Lesson Explainer: Fertilizers Chemistry

In this explainer, we will learn how to describe the production of fertilizers in laboratories and on an industrial scale.

Plants and crops need to absorb certain elements from the soil that they are growing in to stay healthy. In this context, nitrogen is a very important element, as it is used by plants to make proteins, chlorophyll, and nucleic acids. Plant growth is limited if there is not enough nitrogen available from the soil. Farmers refer to crop yield as the amount of crop grown per unit area of land. Since the fertile land area is limited, crop yields need to be high in order to meet the demand for food products.

Nitrogen is found in soil in the form of inorganic and organic compounds. The amount of nitrogen in the soil will decrease over time as crops grow. Nitrogen needs to be replaced on a regular basis through the application of fertilizers. Without fertilizers, the soil will become infertile.

Fertilizers may be natural products, such as manure, and are referred to as natural fertilizers. Fertilizers can also be synthetic compounds such as artificial “azotic,” or synthetic fertilizers, which are often derived from oil or gas products.

Definition: Artificial Fertilizer

Artificial fertilizers contain many different plant nutrients containing the elements nitrogen, magnesium, phosphorus, and potassium. This sort of fertilizer is usually manufactured from fossil fuel sources.

One important type of fertilizer contains nitrogen, phosphorus, and potassium and is known as an NPK fertilizer, where N, P, and K are the chemical symbols for the three elements.

Definition: NPK Fertilizers

Fertilizers that contain nitrogen, phosphorus, and potassium are called NPK fertilizers.

Phosphorus is used in adenosine triphosphate, ATP, which is a biological chemical used in the energy conversion processes in plants, including photosynthesis. Potassium is absorbed in large amounts by plants and is involved in transporting essential nutrients around the plant structure. This is why potassium is an important component of fertilizers. Since all common potassium salts are soluble, cheap potassium salts are added to fertilizers to provide potassium ions for the soil.

Example 1: Determining the Elements Contained within an NPK Fertilizer

Which three elements should a complete NPK fertilizer contain?

  1. Sodium, phosphorus, and potassium
  2. Nitrogen, palladium, and phosphorus
  3. Nitrogen, platinum, and potassium
  4. Nickel, phosphorus, and kalium
  5. Nitrogen, phosphorus, and potassium


The three elements required for healthy plant growth are nitrogen, phosphorus, and potassium. NPK fertilizers provide these three elements. The element nitrogen has the symbol N, phosphorus has the symbol P, and potassium has the symbol K. Since potassium was discovered after phosphorus, the symbol P was already taken. The Neo-Latin name for potassium is kalium. The symbol K was adopted from this name. NPK, therefore, refers to nitrogen, phosphorus, and potassium, so the correct answer is option E.

Nitrogenous fertilizers contain nitrogen that is in a form that plants can use. Although about 78% of the air is nitrogen gas, plants cannot use it in this form. Nitrogen must be dissolved as soluble nitrates or ammonium salts. Sometimes, compounds such as urea are used. Urea is found in natural fertilizers, as it is a waste product produced by many living organisms. The structure of urea is shown below:

Urea has a relatively high percentage of nitrogen by mass, which makes it a prime example of a nitrogenous fertilizer. It is used in warmer climates, as the rate of decomposition into ammonia and carbon dioxide is faster.

Another type of common fertilizers is liquified ammonia. It has the highest nitrogen content among all fertilizers at 82%. When using liquified ammonia as a fertilizer, it is introduced into the soil at a depth of nearly 12 cm.

Example 2: Identifying and Explaining Why One Fertilizer Is More Effective Than Another

Why is fertilizer A not as effective as fertilizer B at promoting plant growth?

  1. Fertilizer B only contains nitrogen.
  2. Fertilizer B inhibits seed germination.
  3. Fertilizer A raises the pH of the soil.
  4. Fertilizer A does not contain any phosphorus or potassium.
  5. Fertilizer A does not contain any sodium.


Plants need to absorb the elements nitrogen, phosphorus, and potassium from the soil to ensure healthy growth. If plants are deficient in these elements, they may suffer from various diseases. Fertilizer B provides all three of these elements, as it is an NPK fertilizer. N stands for nitrogen, P for phosphorus, and K for potassium. The NPK fertilizer contains a blend of compounds that contain these three elements. The compounds may also contain other elements that are not essential for the plants. Option A is therefore incorrect.

