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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), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), nickel (Ni) as well many other elements. Most of these elements are found as ionic compounds in the soil. This means that plants are absorbing essential ions from the soil through their roots.

Perhaps the three most essential elements from the list above that a plant requires in relatively large amounts are nitrogen (N), phosphorus (P) and potassium (K). The image opposite 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 plan to tackle infection deal with the stresses due environmental issues. Potassium genrally improves the vitality of the plant

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 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. The nitrogen is essential for the manufacture of proteins by the plant.

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 be in a poor condition and crop yields would likely be low. 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) -supplies nitrogen to the plant roots

Most fertilisers are a mixture of many different compounds; most 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.

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. Limestone and mudstone are two common examples of phosphate rocks. 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 + sulfuric acid → calcium sulfate + calcium phosphate

The mixture of calcium sulfate and calcium phosphate is often called superphosphate or single superphosphate.

Phosphate rock + nitric acid → calcium nitrate + phosphoric acid

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

The actual reactions taking place here are fairly complex and are unlikely to be asked for in a gcse chemistry paper. As an example consider the industrial preparation of superphosphate (calcium sulfate/calcium phosphate mixture).

• Step 1- would be the production of sulfuric acid. This would involve burning large amounts of sulfur to produce toxic gases such as sulfur dioxide. These gases would be further processed and then dissolved to form sulfuric acid. These reactions produce heat, they are exothermic. In industry heat must never be wasted and this heat will be used later 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 used would be much more concentrated than that used in the lab, so more heat would be produced.
• Step2 - the phosphate rock would have to be tested to ensure it contains the correct minerals in the correct proportions. It may need further treatment. The phosphate rock will most likely have been imported from various locations around the world and so will 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.
• Step3- the mixed phosphate rocks will be crushed down to produce small grains which will react completely with the sulfuric acid.
• Step4 - the phosphate rock and sulfuric acid would be mixed and allowed to react. The products would be removed, and dried. Finally they would be turned into granules by heating with steam. These chemical plants operate on a continual basis and operate 24 hours a day every day unlike the lab process which is a small scale batch (one-off) process.