Nitrates, Fertilisers and the Environment

1A country that feeds its soil

In Season A of 2024, 64.5% of Rwandan farmers used inorganic (factory-made) fertiliser, up from 38.6% in Season A of 2022, according to the National Institute of Statistics of Rwanda (NISR). On the plots where it was used, the commonest types were DAP (43.7% of plots), urea (40.3%) and NPK (12.9%). Every one of these supplies nitrogen, and most of that nitrogen began as ammonia from the Haber process. This lesson looks at nitrates and the other nitrogen fertilisers: what they contain, how to compare them, and what happens when they escape from the field.

Figure 4.1: Percentage of farmers in Rwanda who used inorganic fertiliser, Season A 2022–2024 (data: NISR, Seasonal Agricultural Survey 2024 annual report).

2Nitrates and the action of heat

Nitrates are the salts of nitric acid; they contain the $\mathrm{N}{\mathrm{O}}_3^{-}\backslash mathrm\{NO\_3^-\}$ ion. All nitrates are soluble in water, which makes them quick-acting fertilisers but also means they wash out of the soil easily. Like carbonates, nitrates decompose on heating, and the products again depend on how reactive the metal is:

Table 4.1: The effect of heat on nitrates

Heating lead(II) nitrate is the classic way to make nitrogen dioxide: the white crystals crackle, brown fumes appear, and a yellow solid is left. The oxygen given off relights a glowing splint.

$$2\mathrm{P}\mathrm{b}(\mathrm{N}{\mathrm{O}}_3{)}_2(\mathrm{s})\to 2\mathrm{P}\mathrm{b}\mathrm{O}(\mathrm{s})+4\mathrm{N}{\mathrm{O}}_2(\mathrm{g})+{\mathrm{O}}_2(\mathrm{g})\backslash mathrm\{2Pb(NO\_3)\_2(s)\; \backslash rightarrow\; 2PbO(s)\; +\; 4NO\_2(g)\; +\; O\_2(g)\}$$

Table 4.2: Tests for the nitrate ion

The first two tests were introduced in S2. In the aluminium test, the nitrate is reduced all the way to ammonia, so check first that no ammonium ions are present: warm the solution with sodium hydroxide alone before adding the aluminium.

3Nitrogen fertilisers

Plants need large amounts of three nutrients: nitrogen (N) for leaves and proteins, phosphorus (P) for roots and ripening, and potassium (K) for flowers, fruit and resistance to disease. Fertilisers are compared by their percentage of nitrogen by mass:

$$\mathrm{\%}\mathrm{N}=\frac{\text{mass of nitrogen in one formula unit}}{M_r}\times 100\backslash \%\backslash ,\backslash mathrm\{N\}\; =\; \backslash frac\{\backslash text\{mass\; of\; nitrogen\; in\; one\; formula\; unit\}\}\{M\_r\}\; \backslash times\; 100$$

Table 4.3: Common nitrogen fertilisers

A compound fertiliser such as NPK 17-17-17 supplies all three nutrients. By convention, the numbers are the percentages of N, of ${\mathrm{P}}_2{\mathrm{O}}_5\backslash mathrm\{P\_2O\_5\}$ and of ${\mathrm{K}}_2\mathrm{O}\backslash mathrm\{K\_2O\}$: a 50 kg bag contains $0.17\times 50=8.5\mathrm{k}\mathrm{g}0.17\; \backslash times\; 50\; =\; 8.5\backslash ,\backslash mathrm\{kg\}$ of nitrogen.

4Environmental issues of nitrogen fertilisers

Fertilisers feed crops, but nitrogen that the crop does not take up does not stay where it was put.

• Eutrophication. Nitrates and phosphates washed into lakes and rivers feed algae and water plants, starting the chain of events in Figure 4.2.
• Nitrate in drinking water. Nitrate that soaks down into groundwater can reach wells and boreholes. The World Health Organization's guideline value is 50 mg of nitrate per litre, set to protect bottle-fed babies, whose blood can lose its ability to carry oxygen.
• Acid soils. Ammonium fertilisers make soil more acidic over the years, so farmers must add lime (as you saw in S2).
• Greenhouse gas. Soil bacteria turn part of the nitrogen into dinitrogen oxide, ${\mathrm{N}}_2\mathrm{O}\backslash mathrm\{N\_2O\}$, a powerful greenhouse gas.

Figure 4.2: How fertilisers washed into a lake cause eutrophication.

5Environmental issues of nitrogen dioxide

In S1 you saw that nitrogen oxides form in hot engines and help cause acid rain. Nitrogen dioxide also harms health directly: it irritates the airways, makes asthma worse and lowers resistance to lung infections, so it is worst beside busy roads. In sunlight it reacts with unburned fuel in the air to form photochemical smog, a brown haze containing ozone. Its main source is vehicle exhaust, so it is reduced by catalytic converters, well-tuned engines, and electric or public transport.

Using fertiliser wisely protects both the farmer's money and the environment: test the soil; apply the right amount at the right time, when the crop is growing and can take it up; split the dose instead of applying it all at once; avoid spreading it just before heavy rain; leave grass strips along rivers and lakes; and combine it with manure or compost and with legumes such as beans, whose root bacteria fix nitrogen.