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.
Cover Crops/Green Manures. Rhizobia bacteria are primarily responsible for pulling nitrogen out of the air and making it available to plants, which then use the nitrogen to create proteins. The legume family (the pea & bean family) of plants are able to form a mutually beneficial relationship with rhizobia bacteria.