SOIL leaves are turning yellow on one plant only?

[Gilly]

I am around just shy of a month into flower with my 2 autos, one is critical haze, the other is guerrilla glue. Both are in 30l pots that are 3/4 full of organic soil, both with the exact same organic additives which consist approximately of the following: blood meal, FBB, bat guano, azomite, dolomite lime, epsom salts and calcified seaweed, all in the required amounts. The soil is made up of 2 different sources of vermicompost, german black peat, baltic white peat, some coir, perlite and vermiculite. I also add a humic and fulvic acid liquid additive with every watering. I have grown with this mix several times previously with some particularly good results. The atmospheric conditions are also close to perfect, and I am growing under a 400w HPS light. My issue is with the heavy yellowing that is appearing on the mid to lower section of one of the plants only. I can't be sure which of the two strains is having the issue, as I accidentally mixed the seeds during germinating and lost track of which one was which. I have generally had this problem in the past at different levels of severity, and although I understand that this will happen naturally to a plants lower set of leaves later on in flower, this time in particular it's occurring earlier than usual, so I am wondering what, if anything, I can do to prevent it, or what I may have done wrong to have caused it. As this is only happening to one of the plants at present, and given that they have the exact same soil mix, with the exact same watering schedule, exact same atmospheric conditions, and are the same distance away from the light source, then I think it rules out a lot of the usual generic causes such as heat/light stress, watering problems and fertiliser imbalance. I appreciate that they are two different strains and that it may just be a case that they are reacting differently as a result, however, I thought I would create this post in the hope that someone would see this and recognise a possible cause, or failing that, in hope that someone can reassure me that it's nothing to worry about. Thanks for your time guys. Peace V.