Potassium

Potassium

• Give the forms of soil potassium (K) & show with a diagram the interrelationship among different forms of K. • Or, Classify soil K on the basis of its availability to plants & establish the relationship among different terms. The various forms of potassium (K) in soils can be classified on the basis of availability to plants: 1. relatively available form 2. slowly available form 3. Relatively unavailable form Although most of the soil-K is in the third form of these three forms, from an immediate practical standpoint the first two are of greater significance. These three forms are discussed below: 1. Readily available form: Only 1-2% of the total soil K is readily available. Available K exists in soils in two forms: a. in the soil solution & b. as exchangeable K adsorbed on the soil colloidal surfaces. Although most of this available K is in the exchangeable (approximately 90%), soil solution K is most readily absorbed by higher plants. Unfortunately, K in soil solution is subjected to considerable leaching loss. The two forms of readily available K are in dynamic equilibrium. When plants absorb soil K from the soil solution, exchangeable K immediately moves into the soil solution until the equilibrium is again established. When water-soluble fertilizers are added to the soil, the equilibrium reverses soil solution K moves onto the exchange complex. 2. Slowly available form: In the presence of vermiculite, smectite and other 2:1 type minerals, the K+ ions (as well as NH4+ ions) in the soil solution (or added as fertilizers) not only become adsorbed, but also may become definitely ‘fixed’ by the soil colloids. The K+ ions are just the right size to fit between

layers in the crystals of these normally expanding clays & become an integral part of the crystal. Those ions cannot be replaced or ordinary exchange methods & consequently are referred to as non exchangeable ions. As such, the ions are no longer readily available to higher plants. This is in equilibrium with the more available forms and consequently acts as an extremely important reservoir of slowly available nutrients. 3. Relatively unavailable forms: 90-98% of all soil-K in a mineral soil is in relatively unavailable forms. The compounds containing most of this form of k are the feldspars and micas. These minerals are quite resistant to weathering & supply relatively small quantities of K during a given growing season. However, their cumulative release of K over a period of years undoubtedly is of some importance. This release is enhanced by the solvent action of carbonic acid & of stronger organic & inorganic acids as well as by the presence of acid clays & humus. The relationship among different forms of K is shown diagrammatically in the following figure---Relatively unavailable K (feldspars, micas, etc.) 90-98% of total K

Slowly available K (non exchangeable = fixed 1-10% of total K

Readily available K (exchangeable & in soil solution) 1-2% of total K.

90% Exchangeable K

Non exchangeable K soil solution

Fig: Relative properties of the total soil-K in unavailable, slowly available & readily available forms.

•

Briefly describe the fractionation procedure of soil K. Or, describe the procedures for the determination of different forms of K Or, how would you fractionate & characterize the soil-K Question:

Answer: it has been done from chemical point of view. SoilK is generally believed to exist in five forms/fractions/categories: Categories Extractant 1. water soluble Deionized water 2. readily exchangeable –K (Krex) 1N NH4OAC(pH 7.0) 3.Difficultly exchangeable-K (Kdex) 1N HNO3(hot) 4.Lattice-K (KL) 1N HCl (conc.) 5.Inert potassium (Ki) relatively HF acid + HClO4 acid digestion mixture Methods of extracting different categories K are as follows— 1. Water soluble-K K that can be extracted by Deionized water is called water soluble- K. the water soluble-K is designated by Kw. the K concentration in soil saturation extracts usually varies from 3 to 156 ppm & the higher figures are found in arid or saline soils. Concentration of water-soluble-K less than 8 ppm is considered deficient soil. In sandy soils, there is no clay to hold K+ and in such soils amount of Kw is high. Water soluble K in Bangladesh soil ranges between 5.07-93.28 ppm. This K is easily taken up by plants. Soil + water (Deionized) ------- Extract; Flame photometer is used for determination of K. 2. Readily exchangeable- K: (Krex) Exchangeable K+ is held around negatively charged soil colloids by electrostatic attraction. They are also held in the interlayer space of 2:1 type mineral.

