Introduction:

Living things (such as plants and animal) need food for their growth and development. The growth and harvest of food and fiber crops is necessary to the survival and well-being of humankind. World population is on the increase and as such there is the challenge to raise the crop production. In order to meet up this challenge, some means has been used amongst which are the introduction of irrigation, enhancement of plant strains and introduction of or increase in the usage of fertilizer. 

For efficient productivity of plants, a few elements have been considered necessarily. The primary and secondary nutrient elements are collectively termed as major plant nutrients whereas the nutrients which are employed by field crops in very small quantities are termed as micronutrients or minor plant nutrients. Amongst such nutrients Nitrogen, Phosphorus and potassium are the most significant element and are thus employed in the production of fertilizers.

Fertilizers:

Fertilizers are those substances that should be added to the soil in order to take away the deficiency of necessary elements needed for the growth of plant. Why the addition of fertilizers? Whenever the soil is repeatedly cultivated, there comes a phase where the soil becomes less productive therefore the need for some elements in the form of their compounds to be added to the soil to improve its productivity. The requirement of fertilizing a land can be concluded as follows:

a) To supplement what has been eaten up via the plants.

b) To supply them an additional tonic and good food, in such a way that they might grow healthier and produce a better result.

c) To keep the pH of the soil in the vicinity of 7 to 8 and thus facilitate optimum growth and health. 

Not each and every compound having nitrogen or phosphorus and so on can be employed as a fertilizer. For a compound to be employed as a fertilizer, it must encompass the requisites:

a) The element present in the compound should be simply available to the plant.

b) The substance should be soluble in water.

c) It must be stable, in such a way that it might be made available to the plant for a long time.

d) It must not be costly.

e) It must maintain the pH of the soil in the vicinity of 7 to 8.

f) It must not be poisonous to the plant.

Classification of Inorganic Fertilizers:

Inorganic fertilizers are categorized on the basis of the role they play in the soil chemistry, the numbers and kinds of nutrient elements they have, their physicochemical properties employed in specific, their solubility in sol water, their physiological effect on the effect on the soil to which the fertilizer has been applied and as well the technique utilized for manufacturing them.

We shall now consider merely some of these categorizations. First fertilizers are categorized as direct or indirect fertilizers according to their agrochemical nature. Direct fertilizers are those that encompass the nutrient elements in the form of compounds that are directly assimilated through plants. According to the nutrient elements present in them, direct fertilizers are further categorized as phosphatic, nitrogenous, potash and so on.

Indirect fertilizers are such substances that are added to the sol in order to enhance the chemical, mechanical or biological properties. Illustrations of indirect fertilizers comprise ground dolomite and limestone that are employed to reduce the soil acidity and gypsum employed to enhance the properties of the soil having a high salt content.

Fertilizers are as well categorized as water soluble or soluble in soil acids in accordance to their solubility in the moisture in the soil. Nitrogenous and potash fertilizers are soluble in water and are readily incorporated by plants however are quickly washed out of the soil via surface water. Most of the phosphates are fertilizers soluble in soil acids. They are dissolved much slower, however are retained in the soil much longer.

Fertilizers can be categorized as physiologically acid, physiologically alkaline or physiologically neutral, in accordance to their physiological effect on the soil to which they are added. The physiologically neutral fertilizers don't change the pH of soil.

On the basis of their form or physical properties, fertilizers are categorized into powder form or granulated form. The granulated fertilizers are less hygroscopic and don't cake throughout storage. They are not subjected to weathering after being introduced to the soil and are retained by the soil for a longer period of time.

The inorganic Fertilizers:

The Inorganic fertilizers can be natural and as well synthetic (that is, artificial). The natural inorganic fertilizers are Chile saltpeter (NaNO₃), rock phosphates and potassium salts. Such fertilizers are not adequate to form the soil productive, therefore the requirement for making of fertilizers artificially. Thus, inorganic fertilizers are as well termed as synthetic or artificial fertilizers. These types of fertilizers are made up and have nutrients which can be readily absorbed via plants. Artificial fertilizers might be studied under three groups according to the nature of the element.

1) Nitrogenous Fertilizers:

The significant nitrogenous fertilizers are nitrates of sodium, calcium and potassium, calcium cyan amides, ammonium sulphate, ammonium nitrate and urea. Such fertilizers are added to the soil in order to take away the deficiency of nitrogen in the soil. Now let us illustrate some of these nitrogenous fertilizers and how they are prepared.

A) Ammonium Nitrate:

This is the most significant nitrogenous fertilizer. Ammonium nitrate includes 32 to 35% nitrogen, half in ammonium form and half in nitrate form having the ammonium form being fairly resistant to the leaching. This is a white crystalline salt, extremely soluble in water and very hygroscopic in nature. It consists of the tendency to absorb moisture from air and it makes cake.

What raw materials are needed for the production of ammonium nitrate? The raw materials are ammonia and nitric acid. The nitric acid is specifically made up by oxidizing ammonia and absorbing the resultant nitrogen oxides in water. There are basically three significant techniques of production ammonium nitrate. These are: (i) Crystallization (ii) Flaking (iii) Prilling. 

