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.
This is a very basic recipe. For crops that prefer more Nitrogen, you will want to add a source of Nitrogen to the tea (such as composted horse manure or cow manure). Clean wood ash can be added for additional Phosphorus, but only in small amounts (wood ash can change the pH of your nutrient solution a lot).