Cal- Mag deficiency?

Hi everyone!

This girls is a little past 4 weeks old. It’s an autoflower Afghan, in soil. I noticed some deficiencies in the leafs. Based on my research I believe it may be cal mag deficiency as I’ve been using RO water with liquid seaweed to feed. I am running to the store for some cal mag and wanted to ask if that is the right solution and what other nutrients should I introduce as it’s going well into flower.

Please also comment on my LST and what I could improve , I know it’s a daily game, but just 5 days of it I’ve seen massive growth it lower branches.

Please help!

Update:

after doing some research, I have concluded that it is rust on the bottom leaves and found a product that can be used to prevent the spread of any further rust. I purchased the Safer's Sulphur dust and mixed with water to apply as a spray. Hopefully this fixes the issues.

Nutes:
Since it is past one month, I ordered nutes yesturday as well so i can begin the feeding cycle. I wanted to see your thoughts on them.
- Organic Kelp Meal (1-0-3)
-Gro4 Organics Eartworm Castings
-Organic Phosporus bat Guano (0-18-0)
-Gia Green Power Bloom (2-8-4)

I plan on top dressing with this, I just wanted to see how often I should do it? even though it is an auto, growers choice website mentions it is a slower growing auto and can be a 90 day grow. I am currently on day 34.

Cheers

If using RO you need to add Cal Mag asap.