What is present in the soil that makes nitrogen

directly available to plants?

In order for plants to assimilate nitrogen directly into their system, the nitrogen must be in the form of nitrates. Nitrogen in the air and ammonia from decayed matter in the soil is converted to ammonium by decomposers and nitrogen-fixing bacteria. The ammonium is further nitrified to nitrates by nitrifying bacteria. Finally, the nitrates can then absorbed by the plants.