Studies show that nitrogen has a positive effect on onion skin quality by increasing the specific weight of the outer skin. This is a very important factor for storage.
Research shows that good nutrition with calcium nitrate will also increase quercetine concentrations within onion bulbs. Quercetine is a yellow pigment that improves the color intensity of the skin and the storage properties of the onion. It is an antioxidant and helps improve human health through decreasing cholesterol levels, controlling blood pressure and increasing tolerance to certain allergens. Calcium deficiency leads to onion bulbs with low density and reduced quality, and with softer skins.
Adequate supplies of copper are important for bulb skin and onion scale development, as a result of the element’s role in lignin production. Trials show that copper can enhance skin color.