2. 2 Road Map
General Introduction
Sources
weathering and K release
Mechanisms of uptake
Fates of the Solution K
Path ways in & through the cells
Factors affecting the K uptake
Functions
3. 3 Facts about Potassium
• Potassium is a soft, highly reactive metallic element, it
occurs only in nature as compounds.
• Potassium was formerly called Kalium, which explains the
symbol “K”
4. 4 Soil Potassium
• Second most in terms of plant use.
• Taken up as cation (K+)
• Mineral sources in soils are important.
• Organic sources in soils are not
important.
•Often limiting and often added; third
number on fertilizer bag
• Roles in plants: stomatal control, cell
division, translocation of sugars, enzymes
5. 5 Potassium Reserves
Feldspar Mineral
• Potassium is found in minerals
like feldspars and micas (90% of Soil K)
• K is fixed inside of clay minerals ( 9% of soil
K)
• K is on the soil exchange sites ( 1%)
• K is in the soil solution (0.1%)
6. 6
Minerals-Illite (mica-like minerals)
• Si8(Al,Mg, Fe)4~6O20(OH)4·(K,H2O)2. Flaky
shape.
• The basic structure is very similar to the mica,
so it is sometimes referred to as hydrous mica.
Illite is the chief constituent in many shales.
potassium K
• Some of the Si4+ in the tetrahedral sheet are
replaced by the Al3+, and some of the Al3+ in
the octahedral sheet are substituted by the Mg 2+
or Fe3+. Those are the origins of charge
deficiencies.
• The charge deficiency is balanced by the
potassium ion between layers. Note that the
potassium atom can exactly fit into the
hexagonal hole in the tetrahedral sheet and
form a strong interlayer bonding.
• The basal spacing is fixed at 10 Å in the
presence of polar liquids (no interlayer
swelling).
Trovey, 1971 ( from
7.5 µm Mitchell, 1993) • Width: 0.1~ several µm, Thickness: ~ 30 Å 6
7. 7
Minerals-Vermiculite (mica like
minerals)
• The basal spacing is from 10 Å to 14
Å.
• It contains exchangeable cations
such as Ca2+ and Mg2+ and two
layers of water within interlayers.
• It can be an excellent insulation
material after dehydrated.
Illite Vermiculite
Mitchell, 1993
7
8. 8 Exchangeable vs.
Non-exchangeable K
Exchangeable K
Readily buffers
soil solution K
Non-Exchangeable K
Slowly buffers
Soil tests measure exchangeable K
soil solution K
9. 9
Potassium
Vermiculite, illite trap K+
Montmorillinite releases K+
K k k k k k k
k
Vermiculite
illite montmorillinite
10. 10 K release during mineral
weathering
Havlin et al. (1999)
11. 11
Weathering & Dissolution of Clay
Minerals
•The CO2 gas can dissolve in water and form carbonic acid,
which will become hydrogen ions H+ and bicarbonate ions,
and make water slightly acidic.
• CO2+H2O → H2CO3 →H+ +HCO3-
•The acidic water will react with the rock surfaces and tend
to dissolve the K ion and silica from the feldspar. Finally, the
feldspar is transformed into kaolinite.
•Feldspar + hydrogen ions + water → clay (kaolinite) +
cations, dissolved silica
• 2KAlSi3O8+2H+ +H2O → Al2Si2O5(OH)4 + 2K+ +4SiO2
•Note that the hydrogen ion displaces the cations.
11
12. 12 Hydrolysis
Feldspar + carbonic acid
+H 2 O
= kaolinite (clay)
+ dissolved K (potassium)
ion
+ dissolved bicarbonate ion
+ dissolved silica
Clay is a soft,
platy mineral, so
the rock
disintegrates
13. 13
Moving nutrients from soil to plants
Nutrients in
soil solution
Plant
Root
Exchangeable K ↔ Solution K
Nutrients on soil clay
and organic matter
15. 15
Fates of potassium in the soil solution
Applied K depends on the CEC and clay
minerals
Plant uptake
lost to leaching
retained by soil particles
precipitated as secondary minerals
Crop removal
18. 18
Cation Exchange Capacity
• Cation exchange capacity
(CEC) is the total amount
of cations that a soil can
retain
• The higher the soil CEC
the greater ability it has
to store plant nutrients
• Soil CEC increases as
– The amount of clay increases
– The amount of organic matter
increases
– The soil pH increases
19. 19
Excessive Nutrient Loading
Nutrients in
soil solution
Plant
Root
Exchangeable K ↔ Solution K
Nutrients on soil clay
Nutrient loss in
and organic matter
drainage water
20. 20
Soil properties: pH
Soil pH:
Influences nutrient
solubility.
K, Ca, and Mg most
available at pH > 6.0.
P availability is
usually greatest in the
pH range of 5.5 to 6.8.
At pH values less
than 5.0, soluble Al,
Fe, and Mn may be
toxic to the growth of
some plants.
Most micronutrients
(except Mo and B) are
more available in acid
than alkaline soils.
