Soil nitrogen · pest pressure · yield across multiple years
Each season a single crop is grown on the field. The model tracks three coupled quantities. Soil nitrogen is a pool updated by each crop's N balance: legumes (clover, beans) fix nitrogen from the air via root bacteria and add to the pool (+N); cereals (wheat, barley) and roots (turnips) are hungry feeders and deplete it (−N); cover crops protect and lightly build the soil.
Pest pressure is tracked per crop family. If the same family is grown repeatedly, family-specific pests and soil-borne diseases build up and impose a yield penalty. When a family is absent, its pest pressure decays — this is exactly why rotation works: it starves specialist pests of their host. Yield for the season is base × f(N) × f(pest), where the nitrogen factor rises with the soil N pool (saturating) and the pest factor falls as pressure builds.
Compare the declining Monoculture with the classic Norfolk four-course (wheat → turnips → barley → clover) that transformed 18th-century British agriculture by eliminating the fallow year. The sustainability score rewards high yield with a stable or rising soil-N trend and low pest pressure.