Carboxyl groups have an electronegative oxygen atom double bonded to a carbon atom. This carbon-oxygen bond is very polar and the fact that its a double bond increases the polarity of the bond.
PEA agar, also referred to as Phenylethyl Alcohol Agar is a medium  used by microbiologists to grow microorganisms. The ingredient that  supplies nitrogen is casein. Making the world better, one answer at a time.
The most familiar examples of nitrogen-fixing symbioses are the root nodules of legumes (peas, beans, clover, etc.). Part of a clover root system bearing naturally occurring nodules of Rhizobium. Each nodule is about 2-3 mm long.