All elements have been mentioned except Potassium for cramp relief.  I read about  it in Prevention Mag. long ago. 45 years ago my daughter complained of nighttime leg cramps; which my mother called Growing Pains.  For the daughter I went to the kitchen and got the Co-Salt, a  USA brand for a mixture of Potassium and Sodium Chlorides.  I gave her a glass  of water and some Co-Salt in the palm of her hand and commanded her to lick it and wash it down.  Nearly instant relief.  PetertheBloke probably can suggest a Latin phase for same.

No one really knows the full physiology of cramps. Cramps in foot arches can be caused by all the usual suspects for cramps generally. More specific causes can be footwear problems [mismatch between the foot shape and that of the shoe], and damage or inflammation to the tissue [fascia] underneath the foot arch. There are also some deeper links: for example bowel problems e.g. persistent diarrhoea, can cause reduced uptake of vitamin B and Mg, and thus contribute to aching feet and muscle spasms. There are so many possible causes that you are best getting a professional to have a look: a Physiotherapist could be a good choice because they handle both soft tissue and skeletal issues and can refer you to other specialists if necessary.

Well I can anticipate the furore this comment will cause but as no-one else has made it... here goes.

Put a cake of laundry soap in your bed before you retire and, bingo, no more cramps.

And before you thousands of antiquacks apply your vitriol to your keyboards have a look at just how many people avow that it works.

As a sufferer from night cramps, particularly after unaccustomed exercise, I found out years ago that special OTC tablets for it were just a mixture of salt and sugar. Nowadays I use (and recommend to anyone who will listen) a glass (about 250~300ml) of water with a rounded dessertspoonful of Tang and the tip of a teaspoon of salt mixed in. It has a practically instant effect.

It's roughly equivalent to Gatorade or Isostar but those both have some potassium - and are a lot more expensive. Using Paul6240's Co-Salt would no doubt be better than plain salt, but I read somewhere that it's adding the sugar of Tang that makes the uptake of salt - and hence the effect - a lot faster.

Cramp is an involuntary contraction of the muscle and is normally caused by muscle fatigue and/or lack of salts in the blood. It usually occurs after prolonged periods of vigurous activity, especially when little exercise has occured before this activity. 

When standing up, muscles are constantly contracting in the foot to enable movement and to keep you balanced. These muscles, like all other skeletal muscles, will fatigue and this is when cramp can occur. Muscles also require certain salts to enable them to contract, and so it removes these from the blood. Eventually most of the salts will have been taken from the blood to enable muscle contracts and cramp may occur. 

Cramp can usually be treated by stretching the muscle whilst massaging it. Also sufficent fluids and electrolytes should be taken on to both treat and prevent it.