Question

Why do plant use the nitrogen present in soil

Answer 1

Certain organisms are capable of converting gaseous nitrogen into more usable compounts such as ammonia and nitrates. These are left in the soil or end up dissolved in ground water. Either way, the compounds are then absorbed through the roots of the plants. Once the plant dies, its nitrogen compounds are also converted back into ammonia or nitrate. They are then returned to the soil and eventually used by another plant.Wikipedia has a fairly decent explanation of how this works, though other sources are no doubt just as suitable. The process of creating nitrogen compounds is known as nitrogen fixation, if you’re interested. You can also look up the nitrogen cycle.