It could be something as simple as a lack of nitrogen in your soil. But in your case if you are using new store bought soil you should be fine there.
The first thing I would check is if the soil is wet enough, you might not be watering your plants enough. Yellow tomato leaves could be a sign of lack of water.
When tomato plants leaves turn yellow, it could be from a fungus or bacterial problem or other garden diseases. If disease is the case, you will want to treat this accordingly.
Sometimes yellow leaves on tomato plants mean that there is a pest problem that needs to be handled. You might have caterpillars or worms affecting the health of the plant and may need to handle this issue with an organic pesticide.