Avoid SOY if you can - though it is high in protein it has a history of causing metabolic problems in humans and with the increase in its use in feeds has come an increase in horses with metabolic problems.
For a really shiny coat add up t 1 cupful daily of freshly ground Flax seed - you can grind it in a coffee grinder. Also looks after joints to if freshly ground.
For ultra white socks.
Wash with a mild washing up liquid and a soft nail brush and hand hot water. Work lather up well untl the skin shows through pink.
Rinse well, towel dry then wash with Blue shampoo, leave on for 20 mins or so before rinsing off. Towel dry and then here is the special tip
Mix Curash baby powder with some warm water to make a soft paste then rub into the sock with a toothbrush. Then bandage over. I use whhite towels as bandage pads and either white or blue bandages to wrap over the towel.
If you cannot get 'Curash' baby powder you need to get the type that also contains zinc - often advertised to help babies with diaper rash.
Once the socks are dry - I usually wait until I'm at the show, remove bandages and gently brush off any exccess dried powder. Make sure you don't use the brush elsewhere
Curash socks on my Clydesdale