I don't agree with the Magnesium being the issue, If you read the info below you will see why, but as I said above, if a pot plant is root bound that's what happens it's no big deal to put in a bigger pot. If root bound your plant will not reach it's potential. Yield will be down also.
Your best bet is to treat with Cal-mag as above. There are lots of minerals that will produce similar problems. Copper, Molybdenum, Boron to name a few another thing you should try is adding molasses (get it at the grocery store 3$) Mix at a rate of 2 table spoons per gallon. Lots of good stuff in there.
Benefits...
Molasses increases the amount of carbon into soil (C:N ratio) and promotes the growth of beneficial soil microorganisms.
It is an effective soluble foliar carbohydrate.
Improves the structural stability of soils by decreasing dispersion and/or slaking.
Improves soil moisture retention.
The addition of molasses to high N applications ameliorates the phytotoxic effect of discrete N application.
Source of potassium, sulphur and trace elements.
Buffers phytotoxic effects of pesticides, herbicides and mineral oil applications.
Stimulates benefi cial microbial activity by providing an excellent food source.
Enhanced microbial activity improves availability of nutrients in the soil and reduces the amount of fertilizer required.
Here is more on Magnesium
Manganese (Mn) Micronutrient and Immobile Element
Manganese Helps enzymes break down for chlorophyll and photosynthesis production, as well as it works with plant enzymes to reduce nitrates before producing proteins.
Having plants that are deficient in manganese can turn the young leaves into spotted (mottled) yellow and or brown areas on young leaves. Dead (Necrotic) yellow spots form on top leaves, while the lower older leaves will or may have gray specks and or spots. Symptoms can include yellowing of leaves while the leaf veins can stay green. Can also produce a chequered effect. As the plant gets newer growths the plant will seem to grow away from the problem, that’s why the younger leaves may be unaffected. On the top of the leaves, brown spots can appear. While the severe areas of the leaves turn brown and wither. Parts Affected by a Manganese deficiency are: Young leaves.
Too much Manganese in the soil will cause an iron deficiency. The blotchy leaf tissue is caused by not enough chlorophyll synthesis. Your plants will seem to have very weak vigor caused by the excessive amount of manganese.
Problems with Manganese being locked out by PH troubles
Soil ph of over 6.5, High iron soils, Low nitrogen Soils, Dry weather
and compacted soil.
Soil
Manganese gets locked out of soil growing at ph levels of 2.0-5.0
Manganese is absorbed best in soil at a ph level of 5.5-6.5 (Wouldn’t recommend having a soil ph of over 7.0 in soil) anything out of the ranges listed will contribute to a Manganese Deficiency.
Peace