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.
Sources and Remedies. The best source of nitrogen is organic matter or compost. This, however, is somewhat slow acting because the micro herd must break it down before your plants can use it. Other good sources of nitrogen are green manures, blood meal, alfalfa meal, and fish emulsion. If your plants are in immediate need of a nitrogen fix, the best thing you can do is mix up a batch of fish emulsion and foliar feed them. Donât go overboard on this because, as explained, a nitrogen excess is almost as bad as a deficiency.