Once the ratio of plant and root mass to soil mass is too large, you have problems that can only be solved by repotting- maintaining consistent soil chemistry under those conditions is nearly impossible, and that little media for that large plant is going to give you wild fluctuations depending on the moisture and fert content of the medium at that moment.
Even a plant in a large grow bag may surprise you with its roots. And if the plant has been in the same pot or bag for more than 5 weeks, expect SIGNIFICANT changes in soil chemistry due to the plant using up the lime. The calcium def on your low leaves is a hint...