Of these three, Prilling is the latest and most significant technique. Four basic operations are comprised in the ammonium nitrate Prilling plant. These are:  

a) Neutralization: Heated ammonia vapor and 60 to 80% nitric acid are introduced at the base of the neutralizer tower operating in a pressure of around 3 to 5 atmospheres and via which neutralized solution is recycled. The reaction in the tower is illustrated below:

HNO₃ + NH₃ → NH₄NO₃; ΔH= -20.6 kcals

b) Evaporation: The solution acquired from the neutralizer includes around 83% NH₄NO₃. This solution is allowed to pass via a heater supplied by steam for the neutralizer. The heated solution from the heater is passed to a vacuum evaporator whenever its concentration is raised to around 95%, 83% solution (if left) is drawn off at this point and employed in making solution. 

c) Prilling: The concentrated solution is now sprayed to Prilling tower (that is, the tower is quite high, around 185ft). The given solution in the tower is cooled via an ascending current of air. The solution is sprayed via spray nozzles in such a way that the liquid breaks up to drops of uniform and adequate size. The prills solidify adequately as they reach the bottom of the tower and should be handled carefully till they are dried.

d) Drying and Finishing: Drying the prills of ammonium nitrate is rather difficult as it is delinquent in nature. There are different methods of drying. One technique is the Bamag process in which the nitric acid and ammonia gas are pumped to a melt of ammonium nitrate at around 150^oC which is then cooled and transformed to solid NH₄NO₃.

The other method is the Stengel process. In this method, super heated ammonia vapor (145^oC) and concentrated nitric acid preheated to 170^oC are allowed to react in the packed tower and the reaction mixture is then passed to a cyclone designed to quickly separate the steam and solution. Air is blown via the melt as an outcome of which water is eliminated from the melt to a greater level and a melt having 0.25% moisture is produced directly. The melt can be employed directly, prilled in short tower or passed to a cooled steel belt to provide a thin layer of solid nitrate that is afterward broken up to provide granular crystals.

B) Ammonium Sulphate:

Ammonium sulphate comprises 21% nitrogen and can be obtained as by product or might be prepared synthetically. This is obtained as by product of steel industry in which the ammonia (the other by product) from coke oven is absorbed in the sulphuric acid. 

Waste streams from different chemical and metallurgical industries are the other source of ammonium sulphate. For illustration the production of pigments and synthetic flames forms by product solutions having ammonium sulphate.

Synthetically, ammonium sulphate is made up by manufactured ammonia. Saturates and crystallizers are as well used in the synthetic manufacture of ammonium sulphate. The heat of reaction of anhydrous ammonia is concentrated sulphuric acid acquired by Contact process in very high. Therefore water evaporated from the crystallizer should be returned either via means of a condenser or through addition of water. The ammonium sulphate is dried methodically to prevent caking. Free sulphuric acid is eliminated either via washing on a centrifuge or filter or via neutralizing by weak ammonia solution.

C) Urea:

The urea occupies the third place among the world's solid nitrogenous fertilizer; however it is the highest in nutrient concentration. This is so as if pure; it includes almost 45 to 47% nitrogen. Urea consists of some properties that make it use unfavorable. One is its tendency to hydrolyze and lose ammonia. This property is desirable in several respects as this should take place before plants can make use of the nitrogen efficiency, however if the urea is applied to the surface of the soil, ammonia formed by hydrolysis can be lost to the atmosphere before it has to join by soil constituents. The other property is the slower conversion of urea to nitrate in the soil as compared by ammonia, ammonium nitrate and ammonium sulphate.

In producing urea, the two raw materials are required namely CO₂ and ammonia. Urea can be prepared by passing liquid CO₂ and liquid NH₃ in a silver special autoclave whenever ammonium carbonate is made up. The latter is heated at 130 to 135^o around 35 atmospheric pressure to get urea.

2NH₃ + CO₂ → NH₂COONH₄

NH₂COONH₄ → NH₂CONH₂ + H₂O

Ammonium            Urea

carbonate   

In its utilization as a fertilizer, urea is first hydrolyzed via soil on water to NH₃ and CO₂. Then nitrosification of NH₃ occurs by the agency of Nitrosomonas and Nitrosococcus bacteria and nutrients are formed.

Nitrification of nitrites is brought about via nitrobacter bacteria to make nitrates via the plants in the soil. 

CO(NH₂)₂ + H₂O → CO₂ + 2NH₃

2NH₃ + 3O₂ → (bacteria) → 2NO₂.2H₂O + 2H+ + Energy

2NO₂^- + O₂ → (bacteria) → 2NO₃^- + Energy 

2) Potassium fertilizers:

Potassium comprising fertilizers are KNO₃, KCl and K₂SO₄. Potassium is most easily deduced as K₂O (potash). Of such three illustrated, potassium chloride is the most general potassium fertilizer having potash in the chloride form. This is obtained by the evaporation of Dead Sea via solar energy. Throughout the procedure of evaporation, the concentration raises from 1.2 in to 23% in liquor which crystallization in the pan. After refining around 97% pure KCl can be obtained. Potassium chloride includes around 63.17% potash and potassium content of around 52.44%, it is as well cheaper as compare to all the potassium fertilizer and therefore it is broadly utilized by farmers and cultivators and applied to a broad range of soils and crops apart from for tobacco and potato in which large doses of KCl influences the quantity of the product negatively (that is, lowers the quantity). In such cases, K₂SO₄ is usually applied whenever large amount of potash are required.

Potash sulphate (K₂SO₄) the other potassium fertilizer comprises 54% of potash whenever pure but comprises 48 to 52% of potash of commercial. It is readily soluble in water and becomes available to crops almost instantly. Potassium sulphate can be made up by any of such methods illustrated below:

a) The action of sulphuric acid on the potassium chloride

2KCl + H₂SO₄ → K₂SO₄ + 2HCl

b) By the reaction of burkeite (Na₂CO₃.2NaSO₄) by KCl

c) Via the reaction of potassium chloride and sulphur.

d) From langbemite, K₂SO₄.2MgSO₄, by ion exchange to take away magnesia. This is manufactured by dissolving langbemite in water and adding concentrated solution of KCl.K₂SO₄ is more soluble as compare to MgCl₂ and is separated through fractional crystallization.

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