This is coolbert:
From the Internet web site Snopes and thanks to same.
True or false? A long-standing question that my American soldiers have asked themselves for decades.
"A common report by male recruits undergoing basic training is their inability to troop the morning colors, so to speak. Erections that came so easily before boot camp become few and far between. This surprising downturn of events is frequently attributed to the presence of saltpeter in the chow [food] rather than to the powerful combination of anxiety to succeed, physical exhaustion, and a radical change of lifestyle. Barracks life also contributes to this suppression, with the knowledge of the close proximity of many others subconsciously working to inhibit that aspect of an individual’s expression."
Saltpeter [potassium nitrate] said to be added to the diet of the U.S. Army recruit during basic training. Causes impotence and lack of any normal sexual desire. So is the rumor and has been so for many decades.
According to Snopes this is just not so. Personally I stay away from the mashed potatoes.