You've hit the problem on its head, but the solution is not to try and change people's behaviour on a large scale, but to incentivise based on the self-interest you've mentioned.

From the consumers perspective, there is little indication as to the quality of soil something has been grown in. But there must be an effect on the plant itself if it is grown in poor soil in the form of lower macro, trace minerals and other indicators of plant health detectable within the plant.

Like the organic standards that cover production methods, we need an opt-in food quality standard. If plants were tested after production for indicators of plant health and labels could be put on vegetables indicating quality. Self-interest of the consumers buying higher quality foods would quickly incentivise farmers into increasing plant quality, which in turn will correct our soil.