The complex relationship between plants and soil is called the soil food web. This describes the connection between roots, soil, and soil organisms. In the past 15 years this topic has been the center of attention for organic gardeners, thanks in large part to Teaming With Microbes by Jeff Lowenfels and Wayne Lewis.
Plants produce sugars (carbohydrates) and proteins via photosynthesis, a well known process. Lesser known is that these photosynthetic products are then exuded into the soil, via the root system, to encourage beneficial bacteria and/or fungi. The bacteria and fungi feed larger soil organisms. It is the poop from these larger organisms that feed our plants. This is organic soil gardening at is core.
Soil organisms include (from tiny to small): bacteria, fungi, algae, slime molds, protozoa, nematodes, arthropods, gastropods, worms, and insects. These, and many unlisted organisms, are partially responsible for decomposition/aeration of soil. Decomposed soil releases nutrients, which are then used by plant’s roots for nutrition. Aeration of soil is a major issue, and has led to many growing with soil-less organic media. Peat/coco/bark, the organic soil-less grow medias, are much more airy than traditional soil.
But soil organisms are not the only ones decomposition and improving soil structure. Plants decompose soil chemically by exuding organic acids, ie citric acid, via their roots system. This is how plants actively take nutrition from soil. Plants also alter soil pH to their liking with these same exudates. Plants, like soil organisms, aerate soil via growing and moving root systems. Plants do exert a level of control over the rhizosphere.
Taken indoors the root soil food web is different. Indoor container growing is done without the larger soil decomposers, worms and insects. Bagged potting mixes may be void of other crucial soil organisms due to processing and/or sterilization. This means indoor potting soil won’t be continuously aerated by worm and insect tunneling. This lack of large decomposers has led to indoor growing being based on mostly lighter (more air) and less nutritionally balanced soil-less organic media instead of actual soil. Familiar examples of soil-less organic media again are: peat, coco, and bark.
Soil-less organic potting mixes made from peat/coco/bark need food supplementation, as these mixes do not provide complete plant nutrition, especially in high yield environments. Normally, in the outdoor soil food web, worms help to feed the plants. But indoors we have to do the worm’s job, so we add bottled and dry nutrients to our indoor food web. Indoors we use a root-soil-nutrient food web instead of a root-soil food web.
The constant addition of vermi-compost (EWC) by worms does not apply indoors, and this plant nutrient source must be replaced for high yield indoor organics. The bottled and dry nutrients replace the worm’s decomposition of soil. The switch from soil to soil-less organic (peat, coco, bark), due to soil-less’ airy structure, means most of the nutrients that plants use must be added by the gardener. The dry and bottled nutes that we water in are eaten by the bacteria and fungi, which are eaten by nematodes and protozoa, who poop plant food. The root-soil-nutrient food web.