Pea soup has been eaten since antiquity, usually mixed with some sort of meat, and often with herbs and other vegetables. It is mentioned in Aristophanes' The Birds, and according to one source "the Greeks and Romans were cultivating this legume about 500 to 400 BC. During that era, vendors in the streets of Athens were selling hot pea soup.
A well-known nursery rhyme which first appeared in 1765 speaks of:
Pease porridge hot,
Pease porridge cold,
Pease porridge in the pot
Nine days old.
"Pease" is the Middle English singular and plural form of the word "pea". Pease pudding was a high-protein low-cost staple of the diet and, made from easily stored dried peas, was an ideal form of food for sailors, particularly boiled in accompaniment with salt pork, which is the origin of pea (and ham) soup. Although pease was replaced as a staple by potatoes during the nineteenth century, the food still remains popular in the national diet in the form of "mushy peas" commonly sold as the typical accompaniment to fish and chips, as well as with meat pies.
This versatile annual legume is also a popular soil building green manure or high protein animal feed when inter-planted with rye or oats. Admiral peas are primarily grown for grain, however they can also be used for forage. Admiral peas are a good, digestible source of protein for livestock. Easier to grow than soybeans in organic farming systems
Much of this information came from the Wikipedia article, "Pea Soup"