Question

how do plants and synthesize protein which are nitrogenous compounds

Answer 1

Nitrogen is present in the atmosphere in large amounts, but plants can not absorb it directly. There are two ways by which plants can absorb nitrogen. They are as follows:
The soil bacterium Rhizobium is able to fix the atmospheric nitrogen in water soluble compounds. Plants absorb these compounds along with water in order to get nitrogen.
Plants can also get nitrogen from the nitrogen-rich fertilizers used by farmers to treat the soil.