21. 21
Environmental Factors Affecting K
Availability to a Plant 78 % of K
supplied
to root via
• Soil moisture diffusion
– Low soil moisture results in more tortuous
path for K diffusion – takes longer to get to
root
– Increasing K levels or soil moisture will
increase K diffusion
– Increase soil moisture from 10 to 28 % can
increase toatl K transport by up to 175 %
• Soil Aeration
– High moisture results in restricted root growth,
low O and slowed K absorption by the root
22. 22
Environmental Factors Affecting K
Availability to a Plant
• Soil temperature
– Low temperature restricts plant growth and rate of K
uptake
– Providing high K levels will increase K uptake at low
temperatures
• Reason for positive response to banded starter
• Soil pH
– At low pH, K has more competition for CEC sites
– As soils are limed, greater amount of K can be held
on CEC and K leaching reduced.
23. 23
Environmental Factors Affecting K
Availability to a Plant
• Leaching related with Texture
– K leaching can occur on course textured or
muck soils particularly if irrigated
– Large fall K applications to sandy or muck
soils discouraged
24. 24
Mechanism of K Acquisition
Mass flow
Diffusion
Root interception
(Richardson et al., 2009)
25. 25
Mass flow
Movement of plant nutrients in flowing soil
solution
26. 26
Diffusion
Migration of nutrient from area of higher
concentration to lower concentration
78% K uptake by Diffusion
27. 27
Root interception
Growth of plant roots into new soil areas
where there are untapped supplies of nutrients
28. 28
Apparent free space (AFS) is the cell wall and intercellular
spaces of the epidermis and cortex of the roots (regions of
the root that can be entered without crossing a membrane;
apoplast space of the root epidermal and cortical cells).
AFS occupied about 10%~25% of root volume.
AFS includes space accessible to free diffusion and ions
restrained electrostatically due to charges that line the
space.
31. 31
Uptake of K+ into cells
• Using 86Rb (radioactive K+ analog) found K+
absorption is biphasic, i.e. there are two types of
K+ transport systems.
(1) high affinity uptake system. This system is
active at low [K+] (≦200µm). It is probably a H+-
ATPase-linked K+-H+ symporter.
(2) low affinity uptake system. This system is
bidirectional.
(1)Fewer of Si 4+ positions are filled by Al3+ in the illite. (2)There is some randomness in the stacking of layers in illite. (3) There is less potassium in illite. Well-organized illite contains 9-10% K2O. (4) Illite particles are much smaller than mica particles. Ferric ion Fe3+
Vermiculite is similar to montmorillonite, a 2:1 mineral, but it has only two interlayers of water. After it is dried at high temperature, which removes the interlayer water, expanded” vermiculite makes an excellent insulation material.
Steady water cannot sustain the reaction.
At any given time, the vast majority of nutrients and trace elements in soil are adsorbed onto the surface of clays and organic matter. Some, however, remain in the soil solution (soil water). These nutrients in solution can move back and forth between the soil surface and the soil solution. This is called ion exchange. When plant roots penetrate into the soil, they begin to remove nutrients from the soil solution to meet their nutrient needs. As plants remove nutrients from the soil solution they often exude other elements into the soil solution. The plant uptake of nutrients disrupts the balance between nutrient ions in the solution and nutrient ions on the soil surface is. To get back into balance, nutrients move from the soil surface out into solution and are then available for root uptake. Adsorption of nutrients, trace elements and other chemicals onto soil surfaces keeps them in the soil, usually in available forms, and limits how much could be lost in drainage water or runoff from the surface.
Minerals are the source of K for plants
Stable soil organic matter is made up of large complex organic molecules that are resistant to further attack from soil microbes. Pieces of soil organic matter appear like coiled, twisted strands. This material coats particles of silt and clay and helps to hold clay and silt together in soil aggregates. The coiled structure also gives organic matter a very large surface area. Soil organic matter is also like a sponge. It can soak up large amounts of water and store it for plants to use. Soil organic matter has a very high cation exchange capacity. Unlike many layer clays, the cation exchage capacity of organic matter changes as soil pH changes. As soil pH decreases (becomes more acid) more and more hydrogen cations stick to organic matter. At low pH this hydrogen is held very tightly and will not exchange with nutrients or other elements. As soil pH increases the hydrogen is held less strongly and readily exchanges with other nutrient and trace element cations like calcium, magnesium, potassium, and sodium. These cations will also exchange with each other at near neutral pH.
Together, clays and organic matter account for most of the cation exchange capacity of the soil. Cation exchange capacity refers to the total amount of cations that a soil can retain. The higher the exchange capacity, the better the soil is able to retain plant nutrients and other elements. The exact cation exchange capacity of soil depends on: what type and how much clay it has, how much organic matter it has, and what its pH is.
When excessive amounts of nutrients are added to soil, the capacity of the soil to adsorb them may be exceeded. Nutrients then cannot move from the soil solution onto soil particles. Nutrient concentrations then build up in the soil solution. In this diagram the orange balls represent nitrate and the light green balls represent phosphate. With increased nutrients in the soil solution there is increased likelihood that nutrients could be lost in runoff water or drainage water.
