High soil nutrient levels is not an adequate measure of soil fertility.
In the same way, counts of specific forms of soil biology are not a measure of the contribution of soil biology to plant nutrition.
The proportions of nitrogen, phosphorus and potassium in the soil are not reflected in the proportions of those elements found in plant leaves through tissue or sap testing.
We seem to be resolutely fixed on counting mg/kg of chemical constituents in soil. Yet we do not consider the fundamental soil fertility processes and find indicators of the strength of their functioning.
It is as though we could characterise the health of a person by the volume of blood in their body, noting small changes. For very good reason, we prefer indicators of the transfer of oxygen to tissues as an indicator and for that we measure the functioning of the circulatory system, relying initially at least on blood pressure and pulse counts.
What matters to measure and observe then are the best indicators we have for those factors which favour essential soil processes. The list might include:
- soil bulk density
- soil aggregate stability
- soil porosity
- biodiversity of soil fungi, bacteria and archaea