The commonly accepted model of plant nutrition holds that plants absorb soluble mineral ions from the soil solution through root hairs. In this model, plants are dependent on soluble ions and adequate water supply in the soil to allow the transport of those ions to plant roots. This dependency on water and solubility has several potential weak links. First, when mineral ions are water soluble and transported by water, they can leach into ground water and leave fields as runoff very easily. Secondly, most crops experience their greatest nutritional requirements when they are filling and ripening fruit, which usually occurs at a period in the growing season when rainfall tapers off and there is limited moisture in the soil.
Dry soil conditions can restrict the flow of soluble nutrients to plant roots when they are most needed.
An alternative model of crop nutrition suggests that plants should absorb a great deal of their nutrition in the form of microbial metabolites. In this framework, the soil microbial community acts as a digestive system for the plants. Bacteria, fungi, nematodes, protozoa, actinomycetes, archae, and other unknown microbes digest and breakdown organic residues and plant root exudates. These root exudates are rich in energy, mostly composed of compounds like carbohydrates and lipids, but contain very limited amounts of the minerals needed by microbes to form their cell compounds. To supply the elements they require, they extract minerals from the soil mineral matrix to build their bodies. As this microbial population cycles, their bodies are decomposed by other soil organisms making the minerals contained in their bodies available to plants in the form of organic and amino acids combined with the mineral elements necessary for plant growth.
In order for plants to achieve level 3 of plant health on the plant health pyramid, plants need an active microbial digestive system in the soil. As long as they are dependent on absorbing simple ions, they will never have the surplus energy needed to store elevated levels of lipids. It requires too much of the plant's energy to convert these simple ions and metabolize them in forms that it can use to build the necessary compounds, as compared to absorbing 'prefabricated' parts put together by soil biology. The difference in energy needed in these two processes is analogous to building a house from raw lumber compared to using prebuilt walls and trusses.