Acquisition of K in Plants

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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

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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”

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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

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Potassium Reserves

• 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%)

Feldspar Mineral

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Minerals-Illite (mica-like minerals)

potassium

• 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.

• 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 Mg2+ 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).

• Width: 0.1~ several µm, Thickness: ~ 30 Å7.5 µmTrovey, 1971 ( from Mitchell, 1993)

K

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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

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Exchangeable vs. Non-exchangeable K

Exchangeable KReadily buffers soil solution K

Non-Exchangeable KSlowly buffers soil solution K

Soil tests measure exchangeable K

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Potassium Vermiculite, illite trap K+

Montmorillinite releases K+

K k k k k k kk

Vermiculiteillite montmorillinite

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K release during mineral weathering

Havlin et al. (1999)

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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.

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Hydrolysis

Feldspar + carbonic acid +H2O= kaolinite (clay) + dissolved K (potassium) ion + dissolved bicarbonate ion+ dissolved sil ica

Clay is a soft, platy mineral, so the rock disintegrates

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Moving nutrients from soil to plants

Exchangeable K ↔ Solution K

PlantRoot

Nutrients on soil clay and organic matter

Nutrients in soil solution

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Interconnection of four K Reserves

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Fates of potassium in the soil solution

Applied K depends on the CEC and clay minerals

Plant uptakelost to leachingretained by soil particlesprecipitated as secondary mineralsCrop removal

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Factors Affecting K availability

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Cation Retention onOrganic Matter

Low pH, 4 - 5(acidic soil)

Neutral pH, 7(“sweet” soil)

HydrogenNutrients

Increasing pHincreases cation exchange capacity of organic matter

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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

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Excessive Nutrient Loading

PlantRoot

Nutrients on soil clay and organic matter

Nutrients in soil solution

Nutrient loss in drainage water

Exchangeable K ↔ Solution K

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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.

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Environmental Factors Affecting K Availability to a Plant

• Soil moisture– 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 O2 and slowed K absorption by the root

78 % of K supplied

to root viadiffusion

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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.

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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

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Mechanism of K Acquisition

Mass flow

Diffusion

Root interception(Richardson et al., 2009)

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Mass flow

Movement of plant nutrients in flowing soil solution

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Diffusion

Migration of nutrient from area of higher concentration to lower concentration

78% K uptake by Diffusion

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Root interception

Growth of plant roots into new soil areas where there are untapped supplies of nutrients

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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.

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Figure 5.11

Root epidermis (rhizodermis), cortical cells

Endodermis + suberized Casparian band

Vascular tissues: vessel elements, parenchyma cells

SymplastApoplast

Symplast ApoplastIon

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Facilitated diffusion

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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+] ( 200≦ µm). It is probably a H+-ATPase-linked K+-H+ symporter.

(2) low affinity uptake system. This system is bidirectional.

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Root epidermis (rhizodermis), cortical cells

Endodermis + suberized Casparian band

Vascular tissues: vessel elements, parenchyma cells

SymplastApoplast

Symplast ApoplastIon

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Consider the cow

Forage K levels > 3% can cause milk fever and other anion balance problems

particularly for early-lactation cows

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Contour cropping

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