Gix is right about the onions. The onion bulb is not a root. The bulb is made up of the base of the leaves. Each layer of the onion is a leaf and it has the top green part attached. Take an actively growing onion and slice it in half lengthwise and look closely at its construction.
OK, I will offer my opinion on chicken dung as fertilizer. Yes, chicken is one of the best fertilizers. As noted it is high in nitrogen.
It is also my experience that all crops benefit greatly from nitrogen fertilizer. Yes, even potatoes and other root crops.
I think it is best to compost manures before adding them to your garden if the plants are already growing. It is ok to put fresh manure directly on the garden in the fall and working it in. I just don't like to put fresh manures on when the plants are growing. There has been too much trouble with e-coli when this is done. And sometimes salmonella. Not that I have ever had a problem, but we hear of it on the news. Best to just avoid the problem since we have the option.
Here is a short quote from [url=https://www.fsis.usda.gov/factsheets/salmonella_questions_&_answers/index.asp]a USDA page.[/url]
"A. Salmonella lives in the intestinal tract of humans and other animals, including birds. Salmonella is usually transmitted to humans by eating foods contaminated with animal feces. Salmonella present on raw meat and poultry could survive if the product is not cooked to a safe minimum internal temperature, as measured with a food thermometer. Salmonella can also cause foodborne illness (salmonellosis) through cross-contamination, e.g., when juices from raw meat or poultry come in contact with ready-to-eat foods, such as salads. "