It is typical for gardeners to replace old soil when repotting plants. With the fear of transmitting diseases to their plants, many opt to use fresh potting soil instead. While it is correct that old soil can carry various diseases, reusing old soil does not mean it will automatically carry and spread the diseases to healthy plants. 


Used soil from healthy plants is unlikely to be the source of infection. Disease spores in old pots do not stay and float around to find new soil and thus, replacing soil does not really affect plants’ health.


Nutrients in the soil are also drained when watering plants so changing the soil is not necessary since nutrients can be replenished. For every potted plant, fertilizer is essential regardless if their soil is used or not as this will provide nutrients to them.


The soil’s age is sometimes linked to its effectiveness and productivity as well. Nature has been existing for over a million years. Therefore, needing to use new soil for new plants has no basis. Having this said, old potting soil can be reused for years. 


(Source link)