The answer is; cannot assimilate nitrogen
Plants require nitrate for growth but cannot absorb and assimilate nitrogen from the atmosphere in its N2 of NO2 form. Therefore they require the help of nitrifying bacteria that are capable of fixing nitrogen in the soil in the form of nitrates that can be absorbed by plants. Leguminous plants have a symbiotic relationship with these bacteria that reside in the nodes –root nodules- of the plant.