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.
Natural Soil Nitrogen Cycle. Dinitrogen is an essential element for the nitrogen cycle. The symbiotic microbes fix the dinitrogen (N2) into volatile ammonia, NH3, by the process of nitrogen fixation. The ammonia is further protonated to ammonium, NH4+, which is uptaken by plants to manufacture amino acids for growth.