Cations held in this manner are easily exchanged when the soil is extracted by the 1N NH4OAC at pH 7.0. Neutral ammonium acetate solution is the standard Extractant for exchangeable K in soils. When 1N NH4OAC is used, not only exchangeable K, but also water soluble K will come into solution. So, the readily exchangeable –K represents by extractable –K minus the water soluble-K. i.e. 1N NH4OAC extractable k-Kw readily exchangeable - K (Krex) Readily exchangeable K in Bangladesh soil ranges between 8-189 ppm. This exchangeable K is available for plants. 3. Difficultly exchangeable- K: Difficultly exchangeable K is the fixed-K in the interlayer spaces of the clay minerals such as illite, vermiculite. Nonexchangeable or fixed K is often determined by extraction with a strong acid such as 1N boiling nitric acid (HNO3). Upon nitric acid extraction, the H+ of the HNO3 enters into the interlayer space & replaces the K+ by breaking down the layer & thus K is emerged from the interlayer space of the clay minerals. So, Non exchangeable K = 1N HNO3 extractable-K -1N NH4OAC extractable –K Difficultly exchangeable K in Bangladesh soil ranges between 119-2465ppm. This difficultly exchangeable K is not readily available to plants. 4. Lattice-K (KL): K presents as a part of structure of the mineral is known as lattice K. potassium located at the interlayer throughout the mineral lattice are feldspar. These lattice- K is extracted by 1NHCl. So, Lattice-K = 1N HCl (conc.) extractable-K- 1N HNO3 (Hot) extractableK In Bangladesh soil, lattice-K ranges between 330-2776ppm. 5. Relatively inert potassium (Ki): The rest of the K i.e. the inert K, in soil can be extracted, when it’s digested by the mixture of HF & HClO4 acid. It is also called HF acid-HClO4 acid digestible K. this type of K in Bangladesh ranges between 9,200-28,600 ppm. Total K present in soil can be extracted by this method. Question Describe the mechanism of K-fixation in soil. Answer: •

Fixation of K in soil may be defined as the process whereby readily soluble K is changed to less soluble forms by reaction with inorganic or organic components of the soil, with the result that the K become restricted in their mobility in the soil & suffer a decrease in their availability to the plant. K fixation does not occur to the same extent in all soils or under all conditions. It is maximum in soil high in 2:1 clays & with large amount of illite. K fixation may result due toa. Substitution of K+ for interlayer cations & b. Hexagonal arrangement of oxygen atoms on exposed surfaces between the sheets of 2:1 lattice type minerals (Hole theory). a. Substitution of K+ for interlayer cations: The interlayer K fixation capacity of soils is related to the presence of 2:1 type clay minerals. Each unit cell of these minerals consists of an alumina octahedral sheet sandwiched between two silica tetrahedral sheets. As a result of isomorphous substitution of Al3+ Si4+ by cations of lower valence during crystallization; the lattice obtains an excess negative charge, which will be balanced by other cations e.g. Ca2+, Mg2+, K+, Na+, H3O+, either inside the crystal or outside the crystal unit. During expansion, K+ containing water pass through the interlayer space & during concentration K+ become fixed. Si Si Al

Al Si

K+

Si

Ca+2, Na+, Mg2+

K+ K+ K+ K+

Si Si Al

Al Si

Si Fig: Substitution of K between interlayer space. +

b. Hole theory: According to ‘lattice hole theory’ of page & Baver (1940), cation fixation is related to the size of the cation as well as to the kind of fixing material. The generally accepted idea is that the exposed

surface & surfaces between sheets of three layer (2:1) type minerals consist of oxygen ions, arranged hexagonally. The opening within the hexagon is equal to the diameter of an oxygen ion (approximately 2.8 Aº). Ions having a diameter of this magnitude (e.g. K+) will fit snugly into the lattice holes & such ions will be held very tightly as they come in contact with the negative electrical charges within the crystal. Owing to this, the layers are bound together, thus preventing dehydration & re-expansion of the lattice.

B Exchangeable

M

m

B Fig: Schematic Pictures of

the structure of vermiculite. (Expanded vermiculite lattice with Mg2+ ions (hydrated) in the interlayer position.

fixed B

M

m

B

Fig: Schematic Pictures of the structure of

vermiculite (Collapsed lattice of vermiculite after K+ Saturation.) Here, B= Brucite layer Cations larger than 2.8 Aº cannot enter the cavities.

