Is this Nutrient Lockout??

I posted this in the outdoors growing section but got no meaningful replies, so i'm hoping for better here.

I have got a nice G13 Sour fem who has just started to go into bud after a bout 1 month of veg. She is grown outside from seed in a sub tropical climate with a rough 12/12 lighting of natural daylight / darkness. She gets a full 8 - 9 hours of good drirect sunlight. Light is not the issue.

She is growing in a soil / perlite mix of about 70/30 with a decent handful of worm castings and guano thrown in.
About three days ago i noticed the discoloration in the leaves - yellow patches on the main fan leaves as shown in the picture. This has now started to spread to other leaves and is getting worse. Can anyone tell me what it is? and how to fix it? Is it nutrient lock out? or a deficiency. Any help would be really appreciated.

We have just had a few tropical rainstorms recently and being in a big city - might it be the 'acid' gases in the rain lowering the pH of the soil??

Well since no one else has given it a shot ill try.....

Im thinking its Magnesium Deficiency,
Try adding a tablespoon of epsom salt to the next gallon water an see if that helps...
Or foliar feed at half strength...

That's where I would start, mag def.

Thanks for the input guys. I'll look into it!