Fertilisers Revision Notes

Fertilisers are substances added to soil to supply essential nutrients that plants need for growth. Nitrogen is one of the most important nutrients because it is a major component of amino acids, proteins, and chlorophyll, all vital for plant development.

Plants cannot use atmospheric nitrogen (N₂) directly. Instead, they absorb nitrogen mainly in the form of nitrate ions (NO₃⁻) and ammonium ions (NH₄⁺) from the soil. Fertilisers containing ammonium salts and nitrates provide these accessible nitrogen forms.

• Nitrogen is essential for plant growth.
• Plants absorb nitrogen as nitrate (NO₃⁻) or ammonium (NH₄⁺) ions.
• Fertilisers supply nitrogen in these forms to improve crop yield.

Ammonium salts contain the ammonium ion (NH₄⁺) combined with various anions. Common ammonium salts used as fertilisers include ammonium sulfate ((NH₄)₂SO₄) and ammonium chloride (NH₄Cl).

Nitrates contain the nitrate ion (NO₃⁻) combined with metal cations such as potassium, calcium, or sodium. Examples include potassium nitrate (KNO₃) and calcium nitrate (Ca(NO₃)₂).

Both ammonium salts and nitrates are soluble in water, allowing nitrogen ions to dissolve in soil moisture and be absorbed by plant roots.

• Ammonium salts contain NH₄⁺ ions, e.g. ammonium sulfate ((NH₄)₂SO₄).
• Nitrates contain NO₃⁻ ions, e.g. potassium nitrate (KNO₃).
• Both are water-soluble, facilitating nitrogen uptake by plants.

Ammonium salts supply nitrogen in the form of ammonium ions (NH₄⁺). These ions can be directly absorbed by plants or converted by soil bacteria into nitrate ions through nitrification.

For example, ammonium sulfate ((NH₄)₂SO₄) dissolves in soil water to release ammonium ions:

(NH₄)₂SO₄ (s) → 2 NH₄⁺ (aq) + SO₄²⁻ (aq) Plants absorb NH₄⁺ ions for protein synthesis and growth.

• Ammonium salts release NH₄⁺ ions in soil.
• NH₄⁺ ions are absorbed directly or converted to NO₃⁻ by bacteria.
• Example: (NH₄)₂SO₄ dissociates to 2 NH₄⁺ and SO₄²⁻ in soil solution.

Nitrate ions (NO₃⁻) are the preferred nitrogen source for many plants because they are readily absorbed and transported within the plant.

Nitrate fertilisers such as potassium nitrate (KNO₃) dissolve in soil water to release nitrate ions:

KNO₃ (s) → K⁺ (aq) + NO₃⁻ (aq) The nitrate ions are absorbed by roots and used to synthesise amino acids and proteins, promoting healthy plant growth.

• Nitrate fertilisers supply NO₃⁻ ions directly.
• NO₃⁻ ions are easily absorbed and transported by plants.
• Example: KNO₃ dissociates into K⁺ and NO₃⁻ in soil solution.

Both ammonium salts and nitrates supply nitrogen essential for plant growth but differ in form and soil behaviour.

Ammonium ions can acidify soil slightly and may be converted to nitrates by bacteria, while nitrates are immediately available but can leach from soil more easily.

Farmers choose fertilisers based on soil type, crop needs, and environmental considerations to optimise nitrogen availability and minimise pollution.

• Ammonium salts provide NH₄⁺ ions; nitrates provide NO₃⁻ ions.
• NH₄⁺ can be converted to NO₃⁻ by soil bacteria (nitrification).
• Nitrates are immediately available but more prone to leaching.
• Choice depends on soil conditions and crop requirements.

NPK fertilisers are formulated mixtures containing three primary nutrients essential for plant growth: nitrogen (N), phosphorus (P), and potassium (K). These elements are supplied in forms that plants can readily absorb from the soil, typically as soluble salts. The balanced provision of these nutrients ensures optimal plant development and maximises crop yield.

• NPK stands for Nitrogen (N), Phosphorus (P), and Potassium (K).
• These are macronutrients required in relatively large amounts by plants.
• Fertilisers supply these nutrients to replenish soil deficiencies.

Nitrogen is a vital component of amino acids, proteins, nucleic acids, and chlorophyll. It primarily promotes vigorous leaf and stem growth, resulting in lush green foliage. Nitrogen deficiency leads to stunted growth and yellowing of leaves (chlorosis).

In fertilisers, nitrogen is commonly supplied as nitrate ions (NO3⁻) or ammonium ions (NH4⁺), which plants absorb through their roots.

• Nitrogen is essential for protein synthesis and chlorophyll formation.
• Promotes rapid leaf and shoot growth.
• Common nitrogen sources in fertilisers include ammonium nitrate (NH4NO3) and potassium nitrate (KNO3).
• Example balanced equation for ammonium nitrate dissolving in soil water: NH4NO3 (s) → NH4⁺ (aq) + NO3⁻ (aq)

Phosphorus is a key element in energy transfer molecules such as ATP, nucleic acids, and phospholipids. It supports root development, flowering, and seed formation. Adequate phosphorus improves plant maturity and crop quality.

Phosphorus in fertilisers is often supplied as phosphate ions (PO4³⁻), for example, from compounds like calcium phosphate or superphosphate.

• Phosphorus aids in root growth and flower development.
• Essential for energy transfer within cells (ATP).
• Common phosphorus sources include single superphosphate (Ca(H2PO4)2) and triple superphosphate (Ca(H2PO4)2·H2O).
• Example reaction for superphosphate production: Ca3(PO4)2 + 2H2SO4 → Ca(H2PO4)2 + 2CaSO4

Potassium regulates the opening and closing of stomata, thus controlling water loss and uptake. It also activates enzymes involved in photosynthesis and respiration, enhancing disease resistance and overall plant health.

Potassium is supplied in fertilisers as potassium salts such as potassium chloride (KCl) or potassium sulfate (K2SO4).

• Potassium controls water regulation and stomatal function.
• Enhances enzyme activation and disease resistance.
• Common potassium sources include potassium chloride and potassium sulfate.
• Example dissolution: KCl (s) → K⁺ (aq) + Cl⁻ (aq)

The combined application of nitrogen, phosphorus, and potassium in NPK fertilisers ensures balanced nutrition for plants. Each nutrient supports distinct physiological processes that collectively improve growth, yield, and resilience.

Farmers select NPK fertilisers with different nutrient ratios depending on soil tests and crop requirements.

• Nitrogen: promotes leaf and stem growth; supplied as ammonium or nitrate salts.
• Phosphorus: supports root and flower development; supplied as phosphate salts.
• Potassium: regulates water balance and disease resistance; supplied as potassium salts.
• Balanced NPK fertilisers enhance overall plant productivity.