Page 19 - Curriculum Visions Dynamic Book

 A wide range of leguminous plants, for example these tropical acacias, exists in the world.
 Blue–green algae in the water that floods a paddy field are able to fix nitrogen.
 Nodules on a nitrogen- fixing legume.
fixing: making solid and liquid nitrogen-containing compounds from nitrogen gas. The compounds that are formed can be used as fertilisers.
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The importance of natural sources of fixed nitrogen About one kilogram of nitrogen may be fixed (made into nitrogen-containing compounds) per hectare of the Earth’s surface per year by lightning flashes.
However, the majority of nitrogen is fixed in the soil. Bacteria in legume nodules extract about fifty kilograms of nitrogen per hectare from air in the soil, while other forms
of bacteria not connected
to legumes fix a further five kilograms per hectare per year.
Yet despite these seemingly large amounts of nitrogen fixed from the air, they only make
up about one-third of the total requirements of growing plants, and the remainder must come from recycling nitrogen as compost and manure or from artificial supplies.