Question: Describe the factors controlling the fixation of K in soils. Or, briefly describe the roles of different factors on K-fixation in soil. Answer: There are several factors that affect the K fixation in soils. These are1. Content & types of clay minerals 2. Soil reaction (pH). 3. Concentration of added K 4. Concentration of added NH4+ 5. Wetting and drying 6. Freezing and thawing 7. Time 8. Moisture 9. Temperature 10.The presence of excess lime 11.Particle size These factors are briefly described below: 1. Content & types of clay minerals: The ability of the various soil colloids to fix K varies widely. Kaolinite and other 1:1 type clays fix very little amount of K. on the other hand clays of the 2:1 type such as vermiculite, fine grained mica (illite), & smectite fix K very readily & in large quantities. Even silt sized fractions of some micaceous minerals fix & subsequently releases K. K+ ions are attracted between layers in the negatively charged clay crystals. The tendency for fixation is greatest in minerals where the major source of negative charge is in the silica tetrahedral sheet. Consequently vermiculite has a greater fixing capacity than montmorillonite.

2. Soil Reaction (pH): Under acid conditions, the fixation of K+ in soil is low due to the presence of Al3+, aluminium-hydroxy cations (e.g. Al(OH)+2 & their polymers, because these cations occupy the exchange sites of the colloid. When acid soils are limed, exchangeable Al3+ & hydroxy aluminium cations such s Al (OH)2+ are converted to insoluble Al(OH)3. This change removes the Al3+ from cation exchange competition with K+, & it frees blocked binding sites so that K+ can compete with Ca2+ with Ca2+ for them. As a consequence, much greater amounts of K+ can be held by clay colloids. So, it can be said that K+ fixation increases with the increase of pH which is shown in the figure. 30 (K fixation(%) 24

18 12

3

4

5

6

7

8

9

10

pH

Fig: The effect of pH on the fixation of K in soil. 3. Concentration of added K+:

The amount of K+ fixed by soil increases with the rate of application of K-fertilizers or other sources. However, the percentage of the added K+ that is fixed, decreases with an increase in the amount applied. In one time, fixation does not occur when all negative charges are filled with K+ ions.

4. Concentration of added NH4+:

Concentration of NH4+ controls the K-fixation. As NH4+ and K+ have the nearly same ionic radius, NH4+ may fix in the interlayer space, where usually K+ is fixes. So with the increasing concentration of NH4+, the fixation of K+ decreases. 5. Alternate wetting & drying: Air drying of some soils high in exchangeable k can results in fixation & a decrease in exchangeable K in contrast, drying of field-moist soils low in exchangeable K, particularly subsoils will frequently increase exchangeable K. The release of K upon drying is thought to be caused by cracking of the clay edges & exposure of interlayer K, which can then be released to exchange sites. 6. Freezing & thawing: The freezing & thawing of moist soils may also be important in K release & fixation. With an alternate freezing & thawing certain soil release K, while in other soils, particularly those high in exchangeable K, no K release is observed. This phenomenon was observed in soils that contained appreciable quantities of illites. 7. pH: K+ fixation increases with the increase of soil pH. In strong acid soils, the tightly held Al3+, H+ and hydroxy aluminium ions prevent K+ ions from being closely associated with the colloidal surfaces, which reduce their susceptibility to fixation, as the pH increases, the H+ & hydroxyl alumina ions are removed or neutralized, and it is easier for potassium ions to closer to the colloidal surfaces, where they are more susceptive to fixation 2:1 clays.

30 (K fixation (%) 24

18 12

3

4

5

6

7

8

9

10

pH

Fig: The effect of pH on the fixation of K in soil.

8. Presence of excess lime: Applications of lime sometimes results an increase in Kfixation of soils. Liming increases the C.E.C, this results in an increased K+ adsorption by the soil colloids and a decrease in K level in the soil solution. Furthermore, high Ca levels in the soil solution may reduce K uptake by the plant. Finally, K deficiency has been noted in with excess CaCO3. K-fixation may be responsible for the adverse effects on Ca soils. 9. Particle size: If particles become finer, fixation capacity decreases adsorption capacity increases, because of the breakdown of sheet structure. 10. Moisture: In drying condition, soil solution becomes concentrated. Then ions gathered/heaped together on colloids, i.e., the ions become closer to the clay particles. In more dry condition, those ions will be locked or fixed. So, in low moisture content of the soil, i.e., in dry condition, the K-fixation rate increases. 11. Time: K- Fixation increases with time.

