3.What is found around the roots of some plants that turns nitrogen from the form we cannot use to the form we can use?

Bacteria

Explanation:

Some plants, specifically leguminous plants form a symbiotic relationship with certain bacteria species called NITROGEN-FIXING BACTERIA in their root nodules. In this relationship, the bacteria species e.g nitrosomonas, nitrobacter etc help the plants fix atmospheric nitrogen (N2) or ammonium compounds, which are forms of nitrogen that are unusable to the plant into nitrates, a usable form.

Therefore, BACTERIA are the organisms found around the roots of these set of plants (legumes) that turns nitrogen from the form we cannot use to the form we can use.