Mustard greens are an excellent source of many vitamins including vitamin K, vitamin A (in the form of beta-carotene), vitamin C, and vitamin E. They are also an excellent source of the minerals copper, manganese and calcium. They are a very good source of dietary fiber, phosphorus, vitamin B6, protein, vitamin B2, and iron as well as a good source of potassium, vitamin B1, magnesium, niacin, pantothenic acid, and folate.