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 for plants is essential to your garden but can be difficult to add without chemical assistance, which is not desirable for some gardeners. This is when nitrogen fixing plants are useful. Try planting a winter cover crop of legumes, such as clover or winter peas.