Tomatoes require a deep root system to help pull water from the soil and keep full plants structurally sound. Helping the plant create a deep root system starts at planting. Deep root growth is increased in tomatoes when the seedlings are set deep in the soil, given plenty of water and given adequate space between plants. Tomatoes with deep root growth are more likely to also have an increase in fruit production over shallow-rooted plants.
Roots that grow deep below the plant help support the above-ground stem and branches. Deep roots are also able to maximize water absorption. Plants with shallow roots have an increased risk of drying out, developing fruit that forms poorly and developing stems that can't support heavy fruit production.
Planting tomato seedlings deep into the garden soil increases the depth of root growth. The ideal depth for root growth on an mature plant is at least 18 inches, even though the main portion of the root system is found in the first 12 inches. To obtain this depth of root growth, dig a planting hole that allows you to set the seedling so the first set of true leaves are just above the soil line. Additional roots will form off the portion of stem that is below the soil level for a sturdy and deep-set plant.
Loose garden soil assists in root growth by providing less resistance. Work the soil with a tiller or spade to a depth of 18 inches before planting the seedlings. Soil with a high clay content should be amended with compost to increase the organic content. High clay soils pack easily and are not ideal for deep root growth.
Deep root growth requires some work on your part. Water tomato plants regularly to keep the soil evenly moist throughout the growing season. Healthy plants will not only grow tall, but also deep into the soil. Tomato plants that are in the fruit-producing stage require more water, up to 1 gallon per day. It is also ideal to set the plants 18 to 24 inches apart so they have adequate room for healthy and deep root growth.