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.
Nitrogen is available to plants as either ammonium (NH4+-N) or nitrate (NO3--N). Animal manures and other organic wastes can be important sources of N for plant growth. The amount of N supplied by manure will vary with the type of livestock, handling, rate applied, and method of application.