Types of Chemical Fertilizers Available in the Market

This article throws light upon the three main types of chemical fertilizers available in the market. The types are: 1. Nitrogenous Fertilizers 2. Phosphatic Fertilizers 3. Potassic Fertilizers.

Chemical Fertilizers: Type # 1. Nitrogenous Fertilizers:

Nitrogenous fertilizers are those fertilizers that are sold for their nitrogen content.

Classification:

Nitrogenous fertilizers can be classified into four classes on the basis of forms of N present in straight nitrogenous fertilizers as follows:

1. Nitrate nitrogen (NO₃-N) containing fertilizers e.g. NaNO₃-16% N; Ca(NO₃)₂— 15.5% N.

2. Ammonium containing nitrogenous fertilizers (NH₄-N), e.g. (NH₄)₂SO₄-20% N; NH₄Cl—24 to 26% N, anhydrous ammonia, 82% N;

3. Both NH₄ and NO₃-N containing nitrogenous fertilizers; e.g. ammonium nitrate (NH₄NO₃)—33 to 34% N Calcium ammonium nitrate (CAN)—20% N.

4. Amide fertilizers—It is organic form of N-containing fertilizers, e.g. Urea [CO(NH₂)₂]—46% N, Calcium Cyanamide (CaCN₂)—21% N.

The most important and widely used nitrogenous fertilizers are ammonium sulphate and urea.

(i) Ammonium sulphate [(NH₄)₂SO₄]:

Ammonium sulphate about 20% N. It is a white crystalline salt, stable and soluble in water. It can he stored well. The solution form of (NH₄)₂SO₄ is acidic to litmus. In this fertilizer, Nitrogen is present in cationic form (NH). So this form of N can be retained by soil colloids. Consequently loss of N through leaching is less.

It is less hygroscopic and therefore, this possesses fewer problems in handling. All N-fertilizers containing N as NH₄ form or fertilizers which after application in soil produce NH₄ ion, are physiologically acidic in character. It is not subject to loss by denitrification. The application of heavy dose of (NH₄)₂SO₄ or any other NH₄ fertilizers in alkaline soils may lead to the loss of N through volatilization.

Reactions in Soils:

(i) In upland soil conditions. When (NH₄)₂SO₄ is applied to the upland soil, the first reaction is the cation-exchange, in which ammonium (NH₄⁺) displaces some other cations, usually calcium, on the exchange complex of the soil.

The reaction can be represented as follows:

Calcium sulphate [CaSO₄], so formed, may be leached and lost in drainage water, particularly in a humid climate. In this way the loss of soil calcium (Ca²⁺) takes place due to application of (NH₄)₂SO₄ fertilizer. The ammonium (NH₄⁺) on the soil colloid may be taken up directly by the plants or it is nitrified .  The biological process by which oxidation of such NH₄⁺ to NO₃^– is known as nitrification. It is a two-step process in which the ammonium (NH₄⁺) is first converted to nitrite (NO₂^–) and the two nitrates (NO₃^–).

The above reaction indicated that hydrogen (H⁺) ions are formed during nitrification which results in the acidification of the soil.

The sulphate portion of (NH₄)₂SO₄ combines with Ca²⁺ to form CaSO₄. Calcium sulphate being soluble in water is partly absorbed by plants, but mainly it is leached through the soil during heavy rains. Thus loss of CaSO₄ occurs in soil.

Continued use of (NH₄)₂SO₄ will lower the soil pH. The speed and extent of such transformation will be profoundly influenced by soil environment like moisture regimes and temperatures. s a result of continued application of (NH₄)₂SO₄ depletion of exchangeable bases as well as release of H⁺ in the soil solution occurs and that also leads to the development of soil acidity.

That is why NH₄⁺—fertilizers are called physiologically acidic fertilizers. If a soil contains large amounts of free CaCO₃, then the loss of Ca²⁺ will fall upon the CaCO₃ and not on the exchangeable bases. Under such conditions, there will be no development of acidity due to (NH₄)₂SO₄ application.

