3 Main Forms of Potassium in Soils

This article throws light upon the three main forms of potassium in soils. The forms are: 1. Soil Solution Potassium 2. Exchangeable Potassium 3. Non-Exchangeable and Mineral Form of Potassium.

Form # 1. Soil Solution Potassium:

It is recognised as the readily available form of potassium to the plants. The potassium availability in soils is controlled not only by the soil solution potassium but also by its buffering capacity (ability of a soil to maintain potassium intensity). The soil solution potassium (intensity, I) is maintained by the exchangeable potassium (quantity, Q) in a dynamic equilibrium.

The higher dQ/dl or the higher potassium buffer capacity indicates that during active period of crop growth, the potassium concentration in the soil solution will be depleted very rapidly. Soil solution potassium content usually higher in arid region and saline soil ranging from 3 to 156 ppm whereas the content of the same is lower in humid region soils ranging from 1 to 80 ppm.

Concentration of water soluble potassium may be as low as 8 ppm in deficient soils. However, under actual field conditions, the potassium concentration of soil solution varies with concentration and dilution processes brought about by evaporation and rainfall respectively.

The potentiality of soil solution K for plant growth and nutrition is influenced by the presence of other cations like Ca, Mg, and Al in acid soils and Na in salt affected soils. The activity ratio of potassium at equilibrium (AR_e^K) with respect to these above cations is a measure of the “intensity” of labile potassium in the soil indicating instantly available to plant roots.

Soils having same AR_e^K values may have different capacity in maintaining AR_e^K during depletion of K by crop uptake or leaching and hence for the K status of soils it is necessary to specify not only the status of potassium in the labile pool but also the way in which the intensity depends on the amount (quantity) of labile potassium present.

However, the detail discussion about the Q/I relationship of K is presented in the following section.

Buffer capacity indicates how intensity varies with quantity. A simple relationship between K intensity and K quantity for two soils (Soil X and Soil Y) having differential K adsorbing capacity is being depicted in Fig. 21.9. From the figure it is found that for both soils increasing intensity is accompanied by an increase in quantity.

Soil X, however, shows a steeper rise in the slope than that of soil Y. Where an equal amount of K is removed from both soils by plants a similar decrease in the quantity of (∆Q) takes place. The consequent decrease in intensity (∆l), however, varies greatly for both soils (∆Ix and ∆I_Y).

This example shows that the two soils differ in their capacity of replenishing the soil solution with K. Soil X is better able to maintain the K concentration in the soil solution. Soil X is, therefore, more buffered than soil Y.

In quantitative terms the buffer capacity is expressed as the ratio ∆Q/∆I as follows:

B_K = ∆Q/∆I,

where B_K = buffer capacity of K in soils

The higher the ratio of ∆Q/∆I, the more the soil is buffered. Usually, the rate of K uptake by plant roots is higher than the diffusive flux of K towards the roots. The K concentration at the root surface may decrease during the period of plant uptake.

Such decrease in K concentration is dependent on the K buffering capacity of the soil. If the buffer capacity is high, the decrease may be low because of efficient K replenishment of the soil solution.

Again, for spoils having poor K buffer capacity, the concentration of K at the root surface may decrease appreciably throughout the plant growth period. For optimum growth of the plant, the concentration of nutrients in soil solution should be maintained above a certain level.

This concentration is termed as the critical nutrient concentration (CNC) below which the yield of crop is decreased. The critical level of K in the bulk soil solution is related to the buffer capacity of K (buffer power). The critical concentration is higher, the lower K buffer capacity.

In addition to AR_e^K in assessing soil solution K, electro-ultra filtration (EUF) technique, a process of combination of electro dialysis and ultrafiltration, is used most satisfactorily for the characterisation of soil solution K (intensity) particularly in upland soils. The principle of EUF technique consists of utilizing the acceleration imposed upon ions by an electrical field for the separation of ions from soil colloids.

By adequate variation of voltage (50, 200 or 400 V) and timing (0-35 minutes), the total extractable K or any other nutrients can be separated into their water soluble and exchangeable forms with varying bonding energies.

Extraction and fractionation is done automatically. For the EUF-K fraction I (potassium in the extract obtained after 10 min of EUF—the first 5 min at 50 V and the next 5 min at 200V) and total EUF-K fractions (sum of all potassium fractions obtained after 35 min of EUF— the first 5 min at 50 V, the next 25 min at 200 V, and the last 5 min at 400 V).

The EUF-K fraction I considered as soil solution K (intensity factor) whiles the total EUF-K fractions as total amount of effectively available K (quantity factor). This electro- ultra filtration technique is better suited than that of AR_e^K in distinguishing soils of varying K availabilities.

