Do you need sterile potting mixes for growing tomatoes?
The answer is yes and no.
For starting tomato seeds, you need a sterile mix. Its job is to give your tomato seeds a safe environment so they can sprout, develop their first couple of sets of leaves, and remain disease-free. The seed itself provides the initial food for the plant, so seed starting mix need little (if any) fertilizer to do its job.
But when you move seedlings to larger pots – or when you plant tomatoes in containers – the potting mix has a different function. It becomes your tomato plants’ home (if you’re growing tomatoes in pots) or at least until they go into the garden (for maturing seedlings.) That means the mix needs to provide both a good home plus food. The best potting mix for tomatoes must contain nutrients in order to grow healthy plants. You want a healthy start for your seedlings – or in the case of tomatoes grown in pots – for plants as you set them in containers.
They’re pathogen-free growing mediums. A sterile mix is made one of two ways.
2. A garden or potting soil that has undergone heat or chemical processing to destroy pathogens and seeds. Sterilizing the mix reduces the number of organisms that can cause problems.
1. To start tomato seeds: Sterilized soil is airy, which is ideal for germinating seeds. The light tilth makes it easy for roots to grow down and stems to push up through the surface. Plus, non-sterile seed starting mix contains pathogens, making new tomato plants particularly vulnerable to the damping-off disease. A sterile mix alleviates that problem.
2. To repot tomato seedlings: Garden soils are filled with organic matter and is home to active bacteria and fungi. Tomato seeds and tomato seedlings must battle with them for space, air, light, and nutrients. You can use a sterile mix to re-pot seedlings but you will need to fertilize them as they grow, too.
3. To grow tomatoes in containers: Tomatoes in pots can absorb nitrogen, magnesium and potassium faster from a sterile mix because nutrients are readily available. Plants get stronger. That makes them able to fight off diseases more successfully. Plus they are exposed to fewer diseases because the soil is sterilized.
You save time: Sterile mixes mean fewer weeds and fewer diseases mean you spend less time weeding and less time figuring out what is wrong with your plants.
You save money: Since your plants have fewer diseases, you spend less on fungicides and horticultural oils to treat them.
The best seed starting mix is sterile and has little fertilizer. (Make your own potting mix.)
The best potting mix for tomatoes may or may not be sterile, yet it must contain nutrients in order to grow healthy plants. (Compare potting mixes here.)
More about Using and Making Potting Mix for Tomatoes