– Dissolve in 1N ammonium citrate – “available” (printed
on the bag)
– Why citrate? Citrate is mild acid and high buffer capacity
to approximate plant P availability
Total P = all that is present, regardless of availability
Competing Uses for Phosphorus
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Detergents
Pesticides
Explosives (matches to bombs)
Nerve agents
Water softeners
Metals manufacturing (steel and bronze)
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• Manure
P Fertilizers
– Beef manure = 10 + 5 + 10/ton wet
– Hog manure, the same or little higher in P
– Chicken and sheep manure, 2x as much P as cattle
Manure management plans will continue to play a
major role in managing P fertility for plant growth as
rock phosphate reserves are depleted and
transportation costs continue to rise
http://www.iowadnr.gov/afo/mmp.html
Iowa DNR Animal Feeding Operations
http://www.midwestagnet.com/Global/story.as
p?S=12062705
Late Winter Manure Application and Risk
Angela Rieck-Hinz is an extension program specialist for Iowa State University Extension and
is the coordinator of the Iowa Manure Management Action Group (IMMAG). Rieck-Hinz can
be reached at (515) 294-9590 or by emailing amrieck@iastate.edu.
Placement of P Fertilizer
• To Broadcast or Band apply: That is the question?
• P is not mobile and not soluble (placement can be
important):
– Most important if soil test P is low – close to the seed
– As soil test P increases, placement close to the seed is less important
• P fixing soils - highly weathered soils
– Placement in a band, reduces surface soil/fertilizer contact, reduces
the rate of fixation
• Banded Starter placement
– 2” below and 2” to the side of the seed
– Purpose: offset cool soils
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Why Broadcast?
Can apply large amounts conveniently
Ideal for building soil fertility
Minimizes risk of fertilizer injury
On fields of low fertility, banded starter P may
not be adequate for maximum yield
• Combinations of broadcast and band
applications produce the highest yields on low
testing soils
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Placement of P Fertilizer
Band Application
• Overwhelms soil fixation capacity
• Places nutrients near the seedling
– Roots intercept early and proliferate near the
band
– Including ammonium-N, slows P fixation and
lowers pH near the root to improve both P and
micronutrient availability
• Favors the crop over the weeds
• Keeps nutrients away from the surface
– Reduces runoff P concentrations
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Phosphorus and Environmental
Concerns
• P losses from cropland can cause surface
water quality problems
• Soil P levels have increased
• Manure P is a major contributor to soil P
buildup
• Land application of manure is often the only
practical management option
The Fate of Phosphorus Transport
Phosphorus and Water Quality
• Phosphorus losses from agriculture can be a
major source of P entering lakes and streams.
• Phosphorus additions to natural waters can
stimulate weed and algae growth.
• Vegetative growth -> oxygen depletion (aka
hypoxia) -> reduced water quality.
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Eutrophication Caused by Excessive P
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