Which form of nitrogen can be directly absorbed by plants?

Nitrogen is present in abundance in gaseous form in the air. However, plants cannot absorb nitrogen in this form. Soil has certain bacteria that convert gaseous nitrogen into a usable form and release it into the soil.

The atmospheric nitrogen is converted into ammonia in the soil. This ammonia is then converted into nitrite and then into nitrate. These nitrates are converted to protein in the plant and help plant growth.