The relationship between soil nutrients and plants can be complex and vary between plants (e.g. leguminous plants would certainly create a negative feedback loop, enhancing the stability of the system they are thriving in). In general plants left to their own succession would increase organic matter which serves as a nutrient reservoir, further enhancing nutrient availability. In our more controlled agricultural systems, you can see this effect applied in crop rotation and allowing fields to go fallow as a means to increase available nutrients among other potential benefits.

Note that the nature of a feedback loop also depends on how the system's boundaries are defined. This is especially true looking at industrial agriculture and other systems that rely heavily on external inputs or otherwise have far-removed externalities.