I would say calcium uptake issues, it could just be a problem with ph, or even too much salt in the soil that it's creating a lock out for the nutrients. I had the same problem with my grow. Yes, calcium can be yellowing of the leaves, but it is also browning in spots. It really depends on the situation, because it could be from a number of things, like the lock out, or lack of nutrients, or PH levels being slightly off to prevent nutrient up-take