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.
Custom mixes can also be made for addressing the specific needs of your plants. For instance, for nitrogen sources you can use fish meal, chicken feather meal, blood meal, composted chicken manure, bat guano or bird guano. Greensand and kelp are good potassium organic sources and a good phosphorus source is bonemeal.