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Fertilisers

For healthy growth and development plants need to take in a variety of minerals through their roots from the soil. These minerals include: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S) ,iron (Fe), chloride (Cl), manganese (Mn), zinc (Zn), copper (Cu), nickel (Ni) as well many other elements. Most of these elements are found as soluble ionic compounds in the soil. This means that plants are absorbing essential ions from the soil through their roots.

NPK

Perhaps the three most essential elements from the list above that a plant requires in relatively large amounts to stay healthy are nitrogen (N), phosphorus (P) and potassium (K). The image below shows why these three elements are essential. Basically nitrogen will make "everything" above the soil grow well, phosphorus will take care of "everything" below the soil and potassium helps the plant to tackle infections and deal with the stresses due environmental issues such as drought. Potassium generally improves the vitality of the plant

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Nitrogen and nitrates

You may think that plants could simply use the nitrogen from the air as their source of this particular essential element. Unfortunately this is not the case; simply because atmospheric nitrogen is a very unreactive gas and most plants except some plants called leguminous plants are unable to use atmospheric nitrogen. Leguminous plants such as clover and some bean plants have nodules (swellings) on their roots that contain bacteria that can convert atmospheric nitrogen into nitrates (NO₃^-); which acts as a source of nitrogen for the plant. Most plants however rely on a source of nitrate ions (NO₃^-) in the soil which they take in through their roots, these nitrate ions acts as a source of nitrogen for the plant. This nitrogen is essential for the manufacture of proteins by the plant.

Common fertilisers

Farmers need to replace essential minerals that are removed from the soil when crops are harvested. If the soil is not fertilised then any new crops which are planted will likely grow and develop in a poor condition and crop yields would likely be low due to the lack of essential nutrients and minerals in the soil. Farmers will add fertilisers to their fields to ensure there are sufficient minerals present for healthy plant growth. Most fertilisers are solids; normally powders or often they consist of small granules called prills. Fertilisers are mostly ionic compounds such as:

• Ammonium nitrate - NH₄NO₃ - supplies nitrogen to the plant roots
• Ammonium sulfate - (NH4)₂SO₄ - supplies nitrogen to the plant roots
• Ammonium phosphate - (NH4)₃PO₄ - supplies nitrogen and phosphorus to the plant roots
• Potassium sulfate - K₂SO₄ - supplies potassium to the plant roots
• Potassium chloride - KCl -supplies potassium to the plant roots
• Calcium nitrate - Ca(NO3)₂ supplies nitrogen and calcium to the plant roots
• Calcium sulfate - CaSO₄ supplies calcium to the plant roots
• Sodium nitrate - NaNO₃ supplies nitrogen and sodium to the plant roots
• Urea - CO(NH₂)₂ -supplies nitrogen to the plant roots

Different fertilisers for different crops

Most fertilisers are sold as a mixture of many different compounds; they will likely contain many of the substances from the list above. These different mixes of compounds will contain different amounts of nitrogen, phosphorus and potassium. Depending on what crops are being grown and on the condition of the soil the farmer will chose a fertiliser with a specific N:P:K content that will maximise his yield of crops. In the image below you can see that the N:P:K content of each of the fertilisers is different; so each particular fertiliser maybe suitable for use with different crops or soil types.

What do NPK values mean?

In the image above the bags of fertiliser have labels that indicate the amount of nitrogen (N), phosphorus(P) and potassium present in each fertiliser bag, but what do these numbers actually mean and how can we use them to calculate the amount of nitrogen, phosphorus and potassium present in the fertiliser? Well complete the activity below to answer this question:

Making fertilisers from phosphate rock

Phosphorus is one of the three main essential elements needed by plants. Phosphate rock is a sedimentary rock that contains high amounts of minerals rich in phosphorus. It is mined in many parts of the world and contains minerals such as calcium phosphate. However phosphate rock is insoluble and so cannot be taken in by the plants roots. To be useful as a source of phosphorus for the plant the insoluble phosphate rock needs to be converted into soluble compounds containing phosphate ions (PO₄^3-) which will supply the phosphorus needed by the plant. To produce soluble compounds containing the phosphate ion (PO₄^3-) the phosphate rock is reacted with acids e.g.

Phosphate rock reacting with sulfuric acid:

Phosphate rock + sulfuric acid → calcium sulfate + calcium phosphate

The mixture of calcium sulfate and calcium phosphate compounds is often called superphosphate orsingle superphosphate.

Phosphate rock reacting with nitric acid acid:

Phosphate rock + nitric acid → calcium nitrate + phosphoric acid

And finally

Phosphate rock reacting with phosphoric acid:

Phosphate rock + phosphoric acid→ calcium phosphate (often called triple superphosphate).

Self-check: Quick quiz- Do you know your superphosphate from your triple superphosphate? Take the quick quiz below by simply clicking the button.

Industrial preparation of fertilisers

The actual reactions taking place here are fairly complex and are very unlikely to be asked for in a gcse chemistry paper. As an example consider the industrial preparation of superphosphate (calcium sulfate/calcium phosphate mixture). The bullet points below give a rough outline of the processes that need to be considered in the manufacture of superphosphate fertiliser.

• Step 1- Would be the production of the acid needed e.g. sulfuric acid. Manufacturing sulfuric acid on a large scale would involve the oxidation of large amounts of sulfur to produce toxic gases such as sulfur dioxide.
  
  The sulfur dioxide gas would then be further oxidised and then dissolved in water to form sulfuric acid. These reactions produce heat, that is they are exothermic. In industry heat is never be wasted and this heat will be used later in the manufacturing process to evaporate and dry the final solid fertiliser produced or to drive turbines to produce electricity which can be used elsewhere in the chemical plant. The acid produced here would be much more concentrated than that used in the lab.


• Step 2 - The phosphate rock would have to be tested and quality assured to be certain it contains the correct minerals in the correct proportions. It may need further treatment before it can be used. The phosphate rock used will most likely have been imported from various locations around the world and so will likely have different compositions. The phosphate rocks will be blended and mixed to ensure that it contains the required minerals needed to produce a quality product with the correct amounts of all essential minerals and nutrients.


• Step 3- The mixed phosphate rocks will be crushed down to produce small grains which will react completely with the sulfuric acid. Crushing the rock increases its surface area; this will speed up the reactions taking place.


• Step 4 - The phosphate rock and sulfuric acid would be mixed and allowed to react. The products would then be removed and any waste products would have to be separated out and removed. The final solid fertiliser would be dried. Finally the solid fertiliser would be turned into granules by heating with steam. These chemical plants producing the fertiliser will operate on a continual basis, 24 hours a day every day unlike the lab process which is a small scale batch (one-off) process.