Potassium, Sulfur, Calcium, Magnesium

Section KSWES 316

Potassium in Plants

• First or second in amount taken up by most plants (N is usually first)

• Main functions– Osmotic control– Enzyme activation

• Deficiencies– Mobile nutrient--firing, spotting, necrosis of leaf

tips or edges, causes weak stems

K Deficiency

Corn

K DeficiencyAlfalfa

Soybean

Nutrient Removal (kg/ha/yr)

Source: Plant Nutrient Use in North American Agri., PPI, 2002

N P K N/K Ratio

Broccoli (100 lb yield) 0.44 0.07 0.35 1.25

Celery (100 lb yield) 0.19 0.05 0.42 0.45

Corn (bushel of grain – 56 lb) 0.75 0.19 0.24 3.0

Alfalfa (ton) 56 6.6 50 1.12

Oranges (ton) 8.8 0.8 9.1 0.97

Soils Depleted in K

Potassium Fertility• K is most likely to be deficient under

conditions of:– Acid, weathered soils– Sandy soils– With high K-use crops (e.g. alfalfa)

• In most of the ______ part of the U.S., K deficiencies are uncommon

• However, very high crop demand for K can sometimes create deficiencies in any soil

western

% of soils testing medium or lower for KData from PPI, 2001

Cotton Response to K in AZTreatment Lint Yield

Soil† Foliar‡ DPL 90 Pima S-7 Lbs of K2O/acre lbs/acre

0 0 1089 458 0 18.4 1008 511

200 0 986 563 200 18.4 1018 521 400 0 1111 538 400 18.4 1095 510

OSL§ 0.30 0.40 LSD0.05 NS NS CV(%) 10 15

Source: Silvertooth et al., 1996

† Potassium source was soil incorporated after planting by banding.

‡Sum of four 4.6 lb K2O/acre foliar applications using KNO3 as the K source.

§observed significance level for the treatment differences, or the probability that there are no differences among treatments

Grapefruit Response to K in Florida

Source: Obreza, Better Crops with Plant Food, 2003

Initial soil test K = very low

Alfalfa Response to K in Indiana

Source: Joern et al., BetterCrops with Plant Food, 2003.

Initial soil test K = 90 ppm (very low)

Turf Response to K in Georgia

Source: Trenholm et al., Better Crops with Plant Food, 2001

Potassium in Soils

• Almost all soil K is in inorganic forms:• Minerals

– K-feldspars– Clays (often called “fixed” K):

• Primary micas (biotite, muscovite)• Secondary clays (illite)

• Exchangeable– K+ on cation exchange sites

• Available: K+ in soil solution

from PPI

Interlayer (“fixed”) K in Illite

K in soil clays

Edgeposition

Inner position

Inner positionPlanar position

Hydrated and exchangeable

cation

Hydroxy Al (or Fe) islands

Potassium in Clays

• The availability of K+ on clays to plants depends on position on the clay (micas, illite):– inner sites - low availability, “fixed” K in illite– edge sites - moderate availability– planar sites - high availability “readily

exchangeable”

Potassium Equilibrium in Soils

Soln. K+ Exch. K+ Fixed K+ Mineral K+

Solution planar sites inner sites feldspars

edge sites

K in solution is usually <1% of exchangeable K

Exchangeable K is usually <1% of fixed K

K availability to plants depends on adequate K reserves and fast reactions to replenish solution K

K-fixing Soils

• The reverse process of K release from clays is called K “fixation”, and “fixed” K is essentially that in inner positions of 2:1 clays.

• Some clays, especially illite (hydrous mica), and vermiculite, can trap or “fix” added K in inner positions, rendering it unavailable to plants. Montmorillonite doesn’t fix K.

• Soils high in these clays are known as “K-fixing” soils, and pose special management problems, e.g. San Joaquin Valley, CA.

• The most common soil test for K is an ammonium acetate extract, which is then analyzed for K. This test is widely applicable across many soils.– “Critical” value often used is 150 ppm.– This means ____________________.

• Plant tests - total K in plants, or soluble K tests in plant sap. Guidelines exist for many crops.

Soil and Plant Tests for K

Ca2+

K+

Ca2+

Ca2+

Ca2+

Ca2+ K+

K+

K+

K+

K+K+

K+

Ca2+

Ca2+

Soil with exchangeable cations

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+ NH4

+

NH4+

NH4+NH4

+NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+NH4

+

Add a solution of ammonium acetate

NH4+

NH4+NH4

+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+

NH4+NH4

+

NH4+ NH4

+

Ca2+ K+

Measure amountof K+ released

Note: Ammonium acetate extraction will remove only the K+ on planar and edge exchange sites, and K+ in solution.

