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.
Alfalfa is a legume, a nitrogen fixing plant that can extract nitrogen from the atmosphere and incorporate it into plant proteins. Other legumes include clovers, vetches, peas, beans, and birdsfoot trefoil.