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 gardeners through organic sources. Animal waste like chicken manure contains high amounts of nitrogen that is used by lemon trees after it is well composted. Legume plants, which fix nitrogen into the soil, is composted and mixed into the soil to add more nitrogen. Even though these sources enrich the soil, they do not contain enough nitrogen for hungry lemon trees when used alone.