Ca2+ K+

Properties of Selected Soils

Soil Series Total K(g kg-1)

Exchangeable K

(mg kg-1)

Clay Mineralogy

Antho 22.0 366 S>MI>K

Gilman 21.1 280 S>MI>K

Glenbar 20.1 257 S>MI=K>Q

Grabe 24.8 549 S>MI>K=CA

Indio 17.3 315 S>MI=K>Q

Pima 26.0 430 S>MI K Q

Casa Grande

29.9 560 S<MI>K<PG

Mohall 27.7 309 S=MI<K

Superstition 31.0 100 S>MI=K=PG>Q

Gadsden 18.1 460 S>MI=K>Q

S-SMECTITE; MI-MICA; K-KAOLINITE; Q-QUARTZ; CA-CALCITE;PG-LYGORSKITE

Calculated Sufficiency and K Desorption

Soil Series SufficiencyLevel K(mg kg-1)

Difference betweenExchangeable and Sufficiency (mg kg-

1)

K DesorbedPer 30 min.(mg kg-1)

Antho 143 223 18

Gilman 136 144 17

Glenbar 138 119 15

Grabe 171 378 16

Indio 158 157 12

Pima 169 261 13

Casa Grande

137 423 33

Mohall 141 168 16

Superstition 120 -20 11

Gadsden 173 287 13

Summary of Clay Mineralogy in Arizona

• Clay mineralogy was a mixed composition of smectitie, mica, kalonite, palygorskite, calcite, and quartz.

• All soils contained K bearing mica, typically associated with high K release rates.

• These soils contained negligiable amounts of vermiculite, known for a high capacity to fix K.

Cumulative Time, hours

K R

ele

as

e, m

g k

g-1

200

400

600

800

1000

1200

1400

1600Pima Casa Grande Mohall Gilman Indio

Cumulative K released to calcium resin by claysof ten representative soils on a whole soil basis.

0 200 400 600 800

K R

ele

as

e, m

g k

g-1

0

200

400

600

800

1000

1200

1400

1600Gadsden Glenbar Antho Grabe Superstition

Cumulative Time, hours

K R

ele

as

e, m

g k

g-1

0

2000

4000

6000

8000

10000Pima Casa Grande Mohall Gilman Indio

Cumulative K released to calcium resin by ten representative soils.

0 200 400 600 800

K R

ele

as

e, m

g k

g-1

0

1000

2000

3000

4000

5000

6000Gadsden Glenbar Antho Grabe Superstition

K Fertilizers• Inorganic:

– Solids: KCl (0-0-60), K2SO4 (0-0-52), KNO3, (13-0-45) mixed in combination with N, P

– Liquids: Solutions of K solids• Organic:

– Fresh manures average 1 to 2.5% K, all is highly soluble and available. Composted organics contain much less (<0.3% K).

• Remember that K fertilizers are expressed as %K2O

Potassium Applied in Irrigation Water

5071Sweet Corn

5984Onions

2130Lettuce

4260Carrots

5274Melon

5071Broccoli

Irrigation K

(kg ha-1)

Irrigation

Water (cm)

Crop

Irrigation AE=70%

Comparison of K Applied in Irrigation Water and Amount Accumulated by

Crop

Crop Irrigation K(kg ha-1)

Crop Accumulation

(kg ha-1)

Broccoli 50 238

Melon 52 176

Carrots 42 409

Lettuce 21 192

Onions 59 196

Sweet Corn 50 119

100 200 300 400 500 600

Soil Test K (mg/dm3)

100

200

300

400

500

600

So

il T

est

Na

(mg

/dm

3)

• Celery (Harmer and Benne, 1945; Harmer et al. 1953).

• Cabbage (Costigan and McBurney, 1983; Costigan and Mead, 1987).

• Lettuce (Pereira and Westerman, 1978; Burns, 1986; Burns and Hutsby, 1986; 1987; Costigan and Mead, 1987).

• Tomatoes (Figdore et al., 1987;1989).

Other Vegetable Crops Showing Responses to Na when K Limiting

Sodium Applied in Irrigation Water

Crop IrrigationWater (cm)

Irrigation Na(kg ha-1)

Broccoli 55 719

Melon 60 785

Carrots 62 806

Lettuce 27 355

Onions 79 1030

Sweet Corn 60 780Irrigation =ET/(1-LR)

Tips for Managing K

• Soil test first!!! Many soils don’t need K.• K loss is not usually a problem in most soils,

except very sandy soils.• K does not cause environmental problems.• Beware of K-fixing soils!• K can be applied significantly in advance of

plant needs, because of the low risk of loss.

• Plants use about 10% as much S as N.• S is a lot like N:

– 50-90% in soils is organic (humus, microbes)– Subject to many microbial transformations

• Atmospheric deposition is important (acid rain)• Highly weathered soils are most likely to be S-

deficient (except with SO2 deposition)

Important Things About Sulfur!

