Potassium-forms, equilibrium in soils and its agricultural significance; mechanism of potassium fixation; management of potassium fertilizer under field condition

1. PRIYA P. GURAV
Scientist
(Soil Science)
ICAR – Central Research Institute for
Dryland Agriculture, Hyderabad
UNIT V
Potassium-forms,equilibriuminsoilsanditsagriculturalsignificance;mechanismof potassiumfixation;
managementof potassiumfertilizerunderfieldcondition

2. POTASSIUM
• It is key plant nutrient in the soil
• It constitutes about 2.5% of the earth crust
Potassiumsources
Common minerals
• Sylvite
• Carnallite
• Kainite
• Langbeinite
• Leonite
• Schoenite
• Polyhalite
Major K- bearing primary minerals
• Feldspars
- Orthoclase
- Sanidine
- Microcline
- Leucite
• Micas
- Muscovite
- Biotite

3. Biotite Muscovite
K feldspar

4. 4
Potassium is known as a yield plus quality nutrient
It is involved in the working of a large number of enzymes
 In the production and movement of photosynthesis from leaves to storage organs
Water economy and providing resistance against pests, diseases and stresses
Enhances translocation of sugars and starch
Produces grain rich in starch
Increases protein content of plants
Builds cellulose and reduces lodging
IMPORTANCE OF POTASSIUM

5. • Brown scorching and curling of leaf tips as well as chlorosis
(yellowing) between leaf veins
• Plant growth, root development, and seed and fruit
development are usually reduced in potassium-deficient plants
• Potassium deficiency symptoms first appear on older (lower)
leaves because potassium is a mobile nutrient
• Deficient plants may be more prone to frost damage and
diseases
DEFICIENCY OF POTASSIUM

8. DISTRIBUTION OF K IN SOIL
 The readily available K constitutes only 1-2% of total K and exists in soil in
two forms, viz., solution and exchangeable K adsorbed on soil colloidal surface
 According to increasing order of plant availability, soil K exists in four forms:
lattice (5000-25000 ppm), non-exchangeable (50-750 ppm), exchangeable (40-
600 ppm) and solution (1-10 ppm).

9. POTASSIUM CYCLE

10. DYNAMIC REVERSIBLE EQUILIBRIUM BETWEEN SOIL K POOLS

11. Readily available to plant

12. Classification of K-bearing minerals (Malavolta,1985)
Priya/SRS/04-01-13
Mineral Chemical composition K content (g/kg)
Primary
Feldspars
Orthoclase (K, Na) Al Si3 O8
Sanidine Kal Si3 O8
Microcline (Na, K) Al Si3 O8 ~110
Leucite
Micas
Muscovite KAl2(AlSi3)O10(OH)2 ~80
Biotite K(Mg, Fe2+)3(AlSi3)O10(OH)2 ~70
Secondary
Illite
Dioctahedral (K0.58X0.17)(Al1.55Fe3+
0.20Mg0.25Al0.5Si3.5)O10(OH)2nH2O
Trioctahedral (K0.45X0.21)(Al2.61Fe2+
0.10Mg0.29Al1.05Si2.95)O10(OH)2nH2O
Transitional clay minerals
Edge expanded illite
Illite+Montmorillonite
Illite+Vermiculite
Allophane Sio2.Al2O3.2H2O 1.0-1.3
12

13. Minerals Minerals Mechanism transformation
Feldspars
 Surface reaction and replacement of K + by H3O+
 Rupture of Si‐O‐Al bond
Mica
Existence of voids
 Change in inter layer spacing
Drying of lattice in presence of CaCO3
Muscovite
(Dioctahedral)
Difficultly rupture of shortened and strengthened
K‐O bond
Biotite
(Trioctahedral) Oxidation of Fe2+ to Fe 3+ during weathering
13
The weathering rate of feldspar is much slower than mica.
Among the micas tri-octahedral mica (biotite) releases sufficient
quantity of K to soil compared to dioctahedral mica (muscovite) even at a low
intensity of weathering. (Fanning and Keramidas, 1977)
MECHANISM OF TRANSFORMATION OF K BEARING
MINERAL

