Certified GKH Gardening Expert
Depending on where the yellowing occurs, this can be a pH issue or a true nitrogen deficiency.
Make sure to test your soil for pH, and deficiencies before adding anything. This will ensure that you get the proper balance and ratio of nutrients.
These articles will help:
Alternatively, too much moisture in the soil can choke out the plants. This can resemble deficiencies. Make sure to let the soil dry down to about 2 or 3 inches between watering. This will allow the plant to grow at its best!