In Upland Calcareous Soils:

When ammonium sulphate is applied to calcareous soils considerable losses of ammonia through volatilization may occur.

The ammonium carbonate. (NH₄)₂CO₃, is unstable and decomposes easily to give NH₃, CO₂ and H₂O. The losses of nitrogen from (NH₄)₂SO₄, applied as broadcast in calcareous soils, may be appreciable. But when (NH₄)₂SO₄ is drilled, the losses of nitrogen are not likely to be appreciable.

Volatilization of Ammonia:

Ammonium containing [(NH₄)₂SO₄] or forming (urea) fertilizers will react with CaCO₃ in soil to form (NH₄)₂CO₃ and calcium precipitates.

The following general equations may be represented for the above mentioned reactions:

X(NH₄)₂ Y + N Ca CO₃→ N(NH₄)₂CO₃ + Ca_nY_x … (1)

where Y = anion combined with NH ions

N, X and Z are dependent on the valences of the anion and cation.

The final reaction product. (NH₄)₂CO₃ is unstable and decomposes as follows:

The amount of NH₄OH formed in a given time would depend on the solubility of Ca_nY_x and its rate of formation. If Ca_nY_x is insoluble, the reaction will proceed to the right resulting more (NH₄)₂ CO₃ and consequently more NH₄OH is to be formed. If no insoluble precipitate is formed, no appreciable amount of (NH₄) CO₃ will exist.

When (NH₄)₂CO₃ decomposes according to second equation (2) CO₂ is lost from solution at a faster rate than NH₃ thereby producing additional OH^– ions and an increase in the concentration of OH^– ions. Consequently, more solution NH₄⁺ becomes electrically balanced by OH” ions which will favour NH₃ loss as follows:

NH₄⁺ + OH^–DNH₄OH DNH₃↑ + H₂O

This type of reaction occurs when nitrogenous fertilizers like (NH₄)₂SO₄, urea etc. are applied in calcareous and alkali soils.

(ii) Urea [CO(NH₂)₂]:

Urea is an organic solid nitrogenous fertilizer. It contains about 46 per cent N. Urea is manufactured by reacting anhydrous NH₃ and CO₂ gas under very high pressure in the presence of a suitable catalyst.

The reaction is as follows:

This unstable intermediate product is decomposed and urea is recovered. The urea solution is then concentrated to 99 per cent and is sprayed into a chamber where urea crystals are formed.

Urea is a white crystalline substance. At 86° F urea absorbs moisture from the atmosphere if the relative humidity is 72 per cent or above and releases moisture to the atmosphere if the humidity is below 72 per cent.

During concentrating the solution or crystallization or for preparation of granulated urea at high temperature, some amount of biuret may be formed by condensation reaction as follows:

This biuret is toxic to the plant. Biuret concentration in urea should not exceed 1.5 per cent. When urea will be applied as foliar spray on crop canopy, the biuret concentration should not exceed 0.25 per cent. Urea is less acidic compared to (NH₄)₂SO₄. Pure solution of urea is slowly hydrolysed into NH₃ and CO₂ in absence of bacteria or enzyme.

But this reaction is accelerated or enhanced at high temperature in combination with the addition of alkali or acid and also by adding bacteria and enzyme urease. Urea reacts with HN0₂ releasing elemental N and C0₂. Acid equivalent of urea is 84.

Reactions of Urea in Soil:

After application of urea in soil, it undergoes enzymatic hydrolysis to produce an unstable compound designated as ammonium carbamate.

This NH₃ is converted to NH₄⁺ ions by accepting one proton (H⁺) from proton donor and subsequently forms NH₄OH or any other NH compound depending upon the nature of the donor. After formation of NH₄—compound, its behaviour in soil and effect on the soil property will be similar to that of any other N-fertilizers as ammoniacal form.