The amount of K in the soil solution is very low to meet the demand of the crop throughout the growing period and therefore it is necessary for satisfactory potassium nutrition of crops the soil solution K must be continuously replenished from the exchangeable, non-exchangeable and mineral forms of K.

Form # 2. Exchangeable Potassium:

Potassium ion (K⁺) is held by soil colloids through electrostatic attraction similar to other cations. However, potassium held by soil colloids is easily displaced or exchanged when extracting the soil with neutral salt solutions. The amount of K exchanged varies with cations and usually neutral normal ammonium acetate solution is used for the purpose.

A small amount of potassium in this fraction occurs in soils (<1.0% of the total potassium). The distribution of potassium on soil colloids as well as soil solution depends upon nature and amounts of complementary cations, anion concentration and nature and characteristics of clay minerals.

As for an example, if a soil colloid is saturated with potassium and in that condition a neutral salt like calcium sulphate is applied then the following exchange reaction takes place:

Besides, if a soil is saturated with Al and Ca and in that conditions the application of muriate of potash gives the following exchange reaction:

When muriate of potash is applied to soils containing adsorbed calcium and aluminium, calcium is more easily replaced than aluminium by potassium. Coarse textured sandy soils having a greater base saturation lose very little of their exchangeable potassium by leaching as compared to soils containing low basic cations.

Liming is considered as the most common method of increasing the base saturation of soils which results the decrease in the loss of exchangeable potassium.

Sites for K Exchange:

It is evident that the exchangeable potassium on soil colloids is not homogeneous. Usually potassium is held at three binding sites of soil colloids namely p-(planar) position (outer surface of colloids, non-specific), e-(edge) position and i-(inner or inter layer) position (specific for K).

The amount of K held on p-position is in equilibrium with the soil solution K, while the amount of soil solution K in equilibrium with K held on e and i positions of soil colloids is low. However, under actual field situations, potassium concentrations in the soil pollution are probably the net result of three possible equilibria.

It is evident that the exchangeable form of potassium plays an important role in replenishing soil solution potassium removed by either intensive cropping or leaching losses.

In view of the above fact, it is very much essential to establish the quantity relationship between exchangeable K (Q quantity) and the activity of potassium in the soil solution (I intensity) in order to assess the availability of more labile potassium in soils to plants (Fig. 21.10).

The Q/I concept has been developed by Beckett which is used for predicting the status of potassium in soils. Different parameters of the above curve have some practical implications in relation to potassium in soils and plants.

∆K = Amount through which the soil gains or loses potassium in bringing equilib­rium (Q, quantity factor).

AR^K = Activity ratio of potassium (I, intensity factor).

AR_e^K = Activity ratio of potassium at equilibrium

∆K_ex = Exchangeable or labile pool of potassium

K_SP = Specific sites for potassium

PBC^K = Potential buffering capacity

AR^K (Intensity Factor, I):

It is calculated from the determined concentration of calcium, magnesium, potassium and sodium correcting to the appropriate activities with the help of extended Debye-Huckel theory.

AR_e^K (Activity Ratio of K at Equilibrium):

It is a measure of availability or intensity of labile pool of potassium in soil and can be modified by potassium fertilization, being increased due to application of K fertilizers. However, the availability of potassium in soils can either be increased or decreased due to liming which modifies the AR_e^K values either favorably or adversely.

∆K_ex:

It is used more successfully for the estimation of labile soil potassium held in plannar (p) positions. Greater values of labile potassium i.e. more negative (-K_ex) indicate a higher potassium release into the soil solution which results greater amount of potassium in the labile pool. However

the application of potassic fertilizers and lime in the cropped field have been found to be increased the amount of potassium in the labile pool.

K_sp (Specific Sites for Potassium):

It is a curved portion of the Q/I relationship while the linear portion of the curve (Q/I) is attributed to non-specific sites for potassium. Specific sites having high affinity for potassium are believed to exist on edges of clay minerals (e-positions) and in interlayer or wedge zones of weathered micas (i-positions).

Whereas non-specific sites for potassium are associated with planar surfaces of clay minerals (p-positions). The i-position has the greatest specificity for K⁺ which largely account for K⁺ fixation in soils.

PBC^K (Potential Buffering Capacity of Potassium):

It is a measure of ability of a soil to maintain potassium concentration in the soil solution. The potential buffering capacity for potassium is proportional to the cation exchange capacity (CEC) of the soil that means, with an increase in CEC the value of PBC^K increases and vice-versa resulting from changes in AR^K values.

