We often talk about cultivating organic soil that is rich in microbial action, and full of organisms. Soil in your garden combines naturally occurring minerals with the various organic and mineral amendments you add. As the organic matter breaks down, it feeds layer after layer of soil biology — moulds, fungi, arthropods, earthworms, and so on. Right down to the microbial layer of bacteria and protozoa.
The richer the ecosystem, it seems, the more fertile your soil will be. Soil biodiversity also protects your soil, and acts as a kind of cushion against atmospheric stresses like drought or frost. The more lively your soil ecosystem, the better prepared it will be to bounce back from periods of stress.
It may be that this richly biodiverse soil will work well in some container growing situations. Particularly in larger containers like half barrels or raised beds, an ecosystem within the soil may be able to thrive and produce the same kind of natural fertility.
It raises the question then: Container soil vs. garden soil. In some situations, the benefits of keeping things sterile outweigh the benefits of biodiversity. Certainly, if you are planning to grow microgreens on a kitchen counter, in a warm ambient temperature, the introduction of mould and fungi spores are not desirable. Likewise, insects and earthworms are not ideal guests in an indoor setting.
It is difficult to manage a diverse ecosystem in an unnatural environment. I think the best approach is to make the most of what you can if your system does not allow for biodiversity. In this way of thinking, cleanliness is crucial. I recommend using sterilized seed starting mix for indoor sowing and microgreens. For larger projects like larger container gardening, bagged potting soil is probably the safest bet. As this soil is produced, it is heated and cooked to kill off seeds and microbes. What it lacks in biology, it makes up for in structure, drainage, and dependability.
In an indoor setting, or even in a patio container garden, I recommend getting rid of all spent soil by composting it or otherwise allowing it to break down outdoors. After a season of growing, most of its nutrients will be depleted. Any roots or debris from the previous season will activate the biological process, and potentially become home to mould, etc… It would be possible, in theory, for a very frugal container grower to sterilize spent soil, and amend it to the point that it could be reused. To do really large projects like growing microgreens for restaurants, this may be necessary. I prefer the simplicity of using fresh bagged soil, though.
Think of indoor growing as the artificial stage. Employ inert growing media and artificial light as needed. It’s when plant crops are grown out in the garden where biodiversity becomes the key to organic success.