Typically, yellowing leaves result from a nutritional imbalance or disease outbreak, but other causes can play a part.

Nitrogen deficiency is one of the major reasons for yellowing leaves on tomato plants. Tomatoes are heavy feeders, the plant requires twice the amount of fertilizer that a cucumber needs, and almost four times the amount as beans.

If you don’t fertilize enough with nitrogen, the older leaves will begin turning yellow and, in many cases, may fall off. The older leaves turn yellow because they are providing their nitrogen to the younger leaves to survive.

Yellowing of leaves can also be the result of an iron deficiency in the plant, but this will be most prominent in the youngest leaves. A magnesium deficiency however will produce yellowing that looks more like speckles or spots on the older leaves.

Please remember that with the use of a lot of fertilizer, the plant will also require a lot of water. A good rule of thumb is to do a moisture test where you place a finger several inches deep in the soil to test for moisture near the roots. If it feels dry, it’s time to water, and as the tomato plants get closer to full maturity, they will require more and more water.

Hope this helps!