3 : deficient in active properties ; especially : lacking a usual or anticipated chemical or biological actions
aka microbes
http://www.merriam-webster.com/dictionary/inert
There is little to no usable food for plants in the substrate, which is why you need to start feeding almost immediately and continue feeding every single time you water.
Firstly zVice please, please, please don't refer to Merriam-Webster when trying to define English words.
The Merriam-Webster dictionary is NOT an English dictionary but an American-English dictionary making it completely useless, Noah Webster had the audacity to re-write Dr Samuel Johnson's "A Dictionary of the English Language" (which later became the Oxford English) some 70 years after it had been published as he didn't think it right that Americans should be tied down to the linguistic laws of the English and their language if now independent, despite the fact that Dr Johnson's Dictionary had been published 21 years BEFORE the declaration of independence!
But despite that your definition is pretty much what I said ("to have no inherent power of action") you just don't understand it yet, you have to think logically to grasp it.
"There is little to no usable food for plants in the substrate, which is why you need to start feeding almost immediately and continue feeding every single time you water" - Exactly! (except you don't need to feed every time you water, it just needs more earlier)
The simplest way to explain it is.......
If you try to grow a plant in coco with plain water and no nutes it won't grow (despite being colonised by trichoderma) because the coco is inert, the plant does not feed on bacteria so therefore the bacteria and microbes are not contributing to the coco's chemical or nutritional value.
The bacteria/microbes can't perform their biological actions without nutes either, bacteria has to feed on something.