You can use old potting soil as long as you mix it with soil that contains nutrients. However, if the old soil has already been used for growing and the plants were diseased, you should not reuse the soil, because it is likely to spread diseases to your new plants. You should sterilize the old soil if you had any concerns about diseases or pest infestations. You can do this by heating the soil in the sun in plastic buckets, or using other methods.