Problems with Magnolia Stellata leaves

Hello, I have my Magnolia Stellata growing in a large container. Now half of the leaves are yellow and they look as if they are covered with splashes of dirt. A couple of days ago I sprayed the tree with soda +soap in water but it didn't seem to help.
Any idea what is happening? Thank you.

 

Looks like soda/soap burn to me.

Why the hell did you spray with soda??

Looks like chlorosis to me.

 

I thought it was fungus. How to deal with chlorosis?

 

Chlorosis is when a plant that likes acidic conditions is growing in alkaline soil.
I usually just give a watering with sequestered iron for a quick fix.

Probably a bit late to have much effect this year.

 

@Black Orchid A couple of things; firstly Magnolia stellata prefers acid to neutral soil/compost so if it's in a pot make sure in ericaceous potting compost that is well drained; if it's in the ground what is the pH? Secondly M stellata is deciduous so it could be simply shutting down for the winter as the days shorten.
If it was mine and the soil/compost was suitable I would leave until next year and see how it comes back. If in spring the leaves are chlorotic I would treat with Epsom salts (magnesium sulphate) and/or a chelated iron feed like Sequestrene

 

Pete and Nigel, thank you very much for your replies and recommendations.