Importance of Potassium
• Third most limiting nutrient in agricultural
crops: 1 to 4% of plant dry weight
• Not incorporated into cellular structures
(mobile in plant)
• Not a component of organic matter
• Important for:
Soil K and P
Chapter 14
– Osmotic regulation
– Photosynthesis
– Starch formation and sugar translocation
p. 592-637
1
Importance
2
Potassium Deficiency Symptoms
• Helps adapt to environmental stress
• Leaf tips begin to yellow and die
• Can form white necrotic spots along leaf
margins
• Reduced
R d
dd
drought
ht ttolerance
l
• Increased lodging
• K is mobile, deficiency symptoms seen in
older tissues
– Drought tolerance
– Disease resistance
– Insect tolerance
• Animals
– Regulates nervous system
– Circulatory system
3
Potassium Availability
4
Potassium Losses
• Considerable amounts of K are lost to
leaching
• Lots of uptake, similar to N, 5× to 10× P
• Corn silage removal results in loss of over
150 lb K A–1
• Luxury consumption occurs with K (causes
nutrient imbalances in grazing animals)
• Found at high levels in soils
• ~80 to 37,000 mg kg–1 in soil
• Most soil K is in primary minerals or fixed
by layer silicate clays
• A large portion is unavailable to growing
plants
5
6
1
Potassium Losses
Potassium Cycle
• Leaching losses
• Cycle is mostly uptake and return of K+ to
soil solution
• No organic forms
• Solution
S l ti K+ is
i iin equilibrium
ilib i
with
ith
exchangeable and mineral K
– Can be 50 kg ha–1 y–1
– Very low in natural systems
– Acid soils leach more than neutral ((another
reason to lime)
• Plant uptake and removal
– Large amounts removed
– Corn, 160 kg ha–1
– Legumes, up to 400 kg ha–1
7
Potassium Cycle
8
Soil Potassium Pools
• Up to 50,000 kg ha–1 in soil
• Readily available:
– Solution K+: ~ 0.1 to 0.2% (50 to 100 kg ha–1)
Residue K
Fixed K+
– Exchangeable: ~ 1 to 2 % (500 to 1,000 kg ha–1)
• Slowly available: clay fixation: 1 to 10%
Biomass K
• Unavailable: minerals (micas, feldspar, etc.)
90 to 98%
Solution K+
Exchangeable K+
Mineral K
9
Potassium Problems
10
Potassium Problems
• Very large quantities in soil (quartz sand
soils are exception)
• Very little is in the exchangeable or
solution phases
• K+ is supplied to plant by DIFFUSION
• High yield agriculture: must maintain high
exchangeable K+
• Luxury Consumption
– Plants take up far more K than they need
– No yield increase
– This,
This combined with crop removal = large
losses
• 90-98% of soil K is unavailable
• 1-2% of soil K is readily available
11
12
2
Potassium Availability
Potassium Fixation
• Amount fixed determined by:
• Type of clay
• Sandy soils, low CEC: K+ in solution high
after fertilization, but
– 1:1 → very little fixation
– 2:1 clays → high fixation (especially by finefine
grained vermiculites)
– Lost by leaching
– Poor buffering - little exchangeable
• Fine-textured, high CEC
• pH
– Lower solution concentration, BUT,
– Available over long period of time
– Liming increases fixation
– Also increases buffering capacity in soils with
1:1 clay and high organic matter
• CEC and solution K+ are inversely related
13
14
Role of Phosphorus
Practical Management of K+
• Continuous cropping can deplete soil K
• Most fertility programs aim for maintaining
high levels of exchangeable K+ in soil
• Liming
Li i tto pH
H6
6.5
5 reduces
d
lleaching
hi llosses
+
• Fertilizers supply K as KCl or K2SO4.
