Although tomatoes need moist, rich soil to produce healthy fruit, too much water is just as bad as too little. Signs of over watering may mimic disease or other environmental problems, but if several symptoms are present, too much water is the likely cause. Fortunately, tomato plants usually recover within a few weeks from over watering.
Over watering, as well as over fertilizing, causes tomato plants to produce lots of lush, leafy growth, but few tomatoes. Cold weather and drought conditions may also cause blossoms to drop, but won't produce an excess of foliage. If your plant seems to have an overabundance of leaves, but no fruit, suspect too much water or nitrogen.
As a tomato begins to ripen from green to red, the fruit develops a thin, papery shell. If the plant is watered excessively during this time, the shell cracks. Blossom-end rot is a brown or black spot that develops on the bottom of the fruit and spreads, eventually causing the entire fruit to decay. Blossom-end rot is caused by a calcium deficiency, but is exacerbated by inconsistent watering practices.
Too much water and too little water often produce similar results. The leaves may wilt, turn yellow and drop, or the tips may become burnt. Too much fertilizer also causes leaf tip burn. Brown spots on the leaves, cankers or holes are usually caused by disease or insect infestation rather than environmental conditions.
Amend heavy clay soils or sandy soils with compost and manure before planting tomatoes. These amendments improve drainage for both types of soil, reducing the risk of over watering. Water tomatoes when the soil feels dry 1/2 inch under the surface. Stick your finger into the soil to check. If it feels dry at the first joint, it's time to water. Water for at least 20 to 30 minutes to allow the moisture to soak thoroughly into the soil. Check the soil frequently to keep it consistently moist -- neither dry nor soggy.