The value of nutrients
Nutrients are very important for life to exist. An essential nutrient is any chemical element that is needed for a plant to be able to grow from a seed and complete its life cycle. The same is true for animals. A macronutrient is one that is required in large quantities by the plant or animal, while a micronutrient is one that only needs to be present in small amounts for a plant or an animal to function properly.
- Definition 1: Nutrient
A nutrient is a substance that is used in an organism's metabolism or physiology and which must be taken in from the environment.
In plants, the macronutrients include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P) and potassium (K). The source of each of these nutrients for plants, and their function, is summarised in Table 1. Examples of micronutrients in plants include iron, chlorine, copper and zinc.
| Nutrient | Source | Function |
| Carbon | Carbon dioxide in the air | Component of organic molecules such as carbohydrates, lipids and proteins |
| Hydrogen | Water from the soil | Component of organic molecules |
| Oxygen | Water from the soil | Component of organic molecules |
| Nitrogen | Nitrogen compounds in the soil | Part of plant proteins and chlorophyll. Also boosts plant growth. |
| Phosphorus | Phosphates in the soil | Needed for photosynthesis, blooming and root growth |
| Potassium | Soil | Building proteins, part of chlorophyll and reduces diseases in plants |
Animals need similar nutrients in order to survive. However since animals can't photosynthesise, they rely on plants to supply them with the nutrients they need. Think for example of the human diet. We can't make our own food and so we either need to eat vegetables, fruits and seeds (all of which are direct plant products) or the meat of other animals which would have fed on plants during their life. So most of the nutrients that animals need are obtained either directly or indirectly from plants. Table 2 summarises the functions of some of the macronutrients in animals.
| Nutrient | Function |
| Carbon | Component of organic compounds |
| Hydrogen | Component of organic compounds |
| Oxygen | Component of organic compounds |
| Nitrogen | Component of nucleic acids and proteins |
| Phosphorus | Component of nucleic acids and phospholipids |
| Potassium | Helps in coordination and regulating the water balance in the body |
Micronutrients also play an important function in animals. Iron for example, is found in haemoglobin, the blood pigment that is responsible for transporting oxygen to all the cells in the body.
Nutrients then, are essential for the survival of life. Importantly, obtaining nutrients starts with plants, which are able either to photosynthesise or to absorb the required nutrients from the soil. It is important therefore that plants are always able to access the nutrients that they need so that they will grow and provide food for other forms of life.
The Role of fertilisers
Plants are only able to absorb soil nutrients in a particular form. Nitrogen for example, is absorbed as nitrates, while phosphorus is absorbed as phosphates. The nitrogen cycle (Grade 10) describes the process that is involved in converting atmospheric nitrogen into a form that can be used by plants.
However, all these natural processes of maintaining soil nutrients take a long time. As populations grow and the demand for food increases, there is more and more strain on the land to produce food. Often, cultivation practices don't give the soil enough time to recover and to replace the nutrients that have been lost. Today, fertilisers play a very important role in restoring soil nutrients so that crop yields stay high. Some of these fertilisers are organic (e.g. compost, manure and fishmeal), which means that they started off as part of something living. Compost for example is often made up of things like vegetable peels and other organic remains that have been thrown away. Others are inorganic and can be made industrially. The advantage of these commercial fertilisers is that the nutrients are in a form that can be absorbed immediately by the plant.
- Definition 2: Fertiliser
A fertiliser is a compound that is given to a plant to promote growth. Fertilisers usually provide the three major plant nutrients and most are applied via the soil so that the nutrients are absorbed by plants through their roots.
When you buy fertilisers from the shop, you will see three numbers on the back of the packet e.g. 18-24-6. These numbers are called the NPK ratio, and they give the percentage of nitrogen, phosphorus and potassium in that fertiliser. Depending on the types of plants you are growing, and the way in which you would like them to grow, you may need to use a fertiliser with a slightly different ratio. If you want to encourage root growth in your plant for example, you might choose a fertiliser with a greater amount of phosphorus. Look at the table below, which gives an idea of the amounts of nitrogen, phosphorus and potassium there are in different types of fertilisers. Fertilisers also provide other nutrients such as calcium, sulfur and magnesium.
| Description | Grade (NPK %) |
| Ammonium nitrate | 34-0-0 |
| Urea | 46-0-0 |
| Bone Meal | 4-21-1 |
| Seaweed | 1-1-5 |
| Starter fertilisers | 18-24-6 |
| Equal NPK fertilisers | 12-12-12 |
| High N, low P and medium K fertilisers | 25-5-15 |
The Industrial Production of Fertilisers
The industrial production of fertilisers may involve several processes.
- Nitrogen fertilisers
Making nitrogen fertilisers involves producing ammonia, which is then reacted with oxygen to produce nitric acid. Nitric acid is used to acidify phosphate rock to produce nitrogen fertilisers. The flow diagram below illustrates the processes that are involved. Each of these steps will be examined in more detail.
Figure 1: Flow diagram showing steps in the production of nitrogen fertilisers 
- The Haber Process
The Haber process involves the reaction of nitrogen and hydrogen to produce ammonia. Nitrogen is produced through the fractional distillation of air. Fractional distillation is the separation of a mixture (remember that air is a mixture of different gases) into its component parts through various methods. Hydrogen can be produced through steam reforming. In this process, a hydrocarbon such as methane reacts with water to form carbon monoxide and hydrogen according to the following equation:
Note: Interesting Fact :
The Haber process developed in the early 20th century, before the start of World War 1. Before this, other sources of nitrogen for fertilisers had included saltpeter (NaNO - The Ostwald Process
The Ostwald process is used to produce nitric acid from ammonia. Nitric acid can then be used in reactions that produce fertilisers. Ammonia is converted to nitric acid in two stages. First, it is oxidised by heating with oxygen in the presence of a platinum catalyst to form nitric oxide and water. This step is strongly exothermic, making it a useful heat source.
