Fertilisers (AQA GCSE Chemistry): Revision Notes
Fertilisers
What are fertilisers?
Plants require specific nutrients to grow properly. The most important ones are nitrogen (N), phosphorus (P), and potassium (K). These are called NPK fertilisers because they provide all three essential elements.
Fertilisers add these important compounds to soil so that crops can grow better and produce higher yields.
NPK fertilisers are fundamental to modern agriculture because they provide the three nutrients that plants need in the largest quantities. Without adequate supplies of nitrogen, phosphorus, and potassium, crop yields would be severely limited.
Phosphates
What are phosphate rocks?
Phosphate rocks are natural minerals that contain the phosphate ion (). These rocks come in different types with various chemical formulas, but they all contain this important phosphate ion.
The problem is that phosphate rocks are insoluble - plants can't absorb the nutrients directly from them.
The key challenge with phosphate rocks is their insolubility. Even though they contain essential phosphorus, plants cannot access these nutrients until the rocks are chemically treated to create soluble compounds.
Making phosphates soluble
To make fertilisers that plants can use, phosphate rocks must be treated with acids to create soluble phosphate compounds. Here's how this works:
The phosphoric acid can then be mixed with ammonia to make ammonium phosphate, which is an excellent soluble fertiliser.
Chemical Process: Converting Insoluble Phosphate to Soluble Fertiliser
Step 1: Treat phosphate rock with nitric acid
Step 2: React phosphoric acid with ammonia
Result: Water-soluble ammonium phosphate fertiliser that plants can absorb
Why solubility matters
Fertilisers are often applied by spraying a solution onto crops. The compounds must be soluble so they dissolve in water. Potassium chloride () and potassium sulphate () are good examples - they're obtained by mining and are already soluble, so they can be used directly as fertilisers.
The ability to dissolve fertilisers in water makes application much more efficient and ensures even distribution across crops. Liquid fertiliser solutions can be applied using spray systems, allowing farmers to treat large areas quickly and uniformly.
Industrial production of fertilisers
Raw materials and processes
The industrial production of NPK fertilisers uses an integrated manufacturing process with several connected steps:
Industrial Process: NPK Fertiliser Production
Step 1: Raw materials preparation
- Nitrogen and hydrogen are used to make ammonia in the Haber process
Step 2: Making nitric acid
- Some ammonia is oxidised to produce nitric acid
Step 3: Processing phosphate rock
- Raw phosphate rock reacts with nitric acid to make phosphoric acid and calcium nitrate
Step 4: Final fertiliser production
- Phosphoric acid reacts with more ammonia to produce ammonium phosphate
This integrated approach is efficient because products from one step become raw materials for the next step.
Laboratory preparation
In the laboratory, you can make ammonium phosphate salt by reacting a solution of ammonia with dilute phosphoric acid. This demonstrates the same chemical principle used in industrial production, just on a smaller scale.
Calculating percentage by mass
Why this matters
Understanding percentage by mass calculations is essential for determining the actual nutrient content in fertiliser compounds. This helps farmers choose the most cost-effective fertilisers and apply the correct amounts.
You need to be able to calculate what percentage of a fertiliser compound is made up of useful nutrients like phosphorus.
Method
To find the percentage by mass of an element in a compound:
Worked Example: Percentage by Mass Calculation Method
Step 1: Calculate the relative formula mass of the whole compound
- Add up all the relative atomic masses in the formula
Step 2: Find the mass contribution of the element you're interested in
- Multiply the relative atomic mass by the number of atoms of that element
Step 3: Apply the formula
Example
Worked Example: Percentage of Phosphorus in Calcium Phosphate
For calcium phosphate :
Step 1: Find relative atomic masses
- H = 1, O = 16, P = 31, Ca = 40
Step 2: Calculate total relative formula mass
- Ca:
- H:
- P:
- O:
- Total = 40 + 4 + 62 + 128 = 234
Step 3: Find mass of phosphorus atoms
Step 4: Calculate percentage
Key Points to Remember:
- NPK fertilisers provide nitrogen, phosphorus and potassium - the three essential plant nutrients
- Phosphate rocks must be treated with acids to make them soluble for plants to use
- Industrial production uses integrated processes where ammonia is made first, then converted to other useful compounds
- Percentage by mass calculations help us work out how much useful nutrient is in a fertiliser compound
- Soluble fertilisers can be sprayed as solutions, making them easy to apply to crops