Fertilisers as Salts

Introduction

A field of wheat removes large amounts of nutrients from the soil every harvest. To keep yields high, farmers replace them with fertilisers. Most of these are simply salts: ionic compounds made by neutralising an acid with a base. Understanding fertilisers is a chance to see acids, bases and salt preparation working at a scale of millions of tonnes per year, feeding roughly half of the world's population.

Core explanation

What plants need. Plants make sugars from carbon dioxide and water, but they also need elements from the soil. The three needed in the largest amounts are:

- Nitrogen (N) — for proteins and chlorophyll; promotes leaf growth. - Phosphorus (P) — for DNA and energy transfer; promotes root growth. - Potassium (K) — for enzyme activity and water balance; promotes flowers and fruit.

Roots absorb these elements as dissolved ions, such as NH₄⁺, NO₃⁻, H₂PO₄⁻ and K⁺. So a good fertiliser must be soluble in water . Almost all ammonium, nitrate and potassium salts are soluble, which is why they are chosen.

Fertilisers from ammonia. Ammonia, NH₃, is a weak base. It reacts with acids to form ammonium salts:

- With nitric acid: NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq), ammonium nitrate. - With sulfuric acid: 2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq), ammonium sulfate. - With phosphoric acid: 3NH₃(aq) + H₃PO₄(aq) → (NH₄)₃PO₄(aq), ammonium phosphate.

No water is formed in these neutralisations, because ammonia accepts an H⁺ ion directly to become NH₄⁺. In a school laboratory, a small sample of ammonium sulfate can be prepared by titrating dilute ammonia solution against dilute sulfuric acid, then repeating without indicator and crystallising the salt.

Ammonium nitrate contains nitrogen in both its ions, so it has a high nitrogen content (35%). It is widely used, but it must be stored carefully because in large quantities it can decompose explosively if contaminated or heated; strict regulations control how it is stored and sold.

Potassium and phosphorus. Potassium is usually supplied as potassium chloride, KCl, mined from underground deposits. Phosphorus comes from phosphate rock, which is insoluble; it is treated with acids to make soluble phosphates such as calcium dihydrogenphosphate ("superphosphate") or ammonium phosphate.

NPK labels. A bag labelled 20-10-10 contains 20% nitrogen and, by the traditional labelling convention, 10% "phosphate" and 10% "potash" (expressed as P₂O₅ and K₂O). Farmers choose ratios to match the soil and crop.

Environmental costs. Excess nitrate is very soluble and washes into rivers. It causes eutrophication : algae grow rapidly, block light, die and are decomposed by bacteria that use up dissolved oxygen, so fish die. Ammonium fertilisers also slowly acidify soil.

Formulae

Percentage of nitrogen by mass = (mass of N in one formula unit ÷ Mᵣ) × 100

Step-by-step reasoning

To choose the acid and base for a given ammonium fertiliser:

1. Identify the negative ion in the salt (nitrate, sulfate or phosphate). 2. Match it to its acid: nitrate from nitric acid, sulfate from sulfuric acid, phosphate from phosphoric acid. 3. The positive ion NH₄⁺ comes from ammonia. 4. Balance the equation so that the charges in the salt balance.

Visual explanation

Picture a fertiliser factory flow chart. Air and natural gas feed an ammonia plant. Some ammonia is oxidised to make nitric acid. The two streams meet in a neutraliser tank, producing ammonium nitrate solution, which is evaporated and formed into small pellets called prills. A side branch mixes ammonia with sulfuric acid to give ammonium sulfate crystals.

Real-world analogy

An NPK fertiliser is like a balanced meal for a crop. Nitrogen is the protein for building leaves, phosphorus is the energy drink for roots, and potassium keeps the whole system running. Feeding only one nutrient is like eating only bread: growth is limited by whatever is missing.

Real-world example

Garden "tomato feed" is high in potassium because potassium encourages fruit, while "lawn feed" is high in nitrogen to produce lush green leaves. Both are mixtures of soluble salts such as potassium nitrate, ammonium sulfate and ammonium phosphate.

Why?

Why are fertilisers made as soluble salts rather than using the elements themselves? Plants cannot absorb nitrogen gas, solid phosphorus or potassium metal. Roots take in dissolved ions from soil water, so the nutrient must be in an ionic, soluble form.

Common misconception

"Ammonia reacting with an acid gives a salt and water, like every neutralisation." Ammonia gives only a salt, because NH₃ accepts H⁺ to form NH₄⁺. Water forms only when a hydroxide, oxide or carbonate reacts with the acid.

Worked example

Question: Calculate the percentage of nitrogen by mass in ammonium nitrate, NH₄NO₃. (Aᵣ: N = 14, H = 1, O = 16)

Reasoning: Mᵣ = 14 + (4 × 1) + 14 + (3 × 16) = 80. Mass of nitrogen = 2 × 14 = 28. Percentage = (28 ÷ 80) × 100 = 35%.

Answer: 35% nitrogen.

Quick check

1. Which acid reacts with ammonia to make ammonium sulfate? Answer: Sulfuric acid, H₂SO₄.

Exam focus

Learn the three main nutrients and the ions that supply them. Be able to name the acid needed for each ammonium salt and write the balanced equation. Percentage-of-nitrogen calculations are common: take care to count all nitrogen atoms, including the one in NO₃⁻.

Advanced insight

Aqueous solutions of ammonium salts are slightly acidic, because NH₄⁺ can donate H⁺ to water. In soil, bacteria also convert NH₄⁺ into NO₃⁻, releasing further H⁺. This is why regular ammonium fertiliser use lowers soil pH, and why farms using it must also lime their fields.

Summary

Plants need nitrogen, phosphorus and potassium as soluble ions. Many fertilisers are salts: ammonia neutralises nitric, sulfuric and phosphoric acids to give ammonium nitrate, ammonium sulfate and ammonium phosphate, with no water formed. Potassium chloride and treated phosphate rock supply K and P. NPK blends match crop needs, but excess fertiliser causes eutrophication and soil acidification.

Practice questions

1. Write a balanced equation for the formation of ammonium phosphate from ammonia and phosphoric acid. Answer: 3NH₃ + H₃PO₄ → (NH₄)₃PO₄ 2. Calculate the percentage of nitrogen in ammonium sulfate, (NH₄)₂SO₄. (Aᵣ: N = 14, H = 1, S = 32, O = 16) Answer: Mᵣ = 132; mass of N = 28; percentage = (28 ÷ 132) × 100 ≈ 21.2%. 3. Why must a fertiliser be soluble in water? Answer: Plant roots absorb nutrients as dissolved ions from soil water, so an insoluble compound would not be taken up. 4. Explain how fertiliser run-off can lead to fish deaths in a river. Answer: Nitrates cause rapid algal growth; the algae die and bacteria decomposing them use up dissolved oxygen, so fish suffocate.