14. Factors affecting availability of potassium
1. Soil texture
2. Clay mineralogy
3. Soil depth
4. Soil pH
5. Liming
6. Freezing and thawing and wetting and
drying
14

15. 15
Fig. Potassium relationships in soils and plant roots

16. 16
1. Soil texture(Clay content) & Clay minerals:
- It influences both available and non-exchangeable
potassium
- Fine-textured soils possess large amount of both forms
of K compared t coarse textured soils
- Both the quality and quantity of clay important in K
fixation
- Greater the clay content, greater K fixation
- Clay minerals like illite, weathered mica, vermiculite
and smectite, interstratified minerals fix K, while
kaolinite fixes very little

17. 17
Depth :
- Indian soils show characteristic differences in K content
with depth
- Calcareous alluvial soils show a decrease in both the
available and non-exchangeable forms of K with depth
- Alluvial soils from Indo-Gangetic plains show more
available K in the surface soils while non-exchangeable K
is more in the sub-surface soils
- In shrink-swell (Vertisols/black) soils both available and
reserve K decrease with depth

18. Soil pH (soil reaction)
 It has significant role in availability of potassium in
soil
 In acid soils, H+ and hydroxy-aluminium ions
compete with K+ ions for the exchange or adsorption
sites and are able to keep more K+ ions in the
solution phase and reduce their susceptibility to
fixation
 As the pH increases the H+ and hydroxy-aluminium
ions are neutralized or removed, making it easier for
the K+ ions to move closer to soil colloidal surfaces
where they become susceptible to fixation

19. Liming
 Liming of acid soils (with pH-dependent
negative charge) increases the cation
exchange capacity (CEC) of soil which
results in increased K adsorption by the soil
colloids and a decrease in the K level in the
soil solution
 The high calcium concentration in the soil
solution phase may reduce K uptake by
plant, especially in soils containing high
amount of CaCO3

20. Freezing and Thawing
 Alternate freezing and thawing
may result in increased
exchangeable K in some soils;
however, the reverse may also
happen in illitic soils having high
exchangeable K

21. PotassiumFixation
The phenomenon of K fixation or retention
K availability
21
The important forces involved in interlayer reactions in clays
1. Electrostatic attractions between the negatively charged layers,
2. The positive interlayer ions,
3. Expansive forces due to ion hydration (Kittrick, 1966).
affects
are
and

22. PotassiumFixation
 Conversion of freshly applied potassium and / or soil solution potassium to fixed or
non exchangeable forms that can not be extracted with neutral salts is referred to as
K fixation.
 It is maximum in 2 : 1 clays particularly with high amounts of illite. The fixation is
nearly absent in soils dominated by kaolinite, chlorite and unweathered micas; slight
in montmorillonite and substantial in illite and high in vermiculite dominated soils.
 For the clay minerals like illite, vermiculite and weathered mica three different
adsorption sites can be distinguished. These sites are at the planar surfaces (planar
position), at the edges of layers (e positions) and in interlayer spaces (i positions).
 The binding of K+ with organic colloids and kaolinite is at ‘p’ position
which is weak and hence easily replaced by other cations.
22

23. 23
• p – planner surface
• i – interlayer space
• e – edges of layers

24. PotassiumFixation
 In smectite rich soils, K+ is held at i position which has the maximum
specificity for K + . When dehydration occurs, the lattice sheets come closer and
the adsorbed cations lose their water molecules
 According to ‘Lattice Hole’ theory (Page and Baver, 1940), the exposed surface
and surfaces between sheets of minerals consists of oxygen ions arranged
hexagonally
 The opening within the hexagon is equal to the diameter of an oxygen ion
(approximately 2.8 oA)
 Ions having a diameter in this magnitude (eg. For K+ it is 2.66 oA) will fit
snugly into the lattice holes and such ions will be held very tightly as they come
in contact with the negative electrical charges within the crystal
 However, ions like NH4
+ (dia. 2.86 oA ) has nearly the same ionic radius as the
K+ and is subject to similar fixation by 2 : 1 clays
24

