🧂 Potassic Fertilizers

Potassic fertilizers supply essential potassium for crop growth, quality, and stress resistance; this lesson covers sources, properties, and practical use.

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Potassium found to occur very commonly in the combined form as naturally

occurring minerals. Common soil forming rocks like granite and gneiss are found to

have about 3% K. For many years, K has been considered as important plant nutrient

element. The Chinese were the pioneers among the various farmers who used K for

agricultural purposes. The mineral deposits were not discovered and used as fertilizers

until eighteenth century. The first product to be used as K manure was the wood ash

and the practice of using wood ash existed for centuries.

In olden days, the wood ash was primarily used to get the potassium salts. The

wood was burnt and the resultant ash was leached in mud pots to get the salt mixture

containing chlorides, SO4 and CO3 of K, Ca and Mg. (Pot + ash = Potash). It would be

advantageous and more correct to use P and K to refer to P and K, instead of P2O5 and

K2O in the fertilizer technology i.e.,

% K = % K2O x 0.83 and % of K2O = % K x 1.2)

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Source of potassium

The following are the main sources of K.

1. Mineral deposits 2. Industrial by products and 3. Synthetic K salts

Classification of potassic fertilizers

Potassic fertilizer are classified broadly into chlorides and non- chlorides. The

following are the most important K containing materials. Wood ash, tobacco stems, wool

waste, flue dust, by products from sugar and alcohol industry. Sea weed, sea water, K

minerals, processed fertilizer like muriate and SO4 of potash and synthetic fertilizers.

K Fertilizer and manure

Natural Processed Synthetic

1. Potassic minerals 1. Muriate of potash 1. KNO3

2. Wood ash 2. Sulphate of Potash 2. K HP O4

3. Sea weed

A. Organic K fertilizer Sources

1.Wood ash

Use of wood ash as K fertilizer has been there for centuries. Wood twigs and

branches are burnt to get the ash. Sawdust is also used. The burnt material will contain

varying amounts at K2O from 5 to 25%. Since wood ash is highly alkaline it should not

be applied to germinating crops.

2.Tobacco stem

In the manufacture of cigarette, cigar and chewing tobacco, there results

considerable wastes consisting of mainly the stems and midribs of the leaves. This

waste material is collected, ground and sold as manure. It Tobacco stem contains 4 to

10% K2O and 2 to 3% N. It is found to be a valuable conditioner and adds bulk to the

fertilizer mixtures.

3.Wool waste

Before the raw wool could be used for spinning, it must be treated for the

removal of ‘suint’ which is the dried sweat of sheep. This ‘suint’ consists largely of K

salts and contains 1.5 to 5.0 % K2O.

4.Flue dust

It is a by product from the steel and cement industry. For melting the iron a

mixture of iron are, coal, coke and limestone are introduced into a blast furnace. When

ignited, due to tremendous heat, the K contained in the iron ore volatilizes and when the

gases containing volatilized K are burnt, the K dust will settle in the flue through which

the gases pass and from time to time the dust accumulated will be collected and used.

This will have 3 to 18% K2O as Cl, SO4 and CO3 of K.

In the same way in cement manufacture also the raw materials used are found to

have appreciable amount of K and this volatilized during the cement manufacture. The

flue dust obtained from this is found to have 3 to 13% K2O. The dust can be used as a

straight fertilizer.

5.Sea weeds

This is referred to as the poor mans manure. It contains a high amount of K2O

varying with species of weeds. Out of many varieties ‘kelp’ is found to occur extensively

throughout the world. The Sea weeds ash is found to have more than 25% K2O.

Sea water

It contains can inexhaustible amount of K. It contains 0.04 % K, 0.14% Mg, 0.05

% Ca and 1.14 % Na.

B. Potassic minerals

There are as many as 50 minerals that are fond to contain K but all of them are

not economically important. The K containing minerals are roughly classified into

1. Minerals highly soluble in water occurring in brines or salt deposit

Sylvinite - KCl (Sylvite)

Carnallite _ KCl. MgCl2. 6 H2O

Langbeinite - K2SO4. MgSO4

Kainite - KCl. MgSO4. 3 H2O

2. Minerals less soluble in water.

Polyhalite - K2SO4. Mg SO4. 2 CaSO4 . 2 H2O

3. Minerals insoluble in water

Glauconite - KFeSi2O6. 2 H2O

Feldspars - orthoclase - KAlSi3O8

Micas - Muscovite and biotite.

