Title: Plant and Soil Potassium;
Cation Exchange Capacity
Speaker: Bill Pan
online.wsu.edu
Plant and Soil Potassium
Cation Exchange Review
Plant Nutrition-Human Nutrition
Nutrient concentrations %DW
Element
Corn
Potassium, K
0.92
Human Nutrient soln
(mM)
1.09
6
Calcium, Ca
0.23
4.67
4
Magnesium, Mg 0.18
0.16
1
Sulfur
0.78
1
0.17
Pan’s Blood Composition
Patient: WILLIAM PAN
Note: All result statuses are Final unless otherwise noted.
Patient Note: Ordering Dr. ……
Tests: (1) BASIC METABOLIC PANEL
SODIUM
137 mmol/L
POTASSIUM
3.9 mmol/L
CHLORIDE
104 mmol/L
CO2
27 mmol/L
ANION GAP
[L] 6 mmol/L
CREATININE
1.3 mg/dL
UREA NITROGEN
21 mg/dL
GLUCOSE
79 mg/dL
CALCIUM
8.9 mg/dL
! PHOSPHORUS
2.5 mg/dL
ALBUMIN
3.9 g/dL
normal range
135-148
3.6-5.2
98-107
21-32
7-16
0.7-1.5
7-25
70-110
8.4-10.2
2.5-4.9
3.5-5.0
Potassium Accumulation in Major
Crops
Plant K Characteristics
• Absorbed as monovalent K+
• Typical K concentrations: 1- 4%
• Unlike N, P, and S:
- K is not incorporated into biochemical
structures. Instead:
- remains in ionic form or forms anioncation complexes.
Plant K Functions
1) Enzyme activation:
Presence of K stimulates the activity of over
80 plant enzymes, e.g.:
– starch synthase
– nitrogenase
– ATPase
Plant K Functions (cont.)
• 2) Water relations
(osmoregulation):
– Accumulation of soluble
K+ ions in cytoplasm
decreases the osmotic
potential (more
negative)
– Stomatal cells enlarge
with K accumulation,
and close to reduce
water loss
Plant K Functions (cont.)
3) Maintains cation-anion balance, and
helps stabilize pH in some parts of cell.
4) Multiple roles in photosynthesis
– ATP formation
– sugar translocation across membranes
Plant K Functions (cont.)
5) N assimilation
– K is main counter-ion for NO3
transport through xylem
– Cotransporter with NO3 (across
membranes into cells)
– Stimulates protein synthesis
6) increased epidermal cell wall thickness.
Potassium and Crop Quality
A Good K Supply:
• Decreases:
– stalk lodging
– blackspot bruise in potatoes
– uneven tomato ripening
– disease susceptibility and traffic wear in
turf
– Optimal bract size, color in poinsettia
Crop Quality: K Increases Resistance to
Some Diseases
Potassium Deficiency Symptoms
• Weakened stalks,
stems
• Small fruit, seeds
• Tip and marginal
leaf burn of older
leaves
Potassium Deficiency Symptoms
Classic symptom in
legumes:
• Small white
necrotic spots on
leaf margins
• Often mistaken for
insect damage
Potassium deficiency in fruit
Uneven Ripening in
K-Deficient Tomato
Plant and Soil Potassium
Potassium Movement
Through Soils
K analog (Rb) is depleted in rhizosphere
due to rapid uptake that is faster than
soil diffusion of K through soil
to the root surface
• total soil K can
range:100 - 50,000
lb K/A
• only a small portion
is moderately
available to plants.
Major Pools of K availability
• 1)Readily available (minutes):
• Soil solution (approx 0.1% of total K)
• 2) Moderately available (days) :
• Exchangeable K (1-2% of total K)
• 3) Slowly available (mo to yrs):
• fixed K in interlayer spaces of 2:1 clays
• (1-10% of total K)
• 4) Very slowly available (decades):
• incorp. in mineral structure (90-98%)
Reserve K pool feeds Active K pool
• ________ has higher K
in reserve
• equal clay content, but
_____has more
vermiculite and ______
has more kaolinite
• _____ holds onto its K
more tightly
Crop K uptake increases with increasing
active K in solution
Principle K Minerals
• Primary Minerals
– Orthoclase (K feldspar)
– Biotite (dark mica, Fe & Mg rich)
– Muscovite (light colored mica, Al rich)
• Secondary Minerals
– Illite
– Vermiculite
– Chlorite
Weathering of 2:1 silicates
non-hydrated K
hydrated K
Mica
Illite
Vermiculite
CEC=0 cmol/kg CEC=30-50 CEC=150
K in E. Washington Soils
• Palouse soils high in illite, therefore high in
available K. Typically no response to K
fertilization.
• Typical CEC’s range 15 to 25 cmol(+)/kg.
K in Central WA Soils (cont.)
• Sandy soils of Columbia Basin are low in
K and respond to K fertilization.
• Common CEC’s range from 4 to 9
cmol(+)/kg.
K in Western Washington Soils
• Highly variable texture, OM, and
mineralogy.
• While some soils have similar CEC’s to E.
WA soils, the % base saturation is typically
lower (lower % of cation exchange sites
occupied by basic cations).