25. 25
PotassiumFixation
Priya/Ph.D/LRM/Seminar/08-02-13

26. 26
PotassiumFixation
The major clay minerals responsible for K fixation
are
Smectite , vermiculite, and weathered micas
Srinivasa Rao et al. 2000
Priya/Ph.D/LRM/Seminar/08-02-13

27. The Potassium fixation
Kaolinite, chlorite and
unweathered mica Vermiculite
Nearly absent Large in
Montmorillonite Illite
Slight substantial
27
Srinivas Rao et al. 2000; Singh et al. 1987; Sharma and Dubey, 1988;
Chakarvorh and Patniak, 1990; Srinivasa Rao and Khera, 1995)
PotassiumFixation in different mineral
Priya/Ph.D/LRM/Seminar/08-02-13

28. The degree of K fixation
Charge
density
Extent of the
interlayer wedge
zone that is
depleted of K
Moisture
content
Solution K
concentration
The nature and
concentration of
competing cations
in the surrounding
medium
(Rich, 1968; Sparks and Huang, 1985,
Brar et al.1986; Subba Rao, A.Sesha Sai,
M.V.R. and Pal, S. K. 1993)
Depends on
28
factors influencingPotassiumFixation

29.  K Fixation is high when charge density is high
 Vermiculite and illite tend to fix best under relatively wet
conditions while fixation by montmorillonite and the
interstratified clay minerals occur under drier conditions
 Ions like H+ can compete with K+ for fixing K or binding
sites
 If the wedge zone is confined to the edge of the particle,
then only small amounts of K can be fixed
 On the other hand, if the zone penetrates deeply into the
mineral, considerable amount of K can be fixed
 Wetting and drying cycles lead to fixation of K in soils rich
in available K

30. Ions like NH4+ and H+ can compete
30
With
K+ for K fixing or binding site
If
The wedge zone is confined
to the edge of the particle
then
only small amounts of K can
be fixed.
The zone penetrates deeply
into the mineral,
considerable amount of K
can be fixed
factors influencing PotassiumFixation
(Brar et al.1986; Subba Rao, A.Sesha Sai, M.V.R. and Pal, S. K. 1993)

31. The fixing power of 2:1 type clay mineral follows
Vermiculite>Illite> Smectite (group in general)
31
The order
factors influencing PotassiumFixation
(Srinivas Rao et al. 2000; Singh et al. 1987; Sharma and Dubey, 1988; Chakarvorh
and Patniak, 1990; Srinivasa Rao and Khera, 1995)

32.  The phenomena of both fixation of
exchangeable K and release of non-
exchangeable K play an important role in the
dynamics of soil potassium
 The gradual release of K from trapped
positions in the mica lattice to form illite and
eventually vermiculite with concomitant gain
of water and swelling of K lattice is given in
following figure
PotassiumRelease fromSoil Minerals

33. PotassiumRelease fromSoil Minerals
33
Interconversion of soil minerals: A mechanism of K release and fixation
after McLean, 1978

34. Thereleaseof K frommicasproceedsby
34
The transformation
of K-bearing micas
to expansible 2 : 1
layer silicates by
exchanging the K
With hydrated
cations,
The dissolution of
the micas followed
by the formation
of weathering
products.
(Sparks and Huang, 1985; Sparks, 2000).
Potassium Release from SoilMinerals

35.  The low hydration energy of K ion favours its
entrapment
 low concentration of K in soil solution due to
leaching or crop removal favours release of K
 In the absence of external additions of K, plants are
capable of taking up a very large amount of
potassium without bringing about substantial
decrease in exchangeable K
 This means that K which was not initially in
exchangeable form, has changed into exchangeable
form and becomes available to plant.
 The 2:1 type of clay minerals are capable of both
fixing and releasing potassium