In neutral, slightly alkaline and calcareous soils, the chances of formation of nitrites (NO₂^–) are more than in acid soils. When urea is applied in relatively high amounts, nitrites accumulate even in acid soils, because the pH of the soil increases during the hydrolysis of urea.

In high concentration of urea, there is a combined toxic effect of NH₃ and NO₂ (nitrite). Due to higher concentration of NH₃, the conversion of NH₄→ NO₂ will increase. But due to lower population of nitrobacter, the conversion of NO₂→ NO₃ will be inhibited.

Advantages of Urea over Ammonium sulphate:

(i) During the manufacturing process of urea, no raw materials are to be imported; therefore, no foreign exchange is involved. Whereas for ammonium sulphate, (NH₄)₂SO₄, gypsum or sulphur may be imported which involves foreign exchange.

(ii) Urea is a solid organic—N fertilizer containing highest percentage of N. So, for urea cost of transport and handling per unit weight of N is less than (NH₄)₂SO₄.

(iii) Urea is relatively less.-acid producing fertilizer that (NH₄)₂SO₄.

(iv) Urea can be used carefully for foliar application, but ammonium sulphate cannot be used as foliar application.

Slow Release Nitrogenous Fertilizers:

The efficiency of synthetic inorganic N—fertilizers hardly exceeds 60-70 per cent. The rest amount is lost from the root zone by different mechanisms. The maximum amount is lost by the process of leaching in the form of nitrate. Therefore, attempts have been made to produce N-fertilizer materials which release N slowly in available form or to develop materials which control the release of N in available form slowly.

The manufacturing of this slow release N- fertilizers have the following advantages:

(i) Slow release N-fertilizers minimise the loss of N increases the efficiency of the fertilizers.

(ii) It helps to avoid frequent application of N-fertilizers.

(iii)It helps to prevent different harmful effects of plants like germination of seeds and emergence of seedlings.

(iv) It avoids the luxury consumption of N and prevents the imbalances due to different nutrients within the plants.

Classification:

The slow release-N-fertilizers can be mainly classified as follows:

(a) Conventional soluble N-fertilizers coated with some other materials.

(b) N-substances of low water solubility.

(c) Nitrification and urease inhibitors.

(d) Sparingly soluble minerals.

(a) Coated N-Fertilizers:

The most important coated N-fertilizers that the used as slow releasing N fertilizers are sulphur coated urea (SCU), neem coated urea (NCU), lac coated urea (LCU) etc. Due to coating with neem cakes, lac or sulphur, urea comes into the soil solution through diffusion process very slowly and in this way they supply nitrogen to the plants at a controlled rate or slow rate but for a longer period.

In addition, neem cake and lac contains alkaloids which inhibits the activities of nitrifying bacteria. As a result, the rate of nitrification is minimised and the leaching loss of N is reduced. In case of sulphur coated urea, the elemental sulphur which is placed in the soil along with the fertilizers is oxidised by S-oxidising organisms to sulphuric acid (H₂SO₄).

As a result, acidity increases in the micro-zone of the fertilizer resulting reduced activity of the nitrifying bacteria. Thus, the rate of nitrification is controlled and N loss as nitrate (NO₃) is minimised.

(b) N-Substances of Low Water Solubility:

A large number of chemicals have been developed which can be used as slow release N-fertilizers that are given below:

(i) Urea-Formaldehyde or Urea-Form:

Urea-formaldehyde compounds are the major group of compounds that are commercially available. They are white, odorless solids analyzing about 38 per cent N which are made by reacting with formaldehyde in the presence of a catalyst.

A typical urea form may contain 30 per cent of its nitrogen in forms that are soluble in cold water (25°C). Nitrogen in the cold-water fraction nitrifies almost quickly as urea. Solubility in hot boiling water is a measure of the quality of the remaining 70 per cent of its nitrogen.

An activity index is used by the Association of Official Agricultural Chemists to evaluate the suitability of urea-form compounds.

AI = %CWIN – %HWIN/%CWIN × 100

where, AI = Activity index

CWIN = % N insoluble in cold water at 25°C.

