DAVID NUNUK / SCIENCE PHOTO LIBRARY DAVID NUNUK / SCIENCE PHOTO LIBRARY
A tractor spreading pig manure on a field. This is usually done before ploughing and planting a new crop during the winter season. Pig manure contains nitrogen compounds which are used by the plants for their growth. An inappropriate use of nitrogen fertilisers represents a major hazard for the environment if nitrogen compounds find their way to ground waters, rivers and lakes.
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