Are all leaves uniformly yellow? Or just top leaves? Do you have any pictures? Besides mineral deficiencies, yellow leaves could be a sign of overwatering.
Masha

I purchased a huge bag of vermiculite which is supposed to be a good mineral and help the soil. Along with worm castings or worm gold I put the vermiculite and my blooms are outstanding this year..

Sometimes a few shaded interior leaves just die because they are useless. Could that be it? I don't think there is any deficiency that produces a scattering of solid yellow leaves.

Deficiencies in the plant are fairly common. Often they are not caused by a deficiency in the soil, but rather by extreme pH values in the soil or by an excess of some competing element (whether natural or added by the overzealous gardener). So adding stuff because you guess there might be a deficiency is risky business. If you want to manage your soil precisely, you need to get a soil test. Otherwise I'd be cautious about adding stuff like epsom salts. Some soils have too much magnesium.

check this table, might help.
BTW chlorosys = lack of iron is probably the most common deficiency, but iron sulphate and chelate solve it quickly. Once that iron is absobed by the plant, it stays there.

{{gwi:251383}}

About 20 leaves, bright yellow, in a small bunch midway up the plant on the worst one. They fall off at the slightest touch. They look kind of like what you see in late winter here. The rest of the plant is very healthy with lots of blooms and dark green leaves. I thought I've actually been holding back a little on the water this year but I'm never completely sure even with drip.

I could take a pic but some are completely yellow and others have small, random spots of green left.

That's not deficiency symptoms nor waterlogging symptoms, IMO. You could check for fungal spots or maybe a damaged underlying cane, but it sounds like your plant is basically OK.