🥣 Mixed Fertilizers

15. Mixed fertilizers – sources – preparations- their compatibility – advantages

For over hundred years the mixed fertilizers are in use besides straight fertilizers.

Many fertilizer mixtures are made available now and this account for a major portion of

the consumption of N, P and K. Present day statistics show that numerous grades of

fertilizers mixtures are manufactured and the fertilizer mixing industry is being

considered as one of the major agro- industry.

The following are some of the common term frequently used in the mixed

fertilizer industry.

Fertilizer : The substance which is used for the supply of plant nutrients

Mixed fertilizer : A mixture of more than one straight fertilizer which can supply more

than one plant nutrient element

Complete fertilizer : A single fertilizer material containing the entire three major plant

nutrients viz, N, P and K

Fertilizer grade : This refers to the minimum guarantee with regard to the nutrient

content of the fertilizer mixture in terms of N, P and K.

Fertilizer formula : This related to the quantitative expression of the analysis of the

different ingredients included in the mixed fertilizer in terms of N, P and K.

Fertilizer ratio : This indicates the relative percentage of N, P2O5 and K2O in the

manure mixture.

Acidic fertilizer : Fertilizer capable of increasing the acidity of the soil by continued

applications.

Basic fertilizer : Fertilizers which increase the pH and the soil on continued use by

leaving a basic residue in the soil.

Neutral fertilizer : Materials which are neither increasing nor decreasing the pH of the

soil

Filler : It is called as the ‘make-weight’ material added to the fertilizer mixtures.

Filler materials are inert materials like sand, saw dust etc, are added to make up the

difference between weight of ingredients added to supply the plant nutrients in a tonne

and the final weight viz., 1,000 kg.

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Advantages

All the three major plant nutrients are made available in one and the same material.

There is saving of time and labour. The residual effects will not be there. The fertilizer

mixtures are usually prepared taking into account the acidic or alkaline nature of the

ingredients, and other chemical reactions. Hence, some of the residual effects like

acidity

will not be there. Usually mixed fertilizer are prepared to suit a group of crops and soils.

Disadvantages

• Specific needs of crops and deficiency of individual nutrient elements cannot be

satisfied by using mixed fertilizers as efficiently as in the case of straight fertilizers.

• The use of mixed fertilizer in such cases of specific needs will be a waste as other

nutrients are also added to the soil.

• Unit cost of the various nutrients contained in the mixed fertilizer will always be

higher when compared to the unity cost of nutrients contained in the straight

fertilizers.

Improper mixing and storage of fertilizers can result in large nutrient losses. Some

important aspects to consider in fertilizer mixing and storage include the following:

• Urea should not be mixed with ammonium calcium nitrate (CAN), KCl, SSP or

TSP.

• Urea can be mixed with most other fertilizers but fertilizer mixtures containing

urea should be applied immediately after mixing. Do not store fertilizer mixtures

containing urea.

• Ammonium phosphates and super phosphates should not be mixed with lime,

slag, rock phosphate or CAN.

• Potassium chloride and sulfate of potash can be mixed with most fertilizers, but

mixtures of these fertilizers with urea and calcium ammonium nitrate should not

be stored.

• CAN should not be mixed with basic slag but can be mixed with urea, single

superphosphate, and ammonium phosphates immediately prior to application.

Do not store fertilizers in damp or dirty places. Make sure that bags of fertilizer in the

store do not absorb moisture from leaky roofs or water seepage through walls and floors.

Preparation of mixed fertilizers

Many kinds of materials are used in the manufacture of fertilizer mixtures. The

materials are found to be highly varying in their properties. However, only a limited

number of materials are being used like (NH4)2SO4, CO(NH2)2, Super phosphate,

ammonium phosphate, muriate of potash, limestone, gypsum and some fillers.

The manufacture of fertilizer mixtures usually involves the weighing and

proportioning of ingredients that are used, sieving and sizing of the various ingredients,

mixing the different materials and packing. All the above operations are done both

mechanically and by hand operation. Different kinds of machineries are being used.

Guide for mixing

To determine the amount of individual fertilizer in a mixture, the quantity is

calculated as follows.

(Percentage of plant nutrient desired x (Weight of final mixture)

in the mixture)

Quantity =

(The Percentage of the plant nutrient in the straight fertilizer)

R x T

A =

P

R = Percentage in the mixture

T = Final weight of the mixture

P = Percentage in the straight fertilizer.

To prepare a mixture of 10: 5: 10 using (NH4)2SO4, (20% N), Super phosphate

(16% P2O5) and muriate of potash (60% K2O), the following quantities will be required

per tonne (1,000 kg).

10 x 1000

1. Ammonium sulphate = = 500 Kg.

5 x 1000

2. Super phosphate = = 312.5 Kg.

10 x 1000

3. Muriate of potash = = 166.5 Kg.

60 ___

Total = 979.0 Kg .

