In the drier summer months, a good layer of mulch under the plants is essential for optimum production. Tomatoes need a constant and uniform supply of soil moisture to keep the plants growing well and fruit ripening. Bare soil is not a major concern earlier in the season, when there are frequent showers and the soil is still moist from spring. Also, an unmulched soil will warm faster in the early part of the season and get your plants off to a good start. Just about any material can be used for a mulch under tomatoes.
Black plastic is a common choice and works well.
A thick covering of weed-free hay, straw or other organic material will help retain moisture, keep the soil from heating too much in summer and keep the fruit clean as it ripens. Robert Nuss is a horticulturist at Penn State. He coordinates all extension horticulture programs. He has bachelor's, master's, and doctoral degrees in ornamental horticulture and has been on the Penn State faculty since 1966.