- The Nitrophosphate Process
The nitrophosphate process involves acidifying phosphate rock with nitric acid to produce a mixture of phosphoric acid and calcium nitrate:
- Other nitrogen fertilisers
-
Urea ((NH
- Other common fertilisers are ammonium nitrate and ammonium sulphate. Ammonium nitrate is formed by reacting ammonia with nitric acid.
-
Urea ((NH
- The Haber Process
The Haber process involves the reaction of nitrogen and hydrogen to produce ammonia. Nitrogen is produced through the fractional distillation of air. Fractional distillation is the separation of a mixture (remember that air is a mixture of different gases) into its component parts through various methods. Hydrogen can be produced through steam reforming. In this process, a hydrocarbon such as methane reacts with water to form carbon monoxide and hydrogen according to the following equation:
- Phosphate fertilisers
The production of phosphate fertilisers also involves a number of processes. The first is the production of sulfuric acid through the contact process. Sulfuric acid is then used in a reaction that produces phosphoric acid. Phosphoric acid can then be reacted with phosphate rock to produce triple superphosphates.
- The production of sulfuric acid
Sulfuric acid is produced from sulfur, oxygen and water through the contact process. In the first step, sulfur is burned to produce sulfur dioxide.
- The production of phosphoric acid
The next step in the production of phosphate fertiliser is the reaction of sulfuric acid with phosphate rock to produce phosphoric acid (H
- The production of phosphates and superphosphates
When concentrated phosphoric acid reacts with ground phosphate rock, triple superphosphate is produced.
- The production of sulfuric acid
Sulfuric acid is produced from sulfur, oxygen and water through the contact process. In the first step, sulfur is burned to produce sulfur dioxide.
- Potassium
Potassium is obtained from potash, an impure form of potassium carbonate (K
Fertilisers and the Environment: Eutrophication
Eutrophication is the enrichment of an ecosystem with chemical nutrients, normally by compounds that contain nitrogen or phosphorus. Eutrophication is considered a form of pollution because it promotes plant growth, favoring certain species over others. In aquatic environments, the rapid growth of certain types of plants can disrupt the normal functioning of an ecosystem, causing a variety of problems. Human society is impacted as well because eutrophication can decrease the resource value of rivers, lakes, and estuaries making recreational activities less enjoyable. Health-related problems can also occur if eutrophic conditions interfere with the treatment of drinking water.
- Definition 3: Eutrophication
Eutrophication refers to an increase in chemical nutrients in an ecosystem. These chemical nutrients usually contain nitrogen or phosphorus.
In some cases, eutrophication can be a natural process that occurs very slowly over time. However, it can also be accelerated by certain human activities. Agricultural runoff, when excess fertilisers are washed off fields and into water, and sewage are two of the major causes of eutrophication. There are a number of impacts of eutrophication.
- A decrease in biodiversity (the number of plant and animal species in an ecosystem) When a system is enriched with nitrogen, plant growth is rapid. When the number of plants increases in an aquatic system, they can block light from reaching deeper. Plants also consume oxygen for respiration, and if the oxygen content of the water decreases too much, this can cause other organisms such as fish to die.
- Toxicity Sometimes, the plants that flourish during eutrophication can be toxic and may accumulate in the food chain.
Note: Interesting Fact :
South Africa's Department of Water Affairs and Forestry has a 'National Eutrophication Monitoring Programme' which was set up to monitor eutrophication in impoundments such as dams, where no monitoring was taking place.
Despite the impacts, there are a number of ways of preventing eutrophication from taking place. Cleanup measures can directly remove the excess nutrients such as nitrogen and phosphorus from the water. Creating buffer zones near farms, roads and rivers can also help. These act as filters and cause nutrients and sediments to be deposited there instead of in the aquatic system. Laws relating to the treatment and discharge of sewage can also help to control eutrophication. A final possible intervention is nitrogen testing and modeling. By assessing exactly how much fertiliser is needed by crops and other plants, farmers can make sure that they only apply just enough fertiliser. This means that there is no excess to run off into neighbouring streams during rain. There is also a cost benefit for the farmer.
Discussion : Dealing with the consequences of eutrophication
In many cases, the damage from eutrophication is already done. In groups, do the following:
- List all the possible consequences of eutrophication that you can think of.
- Suggest ways to solve these problems, that arise because of eutrophication.
Chemical industry: Fertilisers
Why we need fertilisers
There is likely to be a gap between food production and demand in several parts of the world by 2020. Demand is influenced by population growth and urbanisation, as well as income levels and changes in dietary preferences.
The facts are as follows:
- There is an increasing world population to feed
- Most soils in the world used for large-scale, intensive production of crops lack the necessary nutrients for the crops
Conclusion: Fertilisers are needed!
The flow diagram below shows the main steps in the industrial preparation of two important solid fertilisers.
 |
- Write down the balanced chemical equation for the formation of the brown gas.
- Write down the name of process Y.
- Write down the chemical formula of liquid E.
- Write down the chemical formulae of fertilisers C and D respectively. The following extract comes from an article on fertilisers: A world without food for its peopleA world with an environment poisoned through the actions of man Are two contributing factors towards a disaster scenario.
- Write down THREE ways in which the use of fertilisers poisons the environment.
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