Let's start with the fact that cereals , are made of grains and are
mostly carbohydrates. therefore they contain carbon oxygen and hydrogen.
They also contain proteins so have nitrogen. Plants have the minerals
that are in the soil where they grow, so the list you have is correct.
They also generally have added vitamins ad minerals. The vitamins
contain C, H, O N, and a few others such as sulfur, The added minerals
might contain:copper, iron, zinc, cobalt, selenium, silicon and