Tomato plants thrive in full sun in humus-rich, well-drained soil with a pH between 6.0 and 6.8. Although tomato plants can tolerate slightly dry soil, they perform best when the soil remains evenly moist, as uneven watering can cause blossom-end rot and cat-facing in tomatoes. Knowing exactly how much water they need can be a challenge, but by following simple guidelines you can provide your tomato plants with the right amount of water.
Determine the amount of water your seedlings require by following their lead. Initially, they may require as little as 1/4 cup of water, but as they grow larger, their water needs increase. Watering them until water runs freely through the bottom of the pot and checking them throughout the day give you a good indication of the amount of water they need. Typically, if the soil dries in less than 24 hours, your tomato plants require either more water or transplanting to a larger pot.
Tomatoes in Containers
Full-grown tomatoes grown in containers often require daily watering. The exact amount needed may range from a quart to a gallon or more, depending on the rate of growth, weather conditions and the pot size. As a rule, watering your tomatoes until water runs freely through the bottom of the pot and again when the soil feels dry to the touch 1 inch below the surface provides them with the water they need to thrive.
Tomatoes in the Soil
Because tomatoes grown in the soil can send their roots deep into the soil when the surface soil dries, they often require less frequent watering than tomatoes grown in containers. However, deep watering to moisten the soil to the root level once or twice a week is preferred to daily light watering. Wetting the surface of the soil without saturating the soil at the root level encourages tomato plants to form roots near the surface of the soil, compromising the support system of the plant. As a rule, tomato plants require 1 to 2 inches of water a week.
Weather conditions, plant size and growth rate, and the soil the tomato plant is grown in all affect the amount of water your tomato plant needs. Your plants require more water during hot, dry spells and periods of active growth than they do during wet weather or when the plants are young. How well the soil drains and its water-holding capacity also affect the amount of water they need. Observing your tomato plants throughout the day gives you important clues to their watering needs. Healthy tomato plants that wilt excessively or soil that feels dry to the touch indicate that more water is needed.