Nitrogen Deficiency

• Tomatoes need nitrogen. A lack of it will cause the leaves to turn yellow, usually from the bottom of the plant. A soil test will determine if the soil lacks nitrogen, in which case, a simple nitrogen fertilizer can be added to the soil.
  
  

Magnesium Deficiency

• A magnesium deficiency can cause yellowing that begins on the lower leaves, moving upwards until all the foliage is affected. This is a common disorder that can be made worse by feeding the plant. Tomato feed contains potassium, which inhibits the plant's intake of magnesium. Mixing magnesium in with the plant food can help to rectify this.
  
  

Watering

• Too little watering can lead to a yellowing or wilting of the leaves. It is important that tomato plants are not allowed to dry out. However, the problem can also be caused by overwatering or poor soil drainage. Overwatering can also lead to root rot, which occurs at the base of the stem.