Yellow and crusty leaves

like asnoriu said -- possibly not a physical deficiency and related to lcokout, or misdiagnosed to start. Important to remember Ca and Mg are 2 different molecules. they are not intrinsically linked just because a popular product smashes their names together. Mg is interveinal chlorosis with spots and Ca deficiency is rusty spots by itself. could be long-term effects of poor watering habits. There's just not enough information to give you a proper answer. Visible symptoms are not discrete so you need more than that to properly diagnose. Senescence may be a contributing factor, but much too early for this much damage. This is definitely related to how you treated the plant and not something you can blame on something out of your control. you can figure it out and avoid it in future. the speed at which things occur can help too.. e.g. if the rate of degredation increased after you upped the ca and mg, it ould be related.. could be locking other thigns out. if you weren't irrigating properly for months you could have all sorts of problems in the root zone which would make what you see mostly useless for diagnosing. it's not dead and canopy is still providing 'something' ... so just power through to the end and don't overreact. Take a look at how you treated this plant from seed until now and figure out a few things you want to try differently next time and see how that turns out... systematic trial and error...