HWIN = % N insoluble in hot water (98 to 100°C)

Therefore, the quality of urea-form may be judged as follows:

(i) The quantity of cold water insoluble N which is the amount of N that will be released slowly,

(ii) The quality of the cold water insoluble N which will indicate the rate at which the insoluble N will release into available form.

(ii) Crotonylidene Diurea (CDU):

This slow-acting nitrogen compound is formed by the reaction of urea’s with croton aldehyde or acetaldehyde. Powdered CDU containing 30 per cent N has been directly used as a fertilizer.

The microbial transformation of chemically bound nitrogen in CDU is known to be temperature dependent.

Besides these, there are various other such N-containing slow acting fertilizers like, thiourea, (NH₂)₂CS, 36.8% N; urea-Z (reaction product of urea and acetaldehyde, 33-38 per cent N); Oxamidenon-hygroscopic, 31.8 per cent N, etc. Oxamide is di-amide of oxalic acid and slowly releases ammonium (NH₄) by hydrolysis.

(iii) Nitrification Inhibitors:

A nitrification inhibitor should ideally:

(i) Be non-toxic to plants, soil, micro-organisms, animals, fish and mammals.

(ii) Block the conversion of NH₄⁺→ NO₃^– by inhibiting Nitrosomonas activity.

(iii) Not interfere with the transformation of NO₂^– (nitrite) by Nitrobacter,

(iv) Be able to move with the fertilizer or fertilizer solution so that it will be distributed uniformly throughout the soil zone contacted by nitrogen fertilizer,

(v) Be stable for its inhibitory action to last for an adequate period of time,

(vi) Be relatively inexpensive, so that it can be used on a commercial basis.

There are various nitrification inhibitors, of which N-serve or nitrapyrin and AM are most important.

N-Serve:

It is 2-chloro-6 (trichloromethyl) pyridine and also referred to as nitrapyrin.

AM:

Chemically it is a substituted pyrimidine (2-amino-4-chloro-6-methyl- pyrimidine).

Chemical Fertilizers: Type # 2. Phosphatic Fertilizers:

The plant nutrient content of all phosphatic fertilizers is expressed in terms of percentage of phosphorus rather than the percentage of P₂O₅.

The conversion of % P to % P₂O₅ and vice- versa (% P₂O₅ to % P) is simple and the following expressions are used:

% P = % P₂O_s× 0.43

% P₂O₅ = % P × 2.29

This conversion should be used because most of the commercial grade phosphatic fertilizers are marketed as % P₂O₅ content. Phosphorus has generally three forms namely P₂O₅ (Phosphorus pentaoxide in burning form of P), HPO₃ (Metaphosphoric acid, when dissolves in water) and H₃PO₄ (Orthophosphoric acid when dissolves in warm water). But the salts of H₃PO₄ are most important and it has three replaceable H⁺ ions.

H₃PO₄H₂PO₄^–HPO₄^2- PO₄^3-

Plants absorb phosphorus is H₂PO₄^– and HPO₄^2- forms. Three replaceable ions of H₃PO₄ combine with Ca²⁺ to form three different combined salts of calcium and phosphorus resulting different classes of phosphatic fertilizers.

Classification:

Phosphatic fertilizers can be classified into three groups on the basis of forms in which orthophosphoric acid (H₃PO₄) is combined with calcium (Ca²⁺);

(i) Water soluble mono-calcium phosphate, [Ca(H₂PO₄)₂]

Example:

Superphosphates (SSP, DSP and TSP)

Single Superphosphate (SSP)—16-18% P₂O₅ or 6.88-7.74% P

Double Superphosphate (DSP) —32% P₂O₅ or 13.76% P

Triple Superphosphate (TSP) — 46-48% P₂O₅ or 19.78-20.64% P

They contain water soluble phosphorus and can be easily available to plants as H₂P0₄ ions. However, within a very short time this class of fertilizers is converted into insoluble phosphorus when it is applied to the soil. The magnitude of such reaction with the soil depends on the nature and properties of soil.