Filler = 1000 – 979 = 21 Kg.

If the total calculated weight exceeds the final weight, a mixture of that ratio can

not be prepared.

Changes that occur while manufacturing mixed fertilizer

The ingredients used in fertilizer mixtures vary widely in their physical and

chemical characteristics. When such widely varying materials in physical and chemical

properties are mixed together, naturally many changes are expected to take place during

or after the mixing. Some changes will be of physical nature and some will be of

chemical nature. The following are the most important physical changes that will take

place during or after the mixing of fertilizers.

Hygroscopicity

It is a property of any substance which absorbs from air and gets converted to

semi- solid or liquid condition. Fertilizer like Ca(NO3)2, NH4NO3, NaNO3 and CO(NH2)2

are capable of absorbing moisture from air and become hygroscopic. In such cases

handling will be very difficult for such mixtures.

Caking up

Moisture present in some of the ingredients is responsible for caking up.

Moisture dissolves some of the easily soluble ingredients and forms a saturated solution.

This saturated solution on evaporation gives out crystals which knit together forming

larger lumps.

The caking up can be prevented by the use of certain kind of materials called as

‘conditioners’. The commonly used conditioners are groundnut hulls, lime, clay etc.

Another was of preventing the caking up is manufacture of granulated fertilizer mixture.

The granulation aims at preparation of uniform sized particles with reasonable stability,

which presents caking up.

Segregation

This relates to separation of different sized particles individually. When

ingredients of different sizes and densities are included there will be the tendency for the

segregation (sorting out to different sizes) to take place. To prevent this bad effect,

granulation is conveniently followed.

The following are some of the most important chemical changes that take place

either during or after the manufacture of fertilizers mixtures. These changes are found to

be influenced by temperature, moisture content and particle size of the in gradients.

(a) Double decomposition

The reaction is between two compounds without a common ion in the

presence of moisture. New compounds are formed which may have different

physical and chemical properties.

(b) Neutralization

This reaction takes place when free acids present in some of the fertilizers

are neutralized by alkalis or Ca containing salts included in the mixture.

(c) Hydration

The process of tying up of water by the anhydrous form of salts is called

hydration. Some of the fertilizers are found to have this property.

(d) Decomposition

Under certain conditions of moisture and temperature, there will be break down

in the composition of molecules forming new compounds.

However, the following important principles must be taken into consideration while

preparing mixed fertilizers.

1. All fertilizers containing ammonia are not mixed directly with the basic

fertilizers (e.g. RP, limestone, basic slag, CaCN2) as reaction will take place

resulting in the loss of gaseous NH3.

2. The water – soluble phosphates are not mixed with those materials which

contain free lime (e.g) lime stone, CaCN2) as there will be reaction towards

the reversing of water – soluble phosphate to water – insoluble phosphates.

3. Hygroscopic fertilizers are not included as they will facilitate caking up.

4. The acidic fertilizers are likely to produce some free acids which may

damage the container or packing materials.

Manufacturing process

The principal steps in the manufacture of solid mixed fertilizers are calculating,

weighing, sieving, sizing, mixing the materials and packing the product.

Calculating and weighing

With a good weighing device the calculated quantities of the various

ingredients are weighed accurately for preparing mixtures.

(1) Seiving and sizing

If the raw materials have undergone too much of caking in the storage piles, it

may be necessary to subject them to preliminary grinding, sieving and sizing. The

raw materials must be converted into uniform sized particles to have effective making

and to avoid segregation during subsequent handling.

(2) Mixing

Many kinds of fertilizer mixing machines have been proposed and used at present.

Rotary drum type is found to be more common than the vertical cylinder type. Weighed

quantities of different materials are introduced into the mixer and mixed thoroughly.

(3) Packing

Equipments for packing fertilizer mixture range from the simplest types of hand operated

facilities to highly sophisticated automatic machines.

Granular mixtures

Compared to pulverized or powdered mixture, the advantages of granular

mixtures are reduced caking up tendency, less dusting loss and easy handling. Of late,

attention is found in the manufacture of only granular mixtures.

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Summary Cheat Sheet

Topic	Key exam point
Main meaning	Fertilizers prepared by physical mixing of two or more fertilizer materials
Main advantage	Flexible nutrient grade preparation according to crop need
Main limitation	Problems like segregation, caking, hygroscopicity, and incompatibility can occur
Preparation rule	Mixing should follow compatibility guides
Common physical issues	Hygroscopicity, caking, and segregation
Common chemical issue	Double decomposition or neutralization between components
Grade calculation	Fertilizer-grade calculations are a major exam area in mixed fertilizers
Comparison	Less uniform per granule than complex fertilizers
Handling note	Physical properties of ingredients strongly affect final product quality
Trap	Compatibility must be checked before mixing; not all fertilizers are safe or stable in mixture

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