36.  The activity of K+ ions in soil solution around mica particles greatly influences the
release of K from micas by cation exchange.
 When the K level is less than the critical value, K is replaced from the interlayer by
other cations from the solution.
 On the contrary, when the K level is greater than the critical value, the mica
expansible 2 : 1 mineral takes K from the solution.
 The critical K level is highly mineral dependent, being much higher for the
trioctahedral minerals (Scott and Smith, 1967; Newman, 1969; von Reichenbach, 1973;
Henderson et al., 1976).
 The critical levels for muscovite are so low that even the K impurities in laboratory
chemicals or dissolved from glassware are often sufficient to prevent any K release
(Scott and Smith,1967).
36
PotassiumRelease fromSoil Minerals

37.  Biological activity promotes K release from micas (Mortland et al., 1956; Boyle et al., 1967; Weed et al., 1969;
Sawhney and Voight, 1969).
 The organisms deplete the K in the soil solution, and their action may be analogous to that of
tetraphenylboron (TPB) in artificial weathering of micas.
 Furthermore, the overall action of organisms is more complex when organic acids are produced
(Boyle et al., 1967; Spyridakis et al., 1967; Sawhney and Voight, 1969).
 The importance of organic acids in weathering of rock-forming minerals has been recognized for
a long time (Sprengel, 1826; Bolton, 1882; Huang and Keller, 1970).
 The influence of oxalic and citric acids on the dynamics of K release from micas and feldspars
was studied by Song and Huang (1988).
 They found that the sequence of K release from K-bearing minerals by oxalic and citric acids is
biotite > microcline > orthoclase > muscovite.
37
PotassiumRelease fromSoil Minerals: biologicalactivity

38. Losses of potassium
 1. Luxury Consumption: Some crops tend to absorb K far in
excess of their needs if it is present in sufficiently large quantities
in the soil. This tendency is termed ‘ luxury consumption’ because
the excess K absorbed does not increase crop yields to any
appreciable extent. Wasteful luxury consumption mostly occur in
forage crops
 2. Leaching losses of K: leaching losses occur mainly in sandy
soils, organic soils and kaolinite dominant soils.
 3. Soil Erosion: it leads to considerable loss of total potassium
from the soil. The erosion losses of K are serious and generally
exceed those of any other major nutrient element.

39. Management of potassium fertilizers under field condition
 Determining potassium fertilization need (Soil Test)
 Choosing application rates
 Types of K fertilizer
 Application methods

40. Management of potassium fertilizers under field
condition
 Soil Test for Potassium
40

41.  India has no potash-rich soluble minerals and
incrustations (mineral layer)
 All K fertilizers is imported

42.  Muriate of potash (MOP) is cheaper than sulphate of potash
(SOP) since it is the raw material from which SOP is
manufactured
Crops sensitive to KCL Chloride loving crops
Tobacco, grapes, fruit trees , cotton,
sugarcane, potatoes, tomatoes, straw
berries, cucumber and onions
Oilpalm, and coconuts
Apply SOP as a K fertilizer
Perform well with application
of MOP
• Potassium nitrate is a preferred fertilizer for spraying on fruit
trees and horticultural crops
• Recent studies shown that schoenite (double salt of potassium
and magnesium) is as good source of K as MOP for groundnut,
banaba, rice, wheat and maize

43. Indigenous sources of
potassium
 Wood ash
 Manure
 Crop residue
 Distillery and coir industry waste
 Cement kiln dust
 Etc.
Assignment: Different sources of K and their K content (minerals, fertilizers
Organic and other

44. Methods of potassium application
 Broadcasting and mixing with surface soil
 Band placement is recommended in soils with low available K
and high K fixing capacity
 In some crop situations split application is emerging as an
alternative to basal application
 E.g i) rice grown in light textured soils and acid soils in high
rainfall areas in order to reduce leaching losses
 Ii) low tillering and late maturing varieties, where the natural
supply of K from soil plus irrigation water decreases in the later
stages of crop growth
 Iii) in highly reduced soils where conditions may hinder K
uptake
 Iv) during the monsoon season