Chemistry of the Environment - Fertilisers (NPK)

Plants require three essential elements for healthy growth: Nitrogen ($N$), Phosphorus ($P$), and Potassium ($K$). These are often supplied via NPK fertilisers.

Nitrogen ($N$) is vital for the synthesis of proteins and chlorophyll, promoting healthy leaf and stem growth. Common sources include $NH_{4}NO_{3}$ and $(NH_{4})_{2}SO_{4}$.

Phosphorus ($P$) is essential for root development and energy transfer (ATP). It is often supplied as ammonium phosphate $(NH_{4})_{3}PO_{4}$ or calcium dihydrogenphosphate $Ca(H_{2}PO_{4})_{2}$.

Potassium ($K$) helps in the regulation of water movement and enzyme activation, which is critical for flower and fruit production. It is typically supplied as $KCl$ or $K_{2}SO_{4}$.

Ammonium salts are prepared by reacting ammonia ($NH_{3}$) with an appropriate acid. For example, $NH_{3} + HNO_{3} \rightarrow NH_{4}NO_{3}$.

Ammonia displacement: If an alkali (like lime, $Ca(OH)_{2}$) is added to soil that has been treated with ammonium fertilisers, a reaction occurs that releases ammonia gas ($NH_{3}$), leading to a loss of nitrogen from the soil: $NH_{4}^{+}(aq) + OH^{-}(aq) \rightarrow NH_{3}(g) + H_{2}O(l)$.