Historians largely agree that it was increased nitrogen fixation from new legumes as well as better nutrient cycling from larger livestock herds that allowed England to feed a growing urban population without a proportionate expansion of arable acres. This project explores the forgotten pest suppression benefits of the bright yellow alternative crop Brassica napus (B. napus), which still goes by the unfortunate common name of “rapeseed.” As an alternative crop, B. napus may have been grown at a smaller scale but likely had a significant environmental impact by suppressing pathogens on strained soils during periods of agricultural intensification. Today B. napus and its sister plants in the mustard family blanket the fields of Northern California, England, Sweden, Germany, and other hotspots of sustainable agriculture, where the seeds are crushed for oil and the meals are used as a soil amendment. Field and potting studies have shown that the crushed Brassica oilseed meals and cakes left from the press can be used as a soil amendment to support beneficial bacteria, trigger plant innate immune defenses, and release pest-fighting chemicals that help suppress soil diseases. Both the living B. napus roots and tilled stems also increase soil fecundity. It turns out that eighteenth-century farmers similarly used crushed B. napus as a manure to control pests, and they harnessed the plant for soil improvement in nearly every county in England and Scotland. As a feed, fodder, and green fallowing technique for convertible husbandry, B. napus was also a frost and drought hardy plant that extended the growing season into the winter and spring and protected against crop failure in polyculture with turnips. It performed well in sandy and water-logged soils that could support few plants and was sometimes the first crop planted on drained marsh or moorland to convert acres to pasture and then arable. B. napus was widely dispersed by the early seventeenth century and planted at the scale of four-thousand acres on some farms. By mining more than 200 period improvement publications and casting the nets widely at the archives to locate and review rare planting logs, tithe surveys, enclosure maps, leases, and unpublished improvement correspondence, this work presents one of the first in-depth histories of the environmental impacts of an alternative crop. This case study is among the first to explore the role of pathogen suppression improvements in the British Agricultural Revolution and highlights the need for a more careful analysis of the many other eighteenth-century soil manures and fodders that remain on the sidelines of the historiography. Rapeseed is often excluded from the list of turnips, clover, legumes, and other rotation crops listed along with enclosure, convertible husbandry, and mechanical invention as the defining technologies of the British Agricultural Revolution. However the crop helped remake the practice of Jethro Tull’s seed drill and horse-hoe by encouraging the use of mechanical technologies for fodder planting for grazing, for green fallowing, and for better crop rotation. B. napus cake and meal crushed at the oilseed press was drilled along with seed, and the earliest evidence of rapeseed cake manuring dates to the early seventeenth century. The technology’s wider use in the eighteenth century suggests that plant manures began to play an important role at least 80 to 100 before the 1815 to 1830 dates suggested by other historians. During the eighteenth century, farmers worked to increase the oil in rapeseed cake to ensure a more potent manure. In contrast, when drilling and horse-hoeing were implemented as a Tullian method, they may have increased the risk of soil exhaustion by excluding rotation, manuring, and fallowing. Finally, historical sources show that B. napus plantings were used to control wireworms and molds as early as the mid-eighteenth century, and cake and meal manures were adopted to kill a range of pests by the late eighteenth to mid-nineteenth centuries. Though the multiple species names descried by the common term “turnip” are more ambiguously referenced in period sources, mid-to-late eighteenth-century authors confirm that rapeseed was the B. napus taxonomically identified by Linnaeus. By exploring the combining of B. napus with other new plants and mechanized drills and hoes, this project underscores the innovative designs that integrated technologies into temporal and spatial arrangements to increase fecundity.