Fertilizer A contains a single compound called urea. We can see from the molecular formula on the bag that urea contains the elements carbon, oxygen, nitrogen, and hydrogen. In terms of plant growth, only nitrogen is important here. Fertilizer A does not provide any phosphorus or potassium, so it is not as effective at promoting plant growth as fertilizer B. Option D is the correct answer.

Some fertilizers release ammonium compounds, and they can decrease the soil pH. This way, using urea will not raise the pH of the soil. Option C is incorrect. Fertilizers do not inhibit the germination of seeds; this process is dependent on the health of the seed and the temperature. Although plants need sodium in small amounts, too much can be toxic, and fertilizers are not intended to provide this element. Options B and E are therefore incorrect, leaving the correct answer as option D.

Ammonia is often the starting material for most nitrogenous fertilizers. Anhydrous liquid ammonia can be directly injected into the soil, although it is a toxic gas and is difficult to handle safely. Ammonium nitrate is the most widely used fertilizer in many countries. It contains a relatively high percentage of nitrogen, around 35%, by mass and is highly soluble in water. Ammonium nitrate is produced industrially by the reaction of ammonia with nitric acid, as shown in the following equation: AmmonianitricacidammoniumnitrateNH()+HNO()NHNO()+3343glaq

This is a neutralization reaction. Note here that nitric acid itself is produced by the oxidation of ammonia gas in a separate process.

While fertilizers are extensively used to help promote growth, some care is needed as excess amounts of ammonium nitrate can make the soil acidic. To prevent this, calcium ammonium nitrate fertilizer is sometimes used instead. This is actually a mixture of ammonium nitrate and chalk. Chalk is a naturally occurring calcium carbonate, which neutralizes the soil during application. Slaked lime (Ca(OH)2) is also commonly added to ammonium fertilizers to help neutralize the soil.

Ammonium sulfate may also be produced by the neutralization of sulfuric acid with ammonia gas. Sulfuric acid is a chemical that is made separately in a highly important industrial process called the contact process. The raw material for sulfuric acid production is sulfur. Sulfur is a very important element in plants. It is used by the plants, in small amounts, to form proteins: 2NH()+HSO()(NH)SO()Ammoniasulfuricacidammoniumsulfate324424gaqaq+

A flow chart indicating how many materials are involved in making ammonium sulfate is shown below.

Nitrogenous phosphorus fertilizers, such as ammonium phosphate fertilizer, provide the soil with two essential elements, nitrogen and phosphorus. The ammonium phosphate is manufactured by using ammonia gas to neutralize phosphoric acid, which is made by reacting naturally occurring phosphate rocks with sulfuric acid. Phosphate rock contains calcium phosphate, among other compounds: Ca(PO)()+3HSO()2HPO()+3CaSO()CalciumphosphatesulfuricacidphosphoricacidcalciumsulfateHPO()+3NH()(NH)PO()Phosphoricacidammoniaammoniumphosphate34224344343434sllsaqgaq+++

Phosphate rock is insoluble and cannot be used directly as a fertilizer. It is mined on the surface in many locations around the world as can be seen in the photograph below.

Aerial view of a phosphate mine processing mill

Example 3: Recalling the Chemical Formulas for the Calcium Compounds Used in Fertilizers

Phosphorite, a sedimentary rock containing phosphate minerals, is reacted with sulfuric acid to produce calcium phosphate and calcium superphosphate.

What are the respective chemical formulas for calcium phosphate and calcium superphosphate?

  1. Ca(PO)342 and Ca(HPO)242
  2. Ca(PO)342 and Ca(HPO)243
  3. Ca(PO)243 and Ca(HPO)242
  4. Ca(PO)342 and Ca(HPO)42
  5. Ca(PO)332 and CaHPO4


Phosphorus is an important element for use in fertilizers. A good source of phosphorus can be found in rocks such as phosphorite. However, these rocks are insoluble in water, they need to be treated with acids to produce water-soluble phosphate salts. When phosphorite rock is reacted with sulfuric acid, two phosphate salts can be produced, calcium phosphate and calcium superphosphate. In this question, we need to recall or determine the chemical formula for these two compounds.

From their names, we can deduce that both compounds will contain calcium cations and phosphate anions. The most common ion of calcium is Ca2+, while the phosphate anion has the formula (PO)43. Calcium phosphate is therefore the combination of the calcium cation and the phosphate anion. In order to ensure that the compound is electrically neutral, we need three calcium cations for every two phosphate anions. This, therefore, gives the chemical formula of calcium phosphate as Ca(PO)342. Based on this, we can already determine that options C and E are incorrect.

