No, plants cannot directly fix nitrogen from the air. Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into a form that can be used by living organisms, such as plants. This conversion is carried out by certain microorganisms, primarily bacteria.

Nitrogen fixation is necessary because nitrogen is an essential nutrient for the growth and development of living organisms, including plants. It is a crucial component of proteins, nucleic acids, and other important molecules. However, atmospheric nitrogen is in a form (N2) that cannot be directly utilized by most organisms. It requires conversion into reactive forms such as ammonia (NH3) or nitrate (NO3-) for plants to use it.

There are two primary ways by which plants obtain nitrogen:

So, while plants do not fix nitrogen themselves, they rely on bacteria to convert atmospheric nitrogen into a usable form, either through symbiotic relationships with nitrogen-fixing bacteria or by absorbing nitrogen from the soil.

Mr. Gaurav H. Tandon has rightly answered and I wish to add the following to it :

Eukaryotes do not fix dinitrogen ( i.e. atmospheric N2). Only certain prokaryotes are known to fix dinitrogen. The plants can be benefited from these bacteria called diazotrophs through establishing loose/associative or intimate symbiosis. Because N2 molecule has three covalent bonds connecting two N atoms, therefore, a lot of energy is required to break these bonds before it can be assimilated into ammonia. These prokaryotes encode an enzyme Nitrogenase which converts dinitrogen into ammonia at the expense of 16 ATP molecules per mole of N2 reduced.

Plants, for their growth and development, largely rely on NO3 (nitrate ions) as nitrogen source because it is energetically less expensive and abundantly available in rhizosphere and non-cytotoxic. Ammoniacal form of nitrogen is alternative one but relatively cytotoxic. That is why all forms of nitrogen (other than nitrate) are microbially converted into nitrate so that plants can efficiently use it. However dinitrogen is the key source to all forms of nitrogen.

Though not essential, certain members of plant kingdom e.g. the legumes ( including food legumes) form N2 fixing root nodules with Rhizobium bacteria wherein dinitrogen present in the entrapped air is reduced to ammoniacal form ( through the nitrogenase enzyme of bacterial origin)and then fixed into a plant usable complex organic form. Similarly, blue green algae (a diazotroph) provides benefit to paddy plants grown under flooded condition through associative symbiosis. These processes are called as biological dinitrogen fixation (BNF). It is interesting to note that when a legume plant ( for example) has both nitrate nitrogen through soil and ammoniacal nitrogen through root nodule, then it will prefer Nitrate nitrogen only. Therefore, symbiotic nitrogen fixation is important when legumes ( or food legumes) are grown in nitrogen deficient soils.

Plants which do not form root nodules can thrive only on nitrogen available through soil. When nitrogen present in soil depletes to support plant growth ( e.g. crop ), we add fertilizer nitrogen such as urea. Urea is manufactured through ammoniacal nitrogen which in turn is produced through Haber's process. In this process the dinitrogen of the air combined with hydrogen ( in presence of catalyst and other physical conditions) to form ammonia. Here the cheapest source of getting hydrogen is fossil fuel therefore this makes the whole process of fertilizer production an expensive, threat-prone and non ecofriendly. On contrary, BNF relies on energy of photosynthetic origin which in turn relies in solar energy, a renewable form of energy. Therefore, BNF is most eco-friendly process.

The above is only basics of BNF. It is an active field of research since past 6 decades. Hope it is meaningful addition.

I agree with K Ram Krishna and Gaurav H Tandon that Nitrogen is present as in the atmosphere which cannot be absorbed by plants or animals. Through nitrogen fixation, bacteria like Azotobacter, Rhizobium converts into ammonia which can be assimilated by plants. Hence, nitrogen fixation solves the major issue of non-assimilation of atmospheric nitrogen. Nitrogen fixation occurs when bacteria are in the soil. They convert nitrogen to ammonium and nitrate, which plants absorb. Nitrogen is required by plants to produce amino acids, proteins, and DNA. Nitrogen is necessary because it is a component of chlorophyll. Nitrogen in its gaseous form (N2) can't be used by most living things. It has to be converted or 'fixed' to a more usable form through a process of fixation.Nitrogen is the nutrient that typically enhances the production of crop plants and promotes rapid vegetative development. Nitrogen is part of the chlorophyll molecule, which gives plants their green color and is involved in creating food for the plant through photosynthesis. Most organisms can't break the powerful triple bond of the N2 molecule's two atoms. For plants to grow and animals to thrive, they need the element in a reactive fixed form that is bonded to carbon, hydrogen, or oxygen, most often as organic nitrogen compounds ammonium (NH4), or nitrate (NO3). Over thousands of years they have developed intimate symbiotic relationships with bacteria such as rhizobia. In exchange for sugars produced by the plant, these bacteria take nitrogen gas in the air and convert it into a form that the plant can use directly. To become useful to plants, that nitrogen must first be "fixed," or busted out of its molecular form and linked with hydrogen to make ammonia. The plants can then get at it by catalyzing reactions with ammonia. But plants can't fix nitrogen. Nitrogen gas makes up about 78% of Earth's atmosphere, but like someone dying of thirst while lost at sea, plants are entirely incapable of absorbing it. Bacteria, on the other hand, have mastered the trick of fixing atmospheric nitrogen on multiple occasions.