Fertilisers

Plants need nitrogen (N) for proteins and growth, phosphorus (P) for roots, and potassium (K) for flowers and fruit. NPK fertilisers supply these as soluble salts such as ammonium nitrate, ammonium phosphate and potassium chloride. Ammonia from the Haber process is neutralised by acids to make them. Fertilisers boost crop yields, but run-off causes eutrophication.

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Section 1 of 14

Learning objectives

Name the three main elements in NPK fertilisers and their roles.
Calculate the percentage of nitrogen in a fertiliser.
Explain why bases should not be added with ammonium fertilisers.

Pause and think

A farmer spreads lime and ammonium sulfate on a field at the same time. Soon there's a smell of ammonia. Why is that a waste?

Section 2 of 14

Core idea

Start hereFeeding eight billion

Without synthetic fertilisers, the world could feed only around half its current population. Three elements, N, P and K, are behind that achievement.

The big idea

Plants need nitrogen, phosphorus and potassium. Fertilisers supply them as soluble salts, mostly made by neutralising ammonia with acids.

1. From air to field

Making ammonium nitrate

Drawing diagram…

Nitrogen from the air ends up in crops.

Worked exampleBest value fertiliser

Compare % N in ammonium nitrate ( $M_r$ 80) and ammonium phosphate, $\mathrm{(NH_4)_3PO_4}$ ( $M_r$ 149).

Goal: Which supplies more N per kg?
Strategy: Calculate both percentages.

$\frac{28}{80}\times100=35\%$
Two N in $\mathrm{NH_4NO_3}$ .
$\frac{42}{149}\times100=28.2\%$
Three N in $\mathrm{(NH_4)_3PO_4}$ .

AnswerAmmonium nitrate gives more nitrogen per kg, but ammonium phosphate also supplies phosphorus.

✓ CheckBoth below 50% as expected ✓.

Common misconceptionLime and ammonium salts

\mathrm{NH_4^++OH^-\rightarrow NH_3+H_2O}

: adding a base releases ammonia gas, wasting the nitrogen.

Pause and think

Why are fertilisers soluble salts?

Summary

N, P, K.
% N = mass of N ÷ $M_r$ × 100.
Don't mix with lime.

NPK fertiliser: Fertiliser supplying nitrogen, phosphorus and potassium.
Haber process: Industrial production of ammonia from nitrogen and hydrogen.

HistoryGuano. In the 1800s, nations fought over islands covered in bird droppings (guano), a rich nitrogen fertiliser. The Haber process ended the guano trade.

Fun factNitrogen in you. About half the nitrogen atoms in your body were fixed by the Haber process and reached you through fertilised crops.

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