what kind of salt?

Willy used to have a selenium block available to him all the time. Now, since I've switched his food, and the bag says NOT to supply a selenium block with this food, I purchased an iodized-salt block. Should I have bought a plain salt block? Is iodized bad??

Salt is essential for all horses, even more so in our Arizona environment. It is difficult for a horse to obtain adequate salt from a block, and our high potassium hays can reduce a horse’s “salt hunger” due to the body’s sodium conserving mechanisms.
Loose salt, based on the amount of hay/forage normally fed (i.e., if a horse is consuming some pasture but would otherwise consume 18 lbs of hay per day figure using the 18lbs/day) should be added directly to the feed.
Figure about 2 ounces (approximately 3 tablespoons) iodized table salt for 15 to 20 lbs of hay (1 ounce for 10 lbs total forage – pony, youngster; up to 3oz for 25-30 lbs total forage – large TB, warmblood).
One ounce of iodized table salt provides approximately 2mg iodine. Providing iodized salt at the above levels will provide “insurance levels” of iodine. Many horses with “low thyroid” have had their thyroid test numbers corrected by simply adding iodized salt (eliminating the need for thyroid medications).
Additional plain (not iodized) salt should be available as either loose salt in a pan or a plain white salt block. After hard work, an ounce of salt can be added to a bucket of water and offered for a few hours (making sure there is also plain water also available). Additional electrolytes are not normally needed if your hay provides sufficient calcium and you are feeding magnesium on a daily basis. For most average working horses (not talking endurance here) feeding a small amount of alfalfa hay – just a couple of pounds – can provide “calcium insurance”.
If the salt/potassium has been balanced by adding salt to the feed as above, your horse should have a “normal” salt hunger and should seek extra salt when he needs it.

Here in Oz, it's generally the case, especially on farm land, that the land and therefore feed grown on it is too high in sodium, which in turn binds up and destroys other minerals(esp.potassium). Therefore it is these, not sodium that needs to be supplimented. I would say that if planning on feeding any particular mineral not naturally balanced, a feed/soil analysis &/or blood test should be done to ensure you're not throwing the system into more disarray.

Country girl, you speak with authority & detail which suggests you know exactly what you're talking about I'm interested to know more about the salt/potassium balance & salt hunger? Never heard of this, tho I've done a lot of reading on nutrition & nutritional suppliments. Perhaps it's because I usually read Australian studies and as I've said, Oz is way high in salt, so it is rare that it needs to be supplimented separately. However, chemical farming is a huge part of the problem, raising salt levels which destroy potassium, so I would have thought it was a global problem in farming areas.