Chemistry (0620) | Fertilisers

🌱 Chemistry of the Environment: Fertilisers (Topic 10.2)

Hello future chemists! This chapter is all about how we keep our crops healthy and feed the world, which is a huge part of environmental chemistry. Don's worry if you don't have a green thumb—we just need to understand the chemistry behind why plants need certain elements to thrive!

What are Fertilisers and Why Do We Need Them?

Think of growing a plant like cooking a meal. It needs basic ingredients (carbon dioxide and water for photosynthesis), but also special vitamins and minerals to grow big and strong.

Fertilisers are substances added to soil to increase the supply of essential mineral elements required for plant growth.

Why do we need them if soil already contains nutrients?

• When crops are harvested, the essential elements stored in them are removed from the field.
• Over time, the soil becomes depleted (runs out) of these key nutrients.
• Fertilisers replace these lost elements, ensuring the soil remains fertile for future crops and leading to improved plant growth and higher crop yields.

The Essential Three: NPK

The syllabus focuses on three major elements that plants absolutely must have. When you buy fertiliser from a garden shop, you will often see three numbers on the bag—this refers to the percentage content of N, P, and K. This is known as an NPK fertiliser.

The Role of N, P, and K in Plant Growth

The elements provided by NPK fertilisers are:

1. N (Nitrogen)
   

   Function: Nitrogen is essential for producing proteins and chlorophyll. Chlorophyll is needed for photosynthesis, giving the plant its green colour.
   Use for Plant: Promotes strong, healthy stem and leaf (foliage) growth.

   Memory Trick: N is for Nice and Green Leaves!


2. P (Phosphorus)
   

   Function: Phosphorus is crucial for respiration and energy transfer (like making the plant’s 'fuel'). It is stored as phosphate compounds.
   Use for Plant: Promotes strong root growth and helps with the development of flowers and fruit.

   Memory Trick: P is for Powerful Roots!


3. K (Potassium)
   

   Function: Potassium (supplied as soluble potassium salts) helps regulate water movement and enzyme activity within the plant.
   Use for Plant: Promotes overall health, resistance to disease, and quality of fruits/grains.

   Memory Trick: K is for King Health and Quality!

Key Takeaway: A balanced NPK fertiliser ensures the plant has all the fundamental building blocks it needs for growth, energy, and survival.

Types of Fertilisers Used

To supply these vital NPK elements, chemists rely on specific compounds. The syllabus requires you to know that two major classes of compounds are used to supply the essential Nitrogen (N):

1. Ammonium Salts
   

   These are salts containing the ammonium ion (\( \text{NH}_{4}^{+}\)). When dissolved in water in the soil, the ammonium ion is released, providing nitrogen that plants can absorb.
   Example: Ammonium sulfate, \( \text{(NH}_{4}\text{)}_{2}\text{SO}_{4} \).



2. Nitrates
   

   These are salts containing the nitrate ion (\( \text{NO}_{3}^{-} \)). Nitrates are very soluble and easily absorbed by plant roots. They are often the most effective form of nitrogen for rapid growth.
   Example: Potassium nitrate, \( \text{KNO}_{3} \).

What is an NPK Fertiliser?

An NPK fertiliser is simply a compound or a mixture of compounds that provides all three primary nutrients (Nitrogen, Phosphorus, and Potassium).

• The N often comes from ammonium salts or nitrates.
• The P often comes from soluble phosphates (like ammonium phosphate).
• The K often comes from potassium chloride or potassium sulfate.

Common Mistakes to Avoid

When answering exam questions on fertilisers, remember these points:

1. NPK vs. Chemical Formulae: When asked what elements NPK provides, state the names: Nitrogen, Phosphorus, and Potassium. Do not just write the symbols N, P, K.
2. Source of N: Know that the main chemical types used to provide Nitrogen are ammonium salts and nitrates.
3. Use of Fertilisers: Always link the use of NPK to improved plant growth and higher crop yields, not just "feeding the plant."