• Question: Discuss the quantity-intensity relationship to explain K-availability in soils. Answer: The intensity factor is measure of the K that is immediately available to the root- the K in the soil solution. Since the absorption of the K+ ion by plant roots is affected by the activity in the soil solution of

other cations, particularly of Ca & Mg. some authors prefer to use the ratio K . 2+ 2+ √ [Ca ] + [Mg ] rather than the K concentration alone to indicate the intensity factor. The quantity factor is a measure of the capacity of the soil to maintain the level of K in the soil solution over the period of time the crop is being produced. This capacity is due to mainly to the exchangeable K, although some non-exchangeable forms release sufficient K slowly during a growing season to provide a notable portion of the crop needs. The main point to remember however that is the quantity factor designates the K sources capable of helping to replenish the soil solution (intensity) level of K. The concept of the buffer capacity of the soil, which indicates how the K level in the soil solution (intensity) varies with the amount of labile form of this element (quantity), is useful in explaining differences in K-supplying power of soils. Clay textured soils that are high in the quantity factor are well buffered, whereas soils that are sandy are apt to be poorly buffered.

• Question: What are the sources of K: Answer: The minerals considered to be original sources of K. the K release from these minerals by weathering which depends upon the properties of that minerals & environment. The K-bearing minerals arei. K-feldspar ( Orthoclase & microcline) KAlSi3O8 ii. Muscovite KAl3Si3O10(OH)2 iii. Biotite K(Mg.Fe)3 AlSi3O10(OH)2 iv. Phlogopite KMgAl2Si3O10(OH)2 K is also found in secondary or clay minerals in the soil. These are i. illite or hydrated micas ii. vermiculite iii. Chlorites & v. Inter-stratified minerals, in which two or more of preceding types occur in a more or less random arrangement in the same particles.

At any one time, most of the K is in primary minerals or in the non-exchangeable or fixed form. Chemical fertilizes are also important sources of K. *Effects of K on plant growth * The K-problem in soil fertility: 1. Availability of potassium 2. Leaching losses 3. Plant uptake & removal 4. Luxury consumption * Practical aspects of potassium management: 1. Frequency of application 2. Potassium-supplying power of soils 3. Potassium losses & gains 4. Increased use of K-fertilizers. *Potassium losses & gains: Plant removal of K generally exceeds that of the other essential elements, with the possible exception of nitrogen. Annual losses from plant removal as great as 400 Kg/ha or more of K are not uncommon, particularly if the plant is legume and is cut several times for hay. As might be expected, therefore, the return of plant residues and manures is very important in maintaining soil potassium. The annual losses of available K by leaching and erosion greatly exceed those of N & P. They are generally not as great, however, as the corresponding losses of available Ca & Mg. Such losses of soil minerals have serious implications for sustainable soil productivity. Commercial Fertilizers Plant residues & Animal manures

slowly available potassium minerals

Available Soil K

Plant removal

Leaching

Erosion

Fixation

Losses

losses

Fig: Gains & losses of available soil potassium under average field conditions. The approximate magnitude is represented by the width of the arrows. *Factors affecting K availability: Soil Factors: i. Kinds of clay minerals ii. C.E.C iii. Amount of exchangeable K iv. Capacity to fix K v. Subsoil K and rooting depth vi. Soil moisture vii. Soil temperature viii. Soil aeration ix. Soil pH x. Ca & Mg xi. Relative amounts of other nutrients xii. Loss of K by leaching Plant factors: i. C.E.C of roots ii. Root system & crop iii. Variety of hybrid iv. Plant population & spacing v. Yield level vi. Time factor • NH4 ion reduces K-fixation –Explain: K+ ions are sufficiently small to enter the silica sheets, where they are held very firmly by electrostatic forces. The NH4 ion has nearly the same ionic radius as the K+ ion. As a result, NH4+ ion will compete with K+ for fixation position. It has been found that the effect may vary depending on whether NH4+ is added simultaneously, prior to or after the addition of K+. The NH4+ prior to K+ will depress fixation of K+, because NH4+ can be fixed by clays in a manner similar that of K+. Its presence will alter both the fixation of added K and the release of fixed potassium. The presence of NH4+ can block the release of fixed K. the NH4+ ions, evidently are held in the interlayer positions further trapping the K+ ions already present. The reduction of K+ fixation in relation to the levels of NH4+ is shown below.

NH4+

K+ Figure: Reduction in k+ in relation to levels of NH4+ previously fixed.