(ii) Citric acid soluble, di-calcium phosphate. [Ca₂H₂(PO₄)₂, or CaHPO₄]

Example:

Basic slag, silicates of lime,—14-18% P₂O₅ or 6.02-7.74% P

Dicalcium phosphate—34-39% P₂O₅ or 14.62-16.77%

This class of fertilizers is not readily soluble in water and hence not readily available to plants. But this class of fertilizers is suitable for acidic soils.

(iii) Phosphatic fertilizers not soluble in water or not soluble in citric acid, tricalcium phosphate, [Ca₃(PO₄)₂].

Example:

Rock phosphate, 20-40% P₂O₅ or 8.6—17.2% P

Raw bone meal, 20-25% P₂O₅ or 8.6-10.75% P

Steamed bone meal, 22% P₂O₅ or 9.46% P

This class of fertilizers is suitable for strongly acidic soils or organic soils (peat). In India, most important phosphatic fertilizers used by the farmers are superphosphates and rock phosphates.

Reactions in Soils:

Effectiveness of phosphatic fertilizers is determined by the properties of both the phosphorus salt and the soil being fertilized and by the reactions which occur between the phosphorus fertilizer and various soil constituents.

Superphosphate:

Superphosphate is formed due to the reaction of rock phosphates and conc. H₂SO₄ as follows:

When superphosphate is applied to a moist or to a dry soil before rainfall or irrigation, the mono calcium phosphate gets dissolved in the soil moisture. The roots of the growing plants easily take up this form of phosphorus. But within a very short time, the solution of mono calcium phosphate is precipitated in the soil pores.

Depending on the soil reaction, different soil-fertilizer reaction products are formed that are not soluble in water and becomes unavailable to plants. Thus superphosphate is not leached from the soil by the rainfall.

When superphosphate is applied to strong acid soils, it combines with iron and aluminium to form their respective insoluble phosphates and also di-calcium phosphate as follows:

In this way, phosphate becomes unavailable to plants.

When superphosphate is applied to moderately acid soils, phosphate fixation takes place by the silicate minerals present in soil and becomes unavailable to plants. Aluminium and iron are removed from the edges of the silicate crystals forming hydroxy phosphates.

A part of the reacted phosphate with Fe and AI compounds is also released by anion exchange reactions with OH^– ions.

Again, when superphosphate is applied to neutral and calcareous soils, the following reaction will take place and P becomes unavailable to plants.

The insoluble tri-calcium phosphate thus formed may be converted in the soil to more insoluble compounds like hydroxyl apatite, Ca₁₀(PO₄)₅(OH)₄; carbonatoapatite, Ca₁₀(PO₄)₆(CO₃)₂; and fluorapatite, Ca₁₀(PO₄)₆. F₂ and ultimately ‘P’ becomes unavailable to plants.

When superphosphate is applied to alkaline soils (pH > 8.5 or 9.0, Na⁺ is the dominant cation), it reacts with Na⁺ and forms mono-sodium phosphate which is highly soluble and becomes available to plants.

H₂PO₄ + Na⁺DNaH₂PO₄ (Soluble)

Rock Phosphate:

It is stable and quite insoluble in water. The citrate solubility varies from 5 to about 17 per cent of the total phosphorus content. Effectiveness of rock-phosphate (tri-calcium phosphate) is determined by its chemical reactivity which in turn depends on the degree of carbonate substitution for phosphate in the apatite structure.

Finely ground apatite phosphate rocks are effective only on acid soil (pH 6 or below). But calcined iron-aluminium phosphate ores with much higher citrate solubilities 60-65 per cent of the ‘P’ can be used successfully on neutral and calcareous soils.

In situations where the reactivity of available rock phosphate is inadequate partially acidulated rock phosphate (treating rock phosphate with H₂SO₄) can be used successfully.