The following are commercially used for the manufacture of potassic fertilizers.

1. Sylvinite - KCl - contains 63.1 % K2O

2. Carnallite _ KCl. MgCl2. 6 H2O - contains 17% K2O

3. Kainite - KCl. MgSO4. 3H2O - contains 18.9% K2O

4. Langbeinite - K2SO4. 2MgSO4 – contains 26.6% K2O

5. Polyhalite - K2SO4. MgSO4. 2CaSO4. 2H2O – contains 15% K2O

The deposits are found to occur at a depth of 650 to 2,500 feet below the earth’s

surface.

Processed K Fertilizers

Muriate of potash (KCl)

KCl is commercially manufactured using any one of the minerals; 1. Sylvinite or

sylvite and 2. Carnallite . There are two main steps involved in the manufacture of this

fertilizer.

1. Mining of the K mineral

2. Separation of the main ingredient and purifying.

Mining

The deposits are found to occur usually from 650 to 2,500 feet below the earth’s

surface. Many methods of mining are employed out of which the room and pillar method

of mining is found to be familiar. After a mine shaft is sunk to a point slightly below the

level of the ore body a large tunnel is driven on both sides of the shaft. At right angles to

this tunnel the ore is mined in larger rooms about 25 to 50 feet wide and 300 feet long.

The ore is brought over to the surface by electrically operating elevators and

machineries.

Purification

This involves the following process: 1. Grinding 2. Lixiviation or converting

the ore into solution form 3.Crystallization 4. Separation, drying etc.

Grinding

The potassic ore, after mining and bringing over to the factory site, is ground to

fine size and screened to get only the ore of required size.

Solutioning

The ore is dissolved in hot water and to that is added the NaCl solution (brine) for

obtaining a mixture of KCl + NaCl. The temperature of the mixture is increased by which

more and more of sylvinite will dissolve and the solubility of NaCl will stop after a stage.

This is continued till a saturated solution of KCl is obtained.

Crystallization

The saturated solution containing both KCl and NaCl is suddenly cooled in a

vacuum and the KCl separates out as crystals while NaCl remains in the solution.

Separation

Crystallized KCl can be separated out by mechanical means and it is dried,

powdered and used. The above process is called the crystallization method and the

principle involved is the differential solubility of KCl and NaCl in cold and hot solution.

When KCl is mixed with NaCl it is much more soluble in hot water than in cold water,

whereas NaCl is somewhat more soluble in cold water than in hot water. By increasing

the temperature, therefore, the solubility of KCl increases while that of NaCl remains

virtually constant.

Floatation method

Floatation is a metallurgical process in which small quantities of special reagents

are added to the pulp to coat or film some of the minerals selectively with respect to

other minerals present. When the reagent added pulp is agitated in a mechanical cell in

such a manner that air is drawn into the pulp to create a frothing action or condition, the

finely divided air bubbles attach themselves to the reagent filmed particles and float

them to the surface of the cell, where the froth rich with the selected mineral may be

skimmed off. Some are found to be very effective to NaCl that is present in sylvinite.

First, after grinding and passing through the screen, the desired sized sylvinite

mineral is dissolved in water to get a pulp and the reagent is added to the pulp. The

reagent will form a coating or film round the NaCl molecules and this reagent added pulp

is allowed to pass through a series of floatation cells in which air is introduced at the

bottom in such a manner as to form a small bubble froth that attaches to NaCl. The

commonly used reagents are (1) Sulphonated aliphatic alcohols of KCl and (2) 20% of

mono-n-octadecyl- amines neutralized with acetic acid and a 0.5% solution of this is

used for NaCl.

It is found that at least 90% of the potassic fertilizers consumed is in the form of

muriate of potash. It contains 48 to 62% K2O and 35 to 47% Cl. The commercial

product is either granular or powdery having white or red colour.

2.Potassium sulphate – K2SO4

This is manufactured from kainite ( KCl. Mg SO4, 3H2O) or langbeinite (K2 SO4,

Mg SO4), the latter being commonly used. The raw materials required are the mineral

langbeinite and KCl

The MgCl2 is rejected. The KCl is used in the form of a mother liquor obtained

from KCl manufacturing process.

