Fertilisers

1. Plants need a supply of certain elements from the soil in order to grow well.
2. They take these elements in through their roots as ions dissolved in the water in the soil.
3. When a crop is harvested, the elements stored in the plants are carried away from the field, so the soil gradually runs short of them.
4. Farmers add fertilisers to the soil to replace these elements, which gives improved plant growth and a larger crop.
5. A fertiliser must be soluble in water, so that its ions can dissolve in the soil water and be absorbed by the roots.
6. Ammonium salts and nitrates are used as fertilisers, because they are soluble and they supply the element nitrogen.
7. Plants use nitrogen to make proteins, so a shortage of nitrogen leads to small plants with pale yellow leaves.
8. Common nitrogen-containing fertilisers include the following salts.
   1. Ammonium nitrate, $\mathrm{N}{\mathrm{H}}_4\mathrm{N}{\mathrm{O}}_3$, which supplies nitrogen from both its ammonium ions and its nitrate ions.
   2. Ammonium sulfate, $(\mathrm{N}{\mathrm{H}}_4{)}_2\mathrm{S}{\mathrm{O}}_4$, which supplies nitrogen from its ammonium ions.
   3. Ammonium phosphate, $(\mathrm{N}{\mathrm{H}}_4{)}_3\mathrm{P}{\mathrm{O}}_4$, which supplies phosphorus as well as nitrogen.
   4. Potassium nitrate, $\mathrm{KN}{\mathrm{O}}_3$, which supplies potassium as well as nitrogen.
9. Ammonium salts are made by neutralising ammonia with an acid, for example $\mathrm{N}{\mathrm{H}}_3+\mathrm{HN}{\mathrm{O}}_3\to \mathrm{N}{\mathrm{H}}_4\mathrm{N}{\mathrm{O}}_3$.

NPK fertilisers

1. The three elements that plants need in the largest amounts are nitrogen, phosphorus and potassium.
2. Their symbols are $\mathrm{N}$, $\mathrm{P}$ and $\mathrm{K}$, so a fertiliser that provides all three is called an NPK fertiliser.
3. NPK fertilisers are used to provide these three elements for improved plant growth.
4. An NPK fertiliser is usually a mixture of several salts, because each element is supplied by a different ion.
   1. Nitrogen is supplied by ammonium ions, $\mathrm{N}{\mathrm{H}}_4^{+}$, and nitrate ions, $\mathrm{N}{\mathrm{O}}_3^{-}$.
   2. Phosphorus is supplied by phosphate ions, $\mathrm{P}{\mathrm{O}}_4^{3-}$.
   3. Potassium is supplied by potassium ions, ${\mathrm{K}}^{+}$, from salts such as potassium chloride, $\mathrm{KCl}$, and potassium sulfate, ${\mathrm{K}}_2\mathrm{S}{\mathrm{O}}_4$.
5. Each of the three elements supports a different part of plant growth.
   1. Nitrogen promotes the growth of healthy leaves and stems.
   2. Phosphorus promotes healthy root growth.
   3. Potassium promotes healthy flowers and fruit and helps the plant to resist disease.
6. A farmer chooses a fertiliser by matching the elements it provides to the elements that the soil and the crop are short of.
7. To decide which of the three elements a fertiliser provides, look at the symbols in its formula and pick out only $\mathrm{N}$, $\mathrm{P}$ and $\mathrm{K}$.