Superphosphates are similar to phosphates, but superphosphates contain two atoms of hydrogen. As a result, superphosphates only have a charge of 1, giving it the chemical formula HPO24. Therefore, when combined with a calcium cation, two superphosphate anions are needed to ensure that the compound is electrically neutral. This gives the chemical formula of calcium superphosphate as Ca(HPO)242.

Our two chemical formulas are therefore Ca(PO)342 for calcium phosphate and Ca(HPO)242 for calcium superphosphate. This corresponds to option A. The correct answer is A.

On an industrial scale, vast amounts of starting materials are required to produce large quantities of fertilizers. The starting materials are manufactured from raw materials. The starting materials are made close to the fertilizer production plant so that they are readily available. The processes involved are run continuously, and a lot of energy is consumed in part due to the high temperatures used to increase the reaction rate. The equipment for large scale production is expensive and it is often automated.

Example 4: Describing the Large-Scale Industrial Production of Ammonia

Fill in the blank: The large-scale industrial preparation of ammonia is an example of a .

  1. renewable process
  2. progression process
  3. nitrogenous process
  4. batch process
  5. continuous process


Ammonia is the raw material used to make artificial fertilizers that contain nitrogen. Industrially, it is made from hydrogen gas and nitrogen gas in the Haber process. Ninety-five percent of the world’s hydrogen is produced from the reaction of methane, a fossil fuel product, and steam. It is sourced from nonrenewable resources such as crude oil. Currently, this makes the production of ammonia a nonrenewable activity. The industrial process can run continuously so long as the raw materials can be fed into the reactor vessel. Due to loss of production, it is expensive to shut the process down, and this would normally happen annually for planned maintenance. Unlike a batch process, where the chemicals are made in small batches with shutdown and cleaning in between, the production of ammonia is described as a continuous process. Therefore, the correct answer is option E.

As many of the reactions used to make fertilizers are simple neutralization reactions, they can easily be performed in a laboratory on a small scale. The process would be slower, and the compounds would be produced in batches. Ordinary laboratory glassware would be used, which is relatively cheap. The neutralization reaction would take place at room temperature, followed by the use of a Bunsen burner for the evaporation of liquid.

In the diagram below, a measured amount of ammonia solution would be neutralized by a predetermined volume of sulfuric acid. This could be performed as a titration initially, using an indicator. Once the volume of acid needed is determined, the experiment could be repeated without an indicator. The ammonium sulfate crystals would be recovered from the neutral solution by evaporating the water in an evaporating dish, with gentle heating.

Example 5: Describing the Small-Scale Laboratory Production of Ammonia

Fill in the blank: A small-scale laboratory preparation of ammonia is an example of .

  1. a renewable process
  2. a continuous process
  3. a batch process
  4. an automated process
  5. a nitrogenous process


Ammonia gas is difficult to make from the elements nitrogen and hydrogen, and, if it were synthesized in a laboratory, only small quantities would be produced. The reactants for the synthesis would need to be acquired separately, and the procedure would involve common laboratory glassware.

The procedure would be described as a batch process, as we would need to dismantle the apparatus and clean it out every time we used it.

It is certainly not a continuous or an automated process. In these processes, the chemicals supplied for the synthesis would arrive from a chemical supplier and would be continually running. Here, the preparation of ammonia is completed and then stopped.

Although used in small quantities, these chemicals may originate from petroleum-based sources, which are nonrenewable sources. For this reason, the laboratory synthesis may not be a renewable process. The correct answer is therefore option C.

Key Points

  • Plants require the elements nitrogen, phosphorus, and potassium to grow and remain healthy.
  • Nitrogen is provided by fertilizers containing ammonium salts or nitrates, which are soluble in water. These fertilizers are produced from ammonia gas.
  • Phosphorus is provided in the form of phosphates. The phosphate ion has the formula PO43. Soluble phosphates, such as ammonium phosphate, are produced from phosphate rock, via phosphoric acid.
  • There are many soluble potassium salts that may be added to the soil to provide potassium ions, K+.
  • NPK fertilizers contain the elements nitrogen, phosphorus, and potassium.
  • Excess use of fertilizers can increase the acidity of the soil but can be neutralized by including calcium carbonate or calcium hydroxide in the fertilizer.
  • Fertilizers can be prepared by reacting phosphoric rock or ammonia with acids such as nitric or sulfuric acid.

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