A granular material containing mixture of raw rock phosphate and finely ground elemental sulphur has been developed and designated as “Bio super”. It is inoculated with the sulphur-oxidising bacteria thiohacillusthioxidans to ensure the conversion of sulphur to sulphuric acid (S → H₂SO₄).

This acid in turn reacts with the rock phosphate and releases soluble phosphorus in the soil and thereby increases ‘P’ availability to plants. When rock phosphate is applied to acidic soils and high amount of organic matter, the following reactions may take place due to formation of H₂CO₃ and HNO₃arising from the decomposition of organic matter.

Chemical Fertilizers: Type # 3. Potassic Fertilizers:

Recently the supply of potassium to plants is expressed as K⁺ (potassium) percentage instead of K₂O percentage in potassic fertilizers.

Converting percentage K to percentage K₂O and vice-versa can be accomplished as follows:

% K = % K₂O× 0.83

% K₂O = % K × 1.2

Practically all of the potassium fertilizers are water soluble. Different potassic fertilizers essentially consist of potassium in combination with chloride, sulphate, nitrate or polyphosphates etc.

Muriate of Potash (KCI):

The term muriate is derived from muriatic acid, a common name for hydrochloric acid. Fertilizer grade muriate contains 50-52 per cent potassium (60-63 per cent K₂O) and varies in colour from pink or red to white depending on the mining and recovery process used. Muriate of potash is generally marketed in five particles sizes: special standard, white soluble, standard, coarse and granular, of which coarse and granular size fractions of muriate of potash are widely used.

Muriate of potash is manufactured from potash-bearing minerals namely carnallite, KCl.MgCl₂.6H₂O (17.0 per cent K₂O); Sylvite, KCI, Sylvinite, KCI, NaCl mixture, etc. Recovery of potassium chloride (KCI) from sylvinite ore is made by the mineral floatation process or by solution of KCI, followed by recrystallization. Floatation process is based on the differences in the specific gravities of KCI and NaCl.

The potassium chloride having lesser specific gravity floatation the top of NaCl and thereby KCI is separated out. Whereas in the re-crystallization process, the difference in temperature-solubility relationships of chloride salts of K and Na is the fundamental principle of this method. The solubility of KCI increases rapidly with a rise in temperature, whereas NaCl solubility varies only slightly over a wide temperature range.

Sulphate of Potash (K₂SO₄):

Potassium sulphate is a white salt which contains 41.5 to 44.2 per cent K (50-53.2 per cent K₂O). It is produced by number of processes like, Langbeinite, Trona, Mannheim, Glaserite processes etc.

Langbeinite Process:

Production form Langbeinite minerals (K₂SO₄.2MgSO₄, 22.6% K₂O), occurs according to the following reactions:

K₂SO₄.2MgSO₄ + 4KCl→ 3K₂SO₄ + 2MgCl₂

Trona Process:

Burkite (Na₂CO₃.2Na₂SO₄) is reacted with potassium chloride to form glaserite (Na₂SO₄.3K₂SO₄) and then reacted with KCI brine to give potassium sulphate.

Galserite Process:

The following reactions take place in the production of sulphate of potash from sodium sulphate (Na₂SO₄) and KCI with glaserite as an intermediate product.

Reactions in Soils:

Both chloride and sulphate of K are soluble in water and on application to the soil they ionize into K⁺, Cl^– and SO₄^2- ions. The released K⁺ ion from the fertilizer gets adsorbed on the soil colloids and also available to the plant through cation exchange reactions.

Based on the chemistry of chloride in the soils, it is concluded that under acidic soil conditions CI^– ion replaces the OH^– ions associated with the free iron oxides and therefore, in such soils, muriate of potash is likely to give a greater response than K₂SO₄. Besides, the Cl^– ions are less strongly retained on soil colloids than the SO₄^2- ions.

In alkaline soils, when muriate of potash (KCI) is applied, the accumulation of CI^– ions creates toxic to plants. So in potash deficient soils with alkaline reaction, it should be applied along with organic matter. The application of potassic fertilizers has little or no effect on soil reaction.