Magnesium is naturally very abundant in the earth and rarely is there ever a deficiency. In clay soil, such as in the UK, magnesium is laying right there on the surface. Irises are very shallow rooted plants and normally get all the magnesium they need. They are very tolerant of 'less than perfect' soil but adding a little extra won't harm them either.

Most importantly for irises, don't plant them too deeply. The rhizomes want to be exposed to the sun & open air. They must also have good drainage. Too much water will rot the rhizomes. Here is a link all about irises you may find helpful.

http://www.irises.org/About_Irises/Cultural%20Information/Grow_Bearded.html

Wouldn't recommend using on either the irises or the fuchsia... What makes you think they need feeding Gemzgg?

Bearded Iris need regular feeding where I am...but that is just because of our horrible soil! Unless you are gardening on pure sand or chalk, Gemzgg, it would be smart to look for symptoms and/or test your soil before feeding where you are. The only "magnesium based" plant food that I am aware of is Epsom salts, and that should be used very sparingly. Minute amounts of magnesium are included as a micronutrient in many brands of "rose food", however.

That is true. Sometimes too much or the wrong type of fertilizer is worse than none at all. Where I am, I don't feed my irises anything at all now take a look at these;

http://www.growsonyou.com/photo/slideshow/329794-bearded-irises/member/bathgate

They are doing just fine without me thank you. :)

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If you are planting hardy fuchsias don't worry. Some in our garden do very well in dreadful poor soil with no feeding at all, poor things. If you do ever want to feed them use a weak high potassium fertilizer such as tomato food.
Soils in Essex do vary, some clay and some chalk so bear this in mind when planting or choosing fertilizer.