Kelps are large brown algae seaweeds. There are about 30 different genear. Despite its appearance, kelp is not a plant, it is a heterokont. Kelps grows in underwater forests in shallow water. The organisms require nutrient rich water with temperaturs between 6 and 14 °C. They are known for their high growth rate. Macrocystis and Nereocystis can grow as fast as half a metre a day. In the 19th century the word kelp was associated with seaweeds that could be burned to obtain soda ash. The word kelp was also used directly to refer to these processed ashes.