Nutrients and Fertilisers Revision Notes for Grade 12 NSC Matric Physical Sciences

Nutrients and Fertilisers

Introduction

Plants require specific nutrients to grow and survive. As populations increase and farming demands grow, the soil often cannot naturally provide enough nutrients fast enough to support crop production. This is where fertilisers become essential - they help restore and supplement soil nutrients to maintain healthy crop yields.

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With the world's population continuing to grow, agricultural demands are increasing rapidly. This puts enormous pressure on soil systems to provide adequate nutrition for crops, making fertilisers increasingly important for food security.

What are nutrients?

Nutrients are chemical substances that organisms need for metabolism and proper functioning. Plants absorb these nutrients from their environment to support growth, reproduction, and survival.

Types of nutrients

There are two main categories of plant nutrients:

Non-mineral nutrients

These nutrients come from the air and water, not from the soil:

Carbon (C): Obtained from carbon dioxide in the air
Hydrogen (H): Obtained from water
Oxygen (O): Obtained from water

All three function as components of organic molecules like carbohydrates, lipids, and proteins.

Mineral nutrients

These nutrients must be absorbed from the soil in dissolved form. They are divided into:

Macronutrients - needed in large quantities:

Nitrogen (N): Found in nitrogen compounds in soil, essential for proteins and chlorophyll
Phosphorus (P): Found in phosphate compounds, needed for photosynthesis and flowering
Potassium (K): Found in potassium compounds, helps with cell building and disease resistance

Micronutrients - needed in small amounts, including iron (Fe), chlorine (Cl), copper (Cu), and zinc (Zn).

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The key difference is that macronutrients are required in large quantities whilst micronutrients are only needed in small amounts for plants to function properly. Understanding this distinction is crucial for proper fertiliser application.

What are fertilisers?

Fertilisers are chemical compounds given to plants to promote growth. They typically provide the three major plant nutrients: nitrogen, phosphorus, and potassium.

The fertiliser industry produces fertilisers from raw materials like nitric acid, phosphate rock, and sulphuric acid. These fertilisers supply nutrients that support life on earth.

Types of fertilisers

There are two main types:

Organic fertilisers are made from natural products like manure, compost, bone meal, or seaweed. These materials started as part of living organisms.

Inorganic fertilisers refer to industrially produced compounds like ammonium nitrate or super phosphates. These have the advantage of being in a soluble form that plants can absorb immediately.

Why fertilisers are needed

Plants can only absorb nutrients when they are dissolved in water. For example, nitrogen gas ( $N_2$ ) cannot be absorbed directly - it must be converted to ions like nitrate ( $NO_3^-$ ) or phosphate ( $PO_4^{3-}$ ).

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Critical Solubility Requirement: Plants can ONLY absorb nutrients when they are dissolved in water. Nitrogen gas ( $N_2$ ) cannot be absorbed directly - it must be converted to ions like nitrate ( $NO_3^-$ ) or phosphate ( $PO_4^{3-}$ ). This is why fertilisers must be in a water-soluble form.

Natural processes like the nitrogen cycle do replace soil nutrients, but this takes a long time. Modern farming practices often don't allow enough time for natural nutrient replacement, so fertilisers play a crucial role in restoring soil nutrients quickly.

The NPK ratio

Fertiliser packaging shows a set of numbers called the NPK ratio. These numbers indicate the mass ratio of nitrogen (N), phosphorus (P), and potassium (K) in the fertiliser.

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Understanding NPK Ratios:

A fertiliser with NPK ratio 3:1:5 (38) means:

The ratio of N:P
is 3:1:5
38% of the total fertiliser mass consists of these nutrients
The remaining 62% consists of fillers and other compounds

Understanding fertiliser grades

Different fertilisers have different NPK ratios suitable for various purposes:

Functions of N, P, and K

Each nutrient serves specific purposes:

Nitrogen (N): Promotes leafy growth - best for lawns and leafy plants
Phosphorus (P): Supports root development and flowering - ideal for flower beds
Potassium (K): Enhances disease resistance and fruit development - excellent for fruit-bearing plants

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Choose your fertiliser based on what you want to achieve: high nitrogen for lush green growth, high phosphorus for strong roots and flowers, or high potassium for healthy, disease-resistant plants with good fruit production.

