MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Fertilisers. Two bags of fertiliser stand against a fence. These fertilisers are of the NPK type, so called because they generally contain nitrogen, phosphorus and potassium (chemical symbol K). The bag on the left has a mixture of all three of these ingredients, while the one on the right does not contain any nitrogen. These fertilisers are used as plant foods. Plants require these three nutrients in large amounts to ensure healthy growth. Undernourished plants tend to be stunted with limp, yellow leaves. Nitrogen is sometimes left out of fertilisers due to its effects as a pollutant. If it enters rivers and streams, it can choke them by causing unchecked growth of algae.
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