Lack of iron and magnesium are helped by Epsom salts. Neither deficiency is common in our area, nor very common to Clematis as far as I know. More likely to be seen on Rhododendrons. Both deficiencies leave the veins green on the yellowing foliage and this occurs on the new growth.

Overall paleness or yellowing or yellow then brown leaves that drop on the bottom of the plant are not lacking iron nor magnesium but can be lack of Nitrogen for the paleness and lack of water on the drying foliage.

Leaves that are yellow, chlorotic or otherwise damaged or defective don't usually regenerate. It is the new foliage that comes out nice and green.

Rose fertilizer is recommended because it is higher in Phosporus than Nitrogen. Which encourages blooms. Many are also organic and slower acting, less likely to burn or shock the plants.

Liquids are fast acting and need to be applied often. According to package directions. Always measure them and don't add more than called for or you can burn your plants.

Peters, Miracle Gro and Schultz are several really common ones but you want to look for formulations that are higher phosporus that may say bloom booster or something similar.

My only knowledge of Eleanor's is as a houseplant fertilizer and I have never been impressed with it although I've read posts from many who are.

I use manure and Rose Tone on my Clems. I fertilized with manure when I pruned them in February as it is mild as they are waking up. I put Rose Tone on all that are not budded or blooming last week. Fertilizing budded and blooming Clematis can shorted bloom time.

Clematis, like roses, need to be fertilized throughout the growing season.

Excellent advices, Buyorsell!
I was not impressed by Eleanor's fertilizer either.
I do not think that lack of Iron can be treated with Epsom salt (magnesium sulfate) though. The good thing about Iron deficiency is that chlorotic leaves turn darker green fast, usually in 3-7 days after treatment. For my Clemmies Iron deficiency is a common problem, unfortunately, because I grow them in pots.
Check out the Symptoms of Deficiency In Essential Minerals here.

You are right, Epsom salts do not help iron deficiency, not sure why I typed that. I hate that you can't edit.

Lack of iron requires Chelated Iron or a fertilizer that contains it such as Miracid by Miracle Gro.

Miracid is not usually needed in Western Oregon. Our soils are acidic enough here. The natural iron isn't locked up due to high alkalinity like it is in other areas. Though containers are a different story. I may need to buy some for my potted gardenias in my new greenhouse but have never had to have it for the garden here.

It has been over twenty years since I lived in AZ where Iron Chlorosis was extremely common and I didn't remember that the chlorotic leaves turned green again either. You can see chlorotic plants from your car as you drive around Phoenix. Especially overwatered Eucalyptus in lawns.

From looking at the pictures from the link that Alina posted it looks like they are Nitrogen Deficent. What is a good fertilizer to correct this problem. I have 3 that are having this problem.

Any fertilizer that has the first number greater than the others. The first number is Nitrogen.

Fertilizers are basically the same thing. Nitrogen, Phosporus and Potassium/Potash.

N. P. K.

for Green leafy growth, flowers, roots in that order.

They have different concentrations and some are organic and some chemical and some are slow acting and some are fast acting. They have different trace minerals but in the end, they are all mostly the same.

Phosphorus!