Tomatoes will grow the best if they are provided with readily available and even moisture. One way to accomplish this is to apply a mulch to the tomatoes. Mulch establishes a physical barrier between the soil surface and the direct rays of the sun. This will prevent excessive evaporation of moisture from the soil surface which can result in widely fluctuating levels of soil moisture. A mulch also helps prevent a dry crust from forming on the soil surface. Such a crust can prevent the free circulation of air in and out of the soil as well as inhibiting water from readily infiltrating the soil. Two other benefits of mulch include moderating soil temperatures and weed suppression.
Hay and straw are both very suitable mulches. Bales can usually be acquired at local nursery and garden centers. I have even found straw bales at a local hardware store. There may be some weed seeds in these materials, but the benefit outweighs any drawbacks from using them. Lawn clippings can also be used but this recommendation comes with two precautions. The first, and foremost, is never use lawn clippings if the lawn has been recently treated with an herbicide. According to the University of Kansas Extension Service, you can use lawn clippings if four mowings have taken place since the last application of a homeowner herbicide. If you have a lawn service provider, do not use the clippings if a product containing clopyralid has been applied. The second caution is to use dried clippings. Wet clippings can get moldy and form an impervious layer above the soil thus negating the positive benefits of a mulch.