Question
My leaves are turning yellow near the bottom of the plant.  They are in containers on a mostly sunny cement slab.  The soil was the normal Miracle Grow potting soil.  Also used is a Miracle grow powder mixed with water every other week.  Thank you in advance!

Answer
You do not need to add any more fertilizer.

It is likely the watering practise, but it could also be a couple of more serious problems. These would be either Fusarium wilt and/or Fusarium crown rot.

These diseases begin as yellowing of older leaves (but so does excessive watering or underwatering, so see below). With Fusarium crown rot, the leaves often turn brown or black and eventually wilt. With Fusarium wilt, the yellow leaves turn downward and droop.

Fusarium oxysporum, the cause of both diseases, is a common tomato fungus that lives in the plant's vascular system, which carries water from the roots to the leaves. The vascular system is located inside the stems.

To determine if these diseases are the cause consider following:

Check watering practices. Both over- and underwatering can mimic disease symptoms, but the plants may not have the diseases. Instead you can change watering pattern. Tomatos should be watered at the bottom of the plant and when the top of the soil turns dry. Do not overwater plant (leave on consistently moist soil) but do not let the soil get excessively dry either. Stick your thumb into the soil when it is dry on top. If it is wet 1" below surface, do not water. If it is dry 1" below surface, water and thoroughly soak the growing media. Do not let the media get dry to a dept of more than 1" (e.g. if it is dry 6-8" below surface, you are not watering frequently enough or you are adding too little water when watering).

If you are sure that you have watered properly you can next check for the diseases:

Check the roots. Discolored roots indicate root rot.

Cut the lower or main stem of one plant which shows the symptoms badly and look inside at the vascular tissue. Fusarium wilt causes a dark brown discoloration within the stem (vascular tissue). Fusarium crown rot causes a rot or canker at the base of the stem and possibly a root rot.

If discoloration of the stem or base (vascular system) is noted, pull and destroy affected plants. Do not plant tomatoes, potatoes or eggplant in the affected area for two or three years (or use fresh soil). Wash planters with a 10% bleach solution.

No fungicides are labeled for control.