The method is found to be efficient only in the presence of a proper ratio between

K2SO4 and MgSO4 of the raw material and to maintain a proper ratio, KCl or Kainite is

added. The reactions are found to take place in different ways

• 3 H2O

Langbeinite, ground to pass through a 200 mesh sieve and the mother liquor

from the KCl process is mixed and then the salts are recycled with water and fresh

quantity of langbeinite. The reactions are allowed to take place in interconnected tanks

and the crystallized K2SO4 is separated out by means of centrifuge. The used K2SO4 is

separated out by means of centrifuge. The wet K2SO4 is dried and conveyed to storage

place. It contains 48% K2O. Only limited quantities of K2SO4 are manufactured, as the

cost of manufacture is more, but is preferred for those crops where quality is desired or

where chlorine toxicity will result when K Cl is used.

3.Potassium Magnesium sulphate

In U.S.A. it is manufactured by mere washing of langbeinite (K2SO4. Mg SO4) to

remove the impurities and is sold under the name sulphate of potassium and

magnesium. It contains 18.5% MgO and 22.6% K2O. Since, the langbeinite is soluble in

water; the washing is done very easily. In Germany, it is prepared by dissolving MgSO4

and adding KCl to it.

Manure salts

The ground and partially processed potassium mineral is sold under the name

manure salt or mine run potassium salt. Various grades are available in different

counties; Carnalite of 9-12 % K2O and other salts of 12-15 % K2O, 18-22% K2O, 28-32 %

K2O, 38-42 % K2O are also marketed and used on a limited scale.

The muck, peat, sandy, lateritic and laterite soils are deficient in K and response

will be more in these soils for K fertilizers. Clayey soils are found to give poor response

for K fertilizers. Potassic fertilizers can be added to these soils in which heavy feeders

of K are grown such as tuber crops, coconut, arecanut, banana etc. The vegetables are

found to respond well to K application.

Reactions of potassic fertilizers in the soil

Application of any potassic fertilizer does not change the pH of the soil. When a

potassium salt is applied to a soil it is immediately dissolved and undergoes ionization. A

portion of K [+] ions remains in the soil solution, another portion gets adsorbed to clay

complex by CEC reactions and a third portion is converted into unavailable form.

Ca K

Ca Ca

Hence, in the soil the K will be in the solution, exchangeable and fixed forms.

The solution form is readily available and the exchangeable form is slowly and steadily

available to plants. The fixed K is unavailable but always in any soil there is a constant

equilibrium among these three forms and hence mostly exists no deficiency of K. A small

quantity of K is fixed by clays like illite, and this fixation of K depends upon the type of

clay, temperature, wetting and drying and soil pH. In this case the fixation is beneficial

as it reduces leaching loss and the availability of K will be there for a long time.

Factor affecting K fixation in soils

Nature of Colloids

The ability of various soil colloids to fix K varies widely. Kaolinite fixes

little, while montmorillonite and illite fix more.

Wetting and Drying

When soils containing exchangeable K are alternatively wetted and dried,

a large portion of exchangeable K is converted to the less available form.

This has been explained by assuming an entrapment of K ions between

contracting structural units when the soil dries some release of the ions

thus held occurs upon rewetting but the liberation is slow.

Influence of lime

Soils with high degree of base saturation lose less of their exchangeable

K by leaching than the soils with a low degree of base saturation. Liming

is the common means by which the base saturation of soils is increased

and so liming decreased the loss of exchangeable K. The K ions are

more readily replaced by calcium.

Application

The K fertilizers are usually applied before the crop is planted or sown, that is the

basal dressing. Since the applied K is not lost from the soil either by leaching or by

fixation, K fertilizers can be applied well in advance but it is preferable to apply only just

before planting. Top dressing with K fertilizers can also be done for vegetables and

other crop.

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Summary Cheat Sheet

Topic	Key exam point
Main category	Potassic fertilizers
Major commercial examples	MOP, SOP, and potassium nitrate
MOP meaning	Muriate of potash
SOP meaning	Sulphate of potash
Organic K sources	Wood ash, tobacco stem, wool waste, flue dust, seaweeds
Classification basis	Fertilizers may be grouped by source and chloride/sulphate nature
Agronomic role	Potassium is important in water regulation, quality, and stress tolerance
Crop sensitivity note	SOP is preferred where chloride-sensitive crops are involved
Exam distinction	MOP and SOP differ in accompanying anion, not in the nutrient element K itself
Trap	Do not treat all potassic fertilizers as interchangeable for chloride-sensitive crops

References

3 sources • [1] [2] [3]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook

Book

[3]

Department of Agriculture & Farmers Welfare

Website