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.
Plants require nitrogen to make protein. The atmosphere is 3/4 nitrogen gas, but in this form it is inert. The bacteria oxidize the nitrogen by combining it with oxygen. Nitrates are water soluble compounds and are taken up through the plant's roots. Ammonia is produced when protein decomposes.