Factors influencing K uptake
Plant Factors:
Root system and crop
Variety (hybrid)
Plant population and spacing
Yield potential
Growth stage
Cropping history
Cropping intensity
Factors influencing K uptake
Soil Factors:
Soil mineralogy
CEC (clay and OM contents)
Base saturation %
Subsoil K
Soil aeration
Soil moisture
Soil temperature
Soil pH
other cations in solution (next slide)
Factors Affecting K Uptake
Examples
• Soil temperature and moisture influences K
diffusion rates, root development, K fixation
• Low pH, high Al will inhibit K uptake
• Ca and Mg will antagonize K uptake (next
slide)
Factors influencing K uptake
other cations (esp. Ca2+ and Mg2+)
compete with K+ for entry into plants.
‘Activity Ratio’ (ARK)
ARK =
activity K+
.
(activity Ca2+ + activity Mg2+)1/2
Potassium Fertilizers
Potassium Chloride
•
•
•
•
“Muriate of potash”
Major potash mines in Saskatchewan
50-52% K
readily soluble
Potassium Sulfate
• “Sulfate of Potash”
• 42-44% K, 17% S
• used on Cl sensitive plants like tobacco.
Potassium Magnesium Sulfate
• “Sulpomag”
• 18%K, 11%Mg, 22%S
Water Applied (in.)
Table 6-5. Influence of K Source on Leaching Loss in Turf
K Source
10
20
50
75
100
% of K applied leached below root zone
Potassium chloride,
KCl
Potassium sulphate,
K2SO4
17
75
91
91
94
0
15
53
79
79
Potassium
phosphate, K3PO4
0
0
0
18
33
SOURCE: Sartain, 1988, Soil Sci. Fert. Sheet., SL52, Univ.
Florida, Gainesville, Fla.
K Loss in Turf
Potassium Application for Uniform
Distribution
Potassium Application for Immediate
Availability in the Root Zone
Example Fertilizer Calculation
• You have K2SO4 fertilizer with 0-0-42-17 grade.
How much would you need to apply to fertilize
150 lb K/acre? How much S would you be
adding along with the K?
Review: Cation Exchange
Capacity (CEC)
CEC = the amount of (-) charge
on soil colloids that
attracts soluble cations
A Schematic Look at
Cation Exchange
50 <<<<<<< Typical CEC Range >>>>>>>0
sand
clay
Cations in solution and on
CEC=50:
heavy clay
exch. sites
or high OM
can exchange positions
CEC=0:
pure sand
Units of CEC and exchangeable cations
 CEC Unit: cmol(+) / kg soil
cmol(+) = centimole of + charge
= 1/100 of a mole of charge
= 6.023 x 1021 charges.
 +1 ions (K, Na, NH4, etc):
1 cmol(+) from K+ = 1 cmol K atoms
 +2 ions (Ca, Mg, etc):
1 cmol(+) from Ca2+ = 1/2 cmol Ca atoms
Having trouble with the CEC concept?
Think parking lots!
 There are a finite number of (-) sites in a given
soil (parking spaces).
 Each site can be occupied by one of several
types of (+) ions that can vary in mass per unit
charge. (different weight per unit car, e.g. Prius
vs. Suburban)
 Some ions occupy more than one negative site
(truck takes up 3 spaces, but the mass of the
truck is distributed over 3 spaces)
Units of CEC and exchangeable cations
(cont.)
Question:
• If a soil has 1 cmol K+/kg soil, how much
K+ does that soil have on a mg/kg basis?
(Hint: 1 mole K = atomic weight of K =39
g/mole)
Units of CEC and exchangeable cations
(cont.)
If a soil has 1 cmol K+/kg soil, how much K+ does
that soil have on a mg/kg basis?
(Hint: 1 mole K = atomic weight of K = 39 g/mole)
Answer:
 K is +1,
so, 1 cmol K+/kg soil = 1/100 mole K/kg soil
 1 mole K = 39 g = 39,000 mg/mole
 1/100 mole/kg X 39,000 mg/mole =
= 390mg/kg
How is CEC measured?
1) Add a salt solution such as NH4OAC
-
Mg
Ca
K
K
initial
soil cations
How is CEC measured (cont.)?
All exchange sites saturated with NH4+
-
NH4
NH4
NH4
NH4
NH4
NH4
Ammonium from the
extractant has replaced
the soil cations on the
exchange sites
K Ca Mg are measured
The soil cations are collected
and quantity analyzed
How is CEC measured?
2) Add a second salt solution such as
MgCl2
6 NH4
Mg
Mg
NH4+ now
replaced by Mg2+
Mg
The ammonium is collected
and analyzed
How is CEC measured (cont.)?
3) Calculate CEC
• cmol(+) from NH4 in step 2 =
cmol(-) charge in soil.
• In words: the number of moles of charge
on the soil cations is equal to the CEC of
the soil.
(#neg. charge = # positive ch.)
Are all cations equally attracted
to exchange sites?
Answer: No.
It depends on:
1) cation charge
2) ionic radius
(Follows Coulomb’s Law)
Coulomb’s Law
Attractive Electrostatic Force is
proportional to:
(- charge on soil) (+ charge cation)
(distance between the two charges)
2
Coulombic Attraction
---
Al
-
Na
Ca
Mg
K
Decreasing
strength of
attraction