• Phosphorus is a macronutrient
• The energy currency in the cell
– ATP: adenosine triphosphate
– ADP: adenosine diphosphate
• Integral component of cell walls & membranes
• Inositol phosphates
• Enhances many aspects of plant physiology
– Photosynthesis, maturation
– N-fixation, flowering, fruiting, seed production
15
Phosphorus Cycle
16
Phosphorus Deficiency
• Hard to recognize but:
– Stunted, thin-stemmed, spindly
– Dark, bluish green foliage
Adsorbed P
Solution
HPO42– & H2PO4–
Occluded P
• P iis mobile
bil iin th
the plant,
l t moves ffrom older
ld
plant parts to younger plant parts
• P deficiency symptoms are most
prominent in older tissues
Residue P
Biomass P
Mineral P
17
18
3
Phosphorus and Environmental
Quality
Phosphorus Fertility Problems
• Low total P levels in soil: 35 to 5,300 mg kg–1
[1/10 to 1/4 of N, 1/20 of K]
• P-bearing soil minerals are generally not very
p
p
)
soluble ((Al-,, Fe-,, and Ca-phosphates)
• P is strongly adsorbed by Al- and Fe-oxides,
hydroxides, and oxyhydroxides
• Phytoavailable P in fertilizers is rapidly
converted to adsorbed → occluded →
mineral forms
19
• Too little available soil phosphorus
– Low yields
– Low plant biomass
– Less residue cover
– Accelerated erosion
• Phosphorus in aquatic systems is not toxic
to fish: just causes eutrophication if P in
runoff is bioavailable
20
Phosphorus in Soil
• Exists as orthophosphate (PO4) in
inorganic form; as a phosphate ester in
organic form
• An oxyanion in solutions:
– pH < 7.2, H2PO4– predominates
– pH > 7.2, HPO42– predominates
• Phosphate anions are NOT exchangeable;
they are strongly adsorbed by metal
oxides, hydroxides, and oxyhydroxides
21
22
23
24
Phosphorus in Soil
• In acidic soils (pH < 6), variscite (Alphosphate) and strengite (Fe-phosphate)
have very low solubility
• In alkaline soil (pH > 6),
6) Ca-phosphates
Ca phosphates
(e.g., apatite, OCP) have very low
solubility
• Phytoavailability of soil P at highest level
in the pH 6 to 7 range
4
Water Quality Degradation
Why is Phosphorus Unavailable?
• Highly weathered soils in warm, humid
and subhumid regions
• Point-source pollutant: a clearly definable
point of discharge
– Acidic → reactive Al and Fe
– Little capacity to supply P for plant growth
• Extensive losses during
g intense weathering
g
• Adsorbed P and P in Al- and Fe-phosphates is not
readily available
• Soils in arid and semi-arid regions
– Alkaline → high Ca levels
– Little capacity to supply P for plant growth
– Waste treatment plants, industries
– Easy to regulate
• Nonpoint
Nonpoint-source
source pollutant: without an
obvious single point of discharge
– Watershed-scale
– Surface runoff
• Fertilizers
• Sediments
• Animal wastes (fecal coliforms)
• P in Ca-phosphates is not readily available
25
26
Eutrophication
• Low soil P levels → greater fertilizer
application rates
– Fertilizer P bound in unavailable forms
– Therefore, higher rates are needed
• Manures & biosolids
– Applied at N rate
– Over-application of P
• Erosion must be minimized or excessive P
levels in surface waters will occur
27
28
Phosphorus Losses in Runoff
Phosphorus Losses in Runoff
• Total P (TP): the total concentration of P in
runoff
• Total soluble P (TSP): the total
concentration of P that is dissolved in
runoff water, includes both orthophosphate
and organic P forms
• Soluble orthophosphate (SP): soluble P
that is in the inorganic form in runoff
• Soluble organic P (SOP): soluble P that is
in the organic form in runoff
• Particulate P (PP): the total concentration
of P that is in runoff sediment
• Bioavailable P (BAP): the total
concentration of P that is readily available
to plants and microorganisms (algae);
includes all orthophosphate (SP) and a
portion of the particulate P (PP)
• How does tillage impact P in runoff?
29
30
5
31
Practical Control of P Availability
32
Practical Control of P Availability
• Control erosion (minimize tillage)
• Use adequate fertilizer rates: do not overfertilize
• Saturate the P-fixing capacity
• Combine ammonium and P fertilizers
– Higher pH from ammonium will increase
availability of P
• Choose P efficient plants
– If done all at once,
once erosion must be kept in
check
– Once saturated, fertilizer P will be available
allowing lower application rates
– Mycorrhizal uptake
– Plant efficiency
• Increase cycling of organic P
• Placement of fertilizer P
– If placed as starter, more will be available and
less will be lost to erosion
33
– Crop residues
– Cycling provides constant availability
34
Practical Control of P Availability
• Control soil pH
– pH 6-7 is best (maximizes P availability)
• Enhance mycorrhizal symbiosis
– Appropriate
A
i t soilil conditions
diti
35
6