🌾 Crop Rotation

Each season the soil nitrogen pool is updated as: soilN += cropN + fertiliserN × 0.6 − 4 − harvestExport. The term cropN is crop-specific: clover (+22), beans (+18), cover crop (+4), barley (−14), turnips (−10), wheat (−16). Fertiliser efficiency is set at 60 % to reflect leaching and volatilisation losses. A baseline leaching loss of 4 units per season is always subtracted. For harvested cereals and roots an additional export of 6 × yield units is removed, reflecting nitrogen leaving the field in grain or roots. The pool is clamped between 2 and 140. Yield itself follows a Michaelis-Menten-like saturation: f(N) = soilN / (soilN + 40), so returns diminish at high soil N.

Before the 18th century, English farmers left one-third of their land fallow each year to rest the soil, meaning up to a third of agricultural land produced nothing. The Norfolk four-course rotation — wheat for cash, turnips as livestock fodder and weed-suppressing root break, barley undersown with clover, and clover to fix nitrogen and feed livestock — replaced the fallow entirely. Clover restored soil nitrogen naturally, turnips broke cereal pest cycles, and livestock fed on turnips and clover returned manure to the field. Originating in the Low Countries and popularised in Norfolk by figures such as Viscount Townshend ("Turnip Townshend") in the 1730s, it was a cornerstone of the British Agricultural Revolution.

No — this is a common misconception. While synthetic nitrogen fertiliser compensates for the nitrogen-depletion aspect of monoculture (and you can observe this in the simulator by cranking the fertiliser slider), it does nothing to stop pest and disease build-up. Continuous wheat, for example, causes severe take-all disease (caused by Gaeumannomyces tritici) and eyespot infections that fertiliser cannot cure. High synthetic N inputs also carry environmental costs: nitrous oxide emissions, leaching into waterways, and energy-intensive Haber-Bosch production. Rotation therefore remains agronomically and environmentally superior even when fertiliser is available.

The role of legumes in enriching soils was known empirically by Roman agronomists — Columella and Pliny the Elder both recommended growing beans before wheat — but the mechanism was unknown for nearly two millennia. In 1886 the German scientists Hermann Hellriegel and Hermann Wilfarth proved experimentally that legumes fix atmospheric nitrogen specifically through symbiosis with bacteria in root nodules. The bacteria were later identified and named Rhizobium in 1889 by Martinus Beijerinck. This discovery gave a scientific basis for the ancient practice and laid the groundwork for modern soil microbiology.

Crop rotation connects to several neighbouring topics in ecology and earth science. Predator-prey population dynamics (Lotka-Volterra equations) share the same logic of boom-and-bust cycles that pest management exploits. Nutrient cycling simulations model the broader nitrogen, phosphorus, and carbon cycles of which soil N is one compartment. Soil microbiology simulations explore the microbial communities — bacteria, fungi, nematodes — whose composition rotation strongly shapes. Climate and carbon sequestration models intersect with rotation because organic-matter-building rotations (with cover crops and legumes) sequester carbon in soil. You can explore predator-prey and population models elsewhere on this site.

Modern precision agriculture uses satellite imagery, soil sensors, and machine-learning models to optimise rotation planning at the sub-field level. Decision-support software ingests yield maps, soil survey data, and commodity prices to recommend which crop to plant in each field parcel each year — effectively automating the agronomist's rotation design. Drone-mounted multispectral cameras monitor crop health and pest pressure in-season, feeding data back into rotation-optimisation algorithms. Research institutions also use process-based crop models (e.g. DSSAT, APSIM) that simulate soil-plant-atmosphere interactions at daily time steps to test rotation strategies before field implementation — the approach this simulator captures in simplified form.

Active research areas include the design of rotations optimised simultaneously for yield, carbon sequestration, and biodiversity — goals that can conflict. Scientists are investigating how cover-crop mixes (polycultures) outperform single-species covers in building soil microbial diversity. Genomic tools are enabling plant breeders to develop legume varieties with enhanced nitrogen-fixation efficiency, potentially reducing synthetic fertiliser requirements further. Climate-change adaptation research examines how rising temperatures and shifting precipitation patterns will alter the optimal rotation sequences for different regions. There is also growing interest in perennial grain crops (e.g. Kernza wheat-grass) that could restructure rotation fundamentally by replacing annual reseeding with persistent root systems.