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 Human Nutrient soln
(mM)
Potassium, K 0.92 1.09 6
Calcium, Ca 0.23 4.67 4
Magnesium, Mg 0.18 0.16 1
Sulfur 0.17 0.78
1
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 normal range
SODIUM 137 mmol/L 135-148
POTASSIUM 3.9 mmol/L 3.6-5.2
CHLORIDE 104 mmol/L 98-107
CO2 27 mmol/L 21-32
ANION GAP [L] 6 mmol/L 7-16
CREATININE 1.3 mg/dL 0.7-1.5
UREA NITROGEN 21 mg/dL 7-25
GLUCOSE 79 mg/dL 70-110
CALCIUM 8.9 mg/dL 8.4-10.2
! PHOSPHORUS 2.5 mg/dL 2.5-4.9
ALBUMIN 3.9 g/dL 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 anion-cation 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
Mica Illite
CEC=0 cmol/kg CEC=30-50 CEC=150
Vermiculite
non-hydrated K
hydrated K
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
K Loss in Turf
Table 6-5. Influence of K Source on Leaching Loss in Turf
K Source
Water Applied (in.)
10 20 50 75 100
% of K applied leached below root zone
Potassium chloride,
KCl
17 75 91 91 94
Potassium sulphate,
K2SO4
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.
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?
CEC = the amount of (-) charge
on soil colloids that attracts soluble cations
Review: Cation Exchange Capacity (CEC)
A Schematic Look at
Cation Exchange
Cations in solution and on exch. sites
can exchange positions
CEC=50: heavy clay
or high OM
50 <<<<<<< Typical CEC Range >>>>>>>0
CEC=0: pure sand
sand
clay
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.)?
Ammonium from the extractant has replaced the soil cations on the exchange sites
- - - - - -
NH4
NH4
NH4
NH4
NH4
NH4
All exchange sites saturated with NH4+
K Ca Mg are measured
The soil cations are collected and quantity analyzed
How is CEC measured?
- - - - - -
Mg
Mg Mg
2) Add a second salt solution such as MgCl2
NH4+ now
replaced by Mg2+
6 NH4 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 Ca Mg K Na
Decreasing strength of attraction