Plants do not get their nitrogen directly from the air. Although nitrogen is the most abundant element in the air, every nitrogen atom in the air is triple-bonded to another nitrogen atom to form molecular nitrogen, N2. This triple bond is very strong and very hard to break (it takes energy to break chemical bonds whereas energy is only released when bonds are formed). As a result, even though nitrogen in the air is very common, it is energetically unfavorable for a plant to split the nitrogen molecule in order to get the raw atoms that it can use. The strong triple bond of N2 also makes it hard for molecular nitrogen to react with most other chemicals. This is, in fact, part of the reason there is so much nitrogen in the air to begin with. Also, the stability and symmetry of the nitrogen molecule makes it hard for different nitrogen molecules to bind to each other. This property means that molecular nitrogen can be cooled to very low temperatures before becoming liquid, leading liquid nitrogen to be a very effective cryogenic liquid.