Dump the old potting soil into a small bucket. Allow it to dry completely.
Remove debris, dead leaves and roots from the old potting soil. Lift the root balls and shake over the bucket to collect the potting soil still attached to them. Set aside organic matter, such as leaves, to reuse.
Place one part cow manure and one part composting material in a separate bucket. Use leaves from the old potting soil, crushed egg shells, old coffee grinds and other organic material for the composting items. Stir well with a stick. Allow the mixture to age or sit for 48 to 72 hours. Stir daily.
Add an equal amount of the mixture (from step 3) to bucket with the old potting soil (from step 1). For example, fill up the bucket with the mixture if it's already half-full with the old potting soil.
Add slow-release fertilizer for tomatoes to the bucket of old potting soil and compost mixture. It can be purchased from your local gardening store. Mix according to the manufacturer's directions.
Stir the mixture contents well in the bucket. Fill potting containers and plant tomato seedlings or fill in gardening holes and plant tomatoes. Repeat the process when the tomatoes are finished growing and harvested.