Phosphorus & Potassium
Roles of Phosphorus:
• 1. Essential for plant growth
• 2. Plays a role in photosynthesis, respiration, energy
storage and transfer, cell division and other processes.
• 3. Promotes early root formation and growth
• 4. Vital in seed formation
• 5. Strengthens structural tissue – prevents lodging
• 6. Improves winter hardiness
• 7. Improves forage crop quality
• 8. Increases water use efficiency- reduces water stress
• 9. Can boost yield levels and reduce grain moisture
levels
• 10. Increases uptake of Mg – prevents grass tetany
Plant Utilization:
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Crop
Alfalfa
Bermuda
Corn
Cotton
Wheat
Soybean
Yield P2O5 taken up
8 ton
120 lb
8 ton
96 lb
160 bu
91 lb
1000 lb
51 lb
60 bu
41 lb
60 bu
58 lb
Phosphorus Deficiency:
• Symptoms:
– Overall stunted appearance
– Leaf shape is distorted
– Dead areas arise on leaves
– Purple or reddish color on leaves
• A grower can expect to get 20-30% efficiency
from water soluble Phosphorus fertilizer the
first year after application.
Fertilizer Sources of P
Available P
Unavailable P
Plant Absorption of Phosphorus:
• Plants absorb most of their P in the
orthophosphate ion (H2PO4) dissolved in soil
water.
• It may also be absorbed in the HPO4- ion but in
smaller quantities.
Plants Take Up P As:
Solution P

Primary orthophosphate ion:
H2PO4- (pH < 7.0)

Secondary orthophosphate ion:
HPO4= (pH > 7.0)