A high PBC^K value indicates a good potassium supplying power of soils whereas a low PBC^K value signifies very low potassium supplying power of soils indicating frequent potassium fertilization. However, the higher PBC^K value may be obtained due to time application which probably as a result of increase in pH-dependent cation-exchange capacity.

If PBC^K is low, small changes in exchangeable potassium produce large differences of potassium content in the soil solution. This value is very small coarse textured sandy soils where mainly organic matter is contributed to the CEC value. In such soils, extensive leaching, rapid plant growth etc. deplete available potassium within a few days.

In general, the relation between exchangeable and soil solution potassium is a good measure of the availability of the labile pool of potassium in soils to plants. The ability of a soil to maintain the activity ratio of K against depletion by crop uptake, leaching etc. is controlled by nature of the labile pool potassium as well as the rate of release of fixed potassium and diffusion and transport of K⁺ ions in the soil solution.

Form # 3. Non-Exchangeable and Mineral Form of Potassium:

Potassium in these forms is not readily available to the plants. However, non-exchangeable potassium pools not instantly available to plants, can contribute significantly to the maintenance of the labile pool of potassium in the soil. On the other hand, in some soils these fractions of potassium may become available as water-soluble and exchangeable forms are removed by leaching, crop uptake etc.

These forms of potassium are consisting of different K-bearing minerals namely primary minerals (K-feldspars) and micas (muscovites, biotites etc.), originating from the parent rock and secondary minerals (clays of the illitic group) formed by alteration of micas.

The main source of K⁺ for plants growing under natural conditions is from the weathering of K containing minerals mentioned above. In potash feldspars, potassium occurs in the interstices of the Si, Al—O framework of the crystal lattice and held rigidly by covalent bonds. The weathering of feldspars starts at the surface of the particle.

Initially potassium is released by water and weak acids at a more rapid rate, However, with the progress of weathering, a Si—Al—O residue envelope is formed surrounding the un-weathered core. This layer reduces the rate of potassium loss from the mineral and hence protects K from further degradation.

Minerals of the mica type and also the secondary minerals of 2: 1 layer silicates vary in structure from feldspars and thereby these minerals also differ in their properties of releasing and binding potassium.

The micas consist of unit layers each containing two Si, Al—O tetrahedral sheets between which is an M (Al, Fe, Mg)—O, OH octahedral sheet potassium (K⁺) ions occupy the approximately hexagonal spaces between the unit layers and as a result the distance between unit layers is relatively small i.e. 1.0 nm in micas.

The replacement of un-hydrated interlayer K⁺ by hydrated cations like Na⁺, Ca²⁺ or Mg²⁺ expands the mineral with an increase in the distance between the unit layers i.e., 1.4 nm in vermiculite (Fig. 21.11 and 21.12).

Usually K⁺ of the lattice is vulnerable to weathering and can diffuse out of the mineral in exchange for other cations. High H⁺ concentrations and low K⁺ concentrations in the soil favour the net release of non-exchangeable, inter layer K⁺.

This K⁺ release may be an exchange process associated with diffusion in which K⁺ adsorbed to i-positions of the inter layer zone is replaced by other large cations like Na⁺, Ca²⁺ and Mg²⁺ resulting an expansion of clay lattice and the formation of wedge zones (See in above figure).

“Frayed edge” or “wedge” zone formation is typical of weathering micas which results release of interlayer K⁺. The rate of release of K⁺ by weathering not only depends on the K content of the mineral, but also affected by structural variation between minerals.

The gradual release of potassium from positions of mica lattice results in the formation of illite (hydrous mica) and eventually vermiculite with accompanying gain of water or H₃O⁺ and swelling of the lattice (Fig. 21.13).

There is also an increase in specific surface charge and CEC of clay minerals formed during the weathering of K containing minerals as well as transformation of mica. However, the applied soluble potassic fertilizers are converted to fixed or non- exchangeable forms of K and such conversions are affected by various factors. Besides fixation, the applied potassic fertilizers also undergo leaching loss from soils.

It is evident that a substantial amount of potassium can be lost through leaching in soils containing more amounts of sands due to flooding. However, in case of silty loam and clay loam soils, the loss of K through leaching is less because of fairly higher rate of adsorption of potassium by soil colloids. Again, in organic soils e.g. muck soils have high exchange capacities.

The bonding strength for cations like potassium is not great and the amount of exchangeable K tends to vary with the intensity of rainfall. Therefore, care should be taken for the supply of potassium to crops through its annual application.

Leaching losses can be reduced with the application of lime to the soil by maintaining a favourable pH level. Leaching losses of potassium frequently occurs in coarse textured sandy or organic soils particularly in areas of high rainfall.