If you cast Plant Growth for its 8-hour version to
[...] enrich the land. All plants in a half-mile radius centered on a point within range become enriched for 1 year. The plants yield twice the normal amount of food when harvested.
and then come back the next day and cast it again the same way for the third time, would the total crops harvested be doubled, and then, quadrupled, and then octupled?
The campaign I am in is measured in years, so the impact of a druid (or in this case, nature cleric) who can spend 8 hours to double a 1/2 mile radius circle of food would be super helpful, but I need to know if I can prioritize one spot to make the growth exponential, vs going and casting in a different spot every day to just have a net double.