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