Air Pollution and Sulfur

Sulfur in Plants• Plants about as much S as P (10% as much as N)• Used for:

– amino acids cysteine, cystine, methionine– proteins, enzymes– ferrodoxins (necessary for photosynthesis)

• Deficiencies: semi-mobile nutrient, usually produces chlorosis

S Deficiency - chlorosis of older leaves or whole plant

Wheat

Corn

Sulfur in Soils• In humid regions, as much as 90% of S may be found

in organic forms (humus)– subject to mineralization--immobilization reactions like N

• In arid regions, at least 50% of S is in inorganic forms (minerals, especially ___________________)

• The available form of S is ___________.• Natural (i.e. unfertilized soils) S availability is mostly

related to:– Soil organic matter content (more = more S)– Weathering (more = less S)– Soil minerals– Proximity to pollution sources

Gypsum - CaSO4.2H2O

SO4 2-

Atmospheric inputs

• SO2 is an important component of air pollution. It may react with H2O to form __________.

• Inputs to soil from the atmosphere may be as high as 110 kg S/ha/yr.

• Amount of inputs depend on:– Pollution controls– Proximity to pollution source

H2SO4

The Sulfur Cycle

• Similar to the N cycle:– Mineralization--immobilization of S– Autotrophic S oxidation (pyrites) similar to

nitrification– S reduction--reduction of SO4

2- under anaerobic conditions. Like ______________.

– SO42- can leach, but to a lesser extent than

nitrate. Not an environmental hazard.

denitrification

Sulfide Oxidation

Mine tailings (spoils) may become acidic because of theoxidation of pyrite (FeS2):

FeS2 + H2O + 7/2O2 → FeSO4 + H2SO4

Sulfur Availability

• Related to:– Soil pH and weathering - S is usually depleted in

weathered soils– Irrigation - water often contains lots of sulfate– Redox - flooded conditions result in S losses– Texture - sandy soils tend to have less S than

clayey soils– Pollution - high air pollution with S leads to high

soil availability

Sulfur Fertilizers

• Western soils rarely need fertilization with S:– Soils not highly weathered– Irrigation water contains SO4

2-

– Amendments added for alkalinity control contain S

• Eastern soils may need S:– Gypsum (CaSO4.2H2O)

– Potassium sulfate K2SO4

Fertilizing with S

• S is (mobile, immobile) in soils.• What does this mean about fertilization?• What about environmental considerations?

Nutrient Mobility in Soil

Soil volume exploitedfor mobile nutrients:N, S

Soil volume exploitedfor immobile nutrients:Most others

Important Things about Ca

• Ca is abundant in many soils.• Soil pH is largely related to exchangeable Ca2+

(base saturation)• Most soil Ca is exchangeable Ca2+ or in other

minerals - feldspars, carbonates• Most western soils have enough Ca for crop

needs.

Ca in Soils

• Total Ca in soils ranges from 0.1 to 25%. It is most abundant in ____________ regions.

• Important mineral forms include feldspars and other primary silicates, and _____________ in semi-arid and arid regions.

• Ca2+ is the available form and the dominant exchangeable cation in _____________ soils.

arid, semiarid

CaCO3

non-acid

Mg in Soils

• Total Mg in soils ranges from 0.1 to 4%. It is highest in soils of _______________ regions.

• Important mineral forms include primary silicates, Mg carbonates, and Mg sulfates.

• Mg2+ is the plant-available form and, next to Ca2+, is a dominant cation in ____________ soils.

arid, semiarid

non-acid

Ca and Mg in Plants

• Calcium:– Strengthens cell walls– Needed in large amounts by developing fruits– Immobile in plants, older parts of roots cannot absorb it,

so a supply is needed at growing tip• Magnesium:

– Central atom of chlorophyll– Osmotic regulation– Mobile in plants

Ca Deficiency

Tomato“blossom-end rot” Lettuce

“tip-burn”

Mg Deficiency

Potato Cabbage

Ca Deficiency• Ca is most likely to be deficient under conditions of:

– weathered, acid soils– sandy soils

• However, Ca deficiency can occur in surprising places:– Fruit crops (High Ca demand)– Plant structures out of the transpirational stream– Vegetable crops with small root systems

• Ca deficiency can be difficult to correct:– Highly immobile in plants– Highly immobile in soils– Only taken up by the youngest part of root system

Ca and Mg Fertilization

• The need for Ca and Mg is usually (not always) related to soil pH:– Lime adds Ca2+

– Dolomitic lime adds Ca2+ and Mg2+

• Ca and Mg immobile in most soils (except very sandy soils).

• What does this mean about fertilization?

Ca and Mg Fertilizers

• Ca fertilizers:– Solids - gypsum, calcium nitrate– Liquid - calcium ammonium nitrate (CAN-17)

• Mg fertilizers:– Solids - magnesium sulfate (epsom salts)– Liquid - magnesium nitrate

• Organics - fresh manures are good sources of Ca, composted are not.

Special Notes on Ca

• As mentioned before, Ca deficiencies can occur, even on soils testing “high” in Ca.

• This is most likely to occur with lettuce, tomatoes, and other fruiting crops.

• Supplying adequate Ca can be a problem -

foliar applications of soluble materials, soil side-dressing of Ca fertilizers may be needed.