The form most common is a
function of soil pH – both equally
present at neutral pH
Where does P come from:
• P comes from the chemical weathering of apatite
rock.
• The higher the ph of the soil the more Hydrogen ions
are given up to form the 2 orthophosphate ions.
• Decomposing organic matter, humus and microbes.
– 20-50% of soil P is in O.M.
How Does P Move In The Soil?
• There is no gas form of P
• P does not leach (unless it is in extremely high
amounts)
• It is not subject to oxidation-reduction
• It does not move- except through diffusion
P Movement:
• It has been estimated that P greater than ¼ in
away from a plant root will never move close
enough to be taken up by the root.
Phosphorus mobility in plants:
• P is mobile in plants and moves readily from
older tissue to newer tissue.
Other forms of P:
• Soluble forms of P will form compounds with:
– Calcium in alkaline soils
– Iron in acid soils
– These forms are not available to plants
Factors Affecting Soil P Availability:
• 1. Amount of clay – clays fix P
• 2. Type of clay – Kaolinitic clays fix more P
than other clays
• 3. Time of application – The longer the soil
and added P are in contact the greater the
chances for fixation.
• 4. Aeration – for microbial breakdown and
plant growth.
• 5. Compaction – influences aeration
Factors Affecting Soil P Availability:
• 6. Moisture – inc. soil moisture to optimum
levels make P more available
• 7. Temperature – influence plant growth and
O.M. decomposition.
• 8. Other nutrients – Calcium increases P
uptake
• 9. Soil pH
• 10. Crop root type
Higher P
concentration
Plant root
Lower P
concentration
Key to P Fertility:
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P does not move in the soil
P does not change forms
So
Soil:
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pH
Microbial activity
Weathering
Having crop residues
Using Manures
Commercial fertilizers
Soil pH effect on P
• optimum P availability
– pH 6.0 to 7.5
• acid soil (pH < 6.0)
– lime to bring pH up
• alkaline soils (pH >7.5)
– too costly to acidify
• S can be used
Ammonium phosphates
 most commonly used P-fertilizers
–ammonium orthophosphates
 nearly 100% water soluble
Other sources of P
 monoammonium phosphate
(MAP)
–NH4H2PO4
–10-52-0
 diammonium phosphate (DAP)
–(NH4)2HPO4
–18-46-0
Other sources of P
 ammonium polyphosphate (APP)
–(NH4)3HP2O7 + NH4H2PO4
• orthophosphate
• polyphosphate
–10-34-0
–mainly used in fluid fertilizers
Calcium phosphates
 triple superphosphate (TSP)
–monocalcium phosphate
• Ca(H2PO4)2
–40-46% P2O5
–water soluble
 ordinary superphosphate (OSP)
–monocalcium phosphate and
gypsum
• Ca(H2PO4)2 + CaSO4 • 2H2O
–18-20% P2O5
–10% S
–high water solubility
 phosphate rock
–fluoroapatite
• [Ca3(PO4)2]3 • CaF2
–25-40% P2O5
–insoluble
• more soluble in acid soils
Other P sources
 nitric phosphate
 phosphoric acid
 potassium phosphate
 basic slag
 farmyard manure
–esp. chicken
Fertilizer Production
 strip mining of P-rock
–deposits
• N. Africa
• N. America
–Florida, N. Carolina, Idaho,
Utah
• Russia
Methods of Applying P Fertilizer:
• 1. Banding – applying P in the row with the
seed.
– Plants utilize the P more
– Co-banding with N increases uptake of both
nutrients
• 2. Broadcast- applying to soil surface,
– Less efficient more becomes fixed
• 3. Plow down – mixed deeper into soil
– Good for deep rooted plants
Potassium
Roles of Potassium:
• Essential in photosynthesis, respiration and protein
synthesis.
• Important in translocation of heavy metals (Fe)
• Helps plant overcome effects of disease
• Important in fruit formation
• Improves winter hardiness
• Important in osmotic regulation in plants
• Important in carbohydrate breakdown
• Important in ionic balance
Helps Plant Fight Disease:
• Trials have shown:
– Leaf blight and stalk rot in corn
– Wilt and damping off in cotton
– Mold and mildew in soybeans
– Black spot and stem end rot in potatoes
– Leaf spot and dollar spot in turf
K
• Strengthens stems
• Thickens cuticle
• Reduces the number of shrunken moldy
discolored soybeans or seed.
N to K balance in Forages
Nutrients
Yield lb/a 2nd cutting
• 500-0-0
2,693
• 500-70-0
2,887
• 500-0-60
4,509
• 500-0-120
4,679
• 500-70-60
4,267
• 500-140-140
4,999
– Bermuda grass in Texas
Plant Deficiency Symptoms:
• Increased drought stress
• Burned or scorched appearance on leaf
margins. Older leaves show deficiency
symptoms first in grasses
– Newer leaves on cotton
• Plants grow slowly and have poor root
development
• Weak stems
• Plants are more subject to diseases
Forms of Potassium in Soil:
• 1. Unavailable K – found in rock minerals
– Released as rock weathers – takes hundreds of
years
• 2. Slowly available K – fixed or trapped
between layers of certain soil clays
– Available as clays swell and shrink.
• 3. Available soil K – K found in soil solution
plus K held in exchangeable form by soil
organic matter and clays. About 2% or less of
total K in soils
How K moves in the soil:
• K does not move readily in soils except sandy
or organic soils.
• When it does move it does so by diffusion
How do plants take up K?
• Plants can take up K in two ways:
– 1. Soil solution
– 2. Exchangeable K – loosely held by soil clays and
organic matter.
Factors Affecting Plant Uptake of K:
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1. Any factor affecting/limiting root growth
2. Soil aeration
3. Level of K in soil
4. Fixation of K
5. CEC level
6. Soil temperature – low temps reduce K
availability
• 7. Soil moisture – drought reduces K uptake
Methods of Applying K Fertilizer:
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Broadcast
Broadcast, plow down
Direct seed placement
Fertigation
Deep placement or knifed
Sources of K Fertilizer:
• K deposits occur as beds of solid salts beneath the
earths surface, and brines in dying lakes and seas
• Mined in 4 ways:
• 1. Conventional underground coal mining method
• 2. Continuous mining method
• 3. Solution mining – sodium chloride brine dissolves
KCl pumps to surface for refining
• 4. Surface Brine Recovery- K and other minerals
harvested with solar evaporation from natural brines
– Great Salt Lake, Dead Sea
Minerals K is Mined From:
• 1. Sylvinite – KCl and NaCl (20-30% K2O)
• North American KCl (0-0-60) 90% of K sold in U.S. and
Canada
• 2. Sylvite – KCl (63% K2O)
• 3. Langbeinite – K2SO4 and MgSO4 (23%
K2O)
Fertilizer Grade Particle Size:
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5.
White Soluble – ideal for liquids
Special Standard
Standard
Coarse
Granular – suitable for bulk blending
Other Sources:
• K2SO4 – Sulfate of potash (SOP) 50% K2O and 18% S
– Used in Cl- sensitive crops or where Cl buildup is a problem
• K2SO4-2MgSO4 – Potassium-magnesium sulfate “KMag” or “Sul-Po-Mag” (22% K2O, 11% Mg, 22% S)
– Good source of water soluble K and Mg
• KNO3 – Potassium nitrate (13-0-44)
– Little or no Cl or S