Calculating nutrients from NPK ratios

Worked example 1: Mass calculations

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Worked Example: Mass Calculations

Question: Calculate the mass of nitrogen (N), phosphorus (P), and potassium (K) in 500g of industrial fertiliser with NPK ratio 5:2:3 (40).

Solution:

Step 1: Determine the total nutrient mass 40% of 500g = $0.4 \times 500g = 200g$ contains nutrients

Step 2: Calculate individual masses using the ratio

Total ratio parts = 5 + 2 + 3 = 10 parts
Nitrogen: $\frac{5}{10} \times 200g = 100g$
Phosphorus: $\frac{2}{10} \times 200g = 40g$
Potassium: $\frac{3}{10} \times 200g = 60g$

Worked example 2: Mole ratio calculations

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Worked Example: Mole Ratio Calculations

Question: Calculate the mole ratio of an industrial fertiliser with NPK ratio 5:2:3 (40).

Solution:

Step 1: Use masses from previous example

Mass of N = 100g
Mass of P = 40g
Mass of K = 60g

Step 2: Calculate moles for each element

Moles of N = $\frac{100g}{14.0 \text{ g.mol}^{-1}} = 7.14 \text{ mol}$
Moles of P = $\frac{40g}{31.0 \text{ g.mol}^{-1}} = 1.29 \text{ mol}$
Moles of K = $\frac{60g}{39.1 \text{ g.mol}^{-1}} = 1.53 \text{ mol}$

Step 3: Determine the mole ratio N:P

mole ratio = 7.14 : 1.29 : 1.53

Dividing by the smallest number (1.29): Mole ratio = 5.5 : 1 : 1.2 ≈ 55 : 10 : 12

Worked example 3: Compound ratios

Some countries express NPK ratios in compound form (N:P₂O₅:K₂O).

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Worked Example: Compound Ratios

Question: Calculate the moles of phosphorus (P) in 120g of fertiliser with N:P₂O₅:K₂O mass ratio of 4:3:8 (50).

Solution:

Step 1: Calculate nutrient mass 50% of 120g = 60g contains nutrients

Step 2: Calculate mass of P₂O₅ Total ratio parts = 4 + 3 + 8 = 15 Mass of P₂O₅ = $\frac{3}{15} \times 60g = 12g$

Step 3: Calculate moles of P₂O₅ Molar mass of P₂O₅ = $(2 \times 31.0) + (5 \times 16.0) = 142.0 \text{ g.mol}^{-1}$ Moles of P₂O₅ = $\frac{12g}{142.0 \text{ g.mol}^{-1}} = 0.0845 \text{ mol}$

Step 4: Calculate moles of P Since each P₂O₅ contains 2 P atoms: Moles of P = $2 \times 0.0845 \text{ mol} = 0.17 \text{ mol}$

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Key Points to Remember:

Nutrients are essential chemicals that plants need for growth - they include both mineral nutrients (from soil) and non-mineral nutrients (from air and water)
Fertilisers provide the three major nutrients - nitrogen (N) for leafy growth, phosphorus (P) for roots and flowers, and potassium (K) for disease resistance
NPK ratios show the mass proportion of nitrogen, phosphorus, and potassium in fertilisers, helping you choose the right fertiliser for specific plant needs
Organic fertilisers come from natural sources like compost, while inorganic fertilisers are industrially produced compounds that dissolve quickly
Always convert ratios to actual masses when calculating nutrient content - remember to account for the percentage purity and use molar masses for mole calculations

Nutrients and Fertilisers (Grade 12 NSC Matric Physical Sciences): Revision Notes