Wikipedia
The three-field system is a regime of crop rotation that was used in medieval and early-modern Europe. Crop rotation is the practice of growing a series of different types of crops in the same area in sequential seasons. Under this system, the arable land of an estate or village was divided into three large fields: one was planted in the autumn with winter wheat or rye; the second field was planted with other crops such as peas, lentils, or beans; and the third was left fallow, in order to allow the soil of that field to regain its nutrients. With each rotation, the field would be used differently, so that a field would be planted for two out of the three years used, whilst one year it "rested". Previously a "two field system" had been in place, with half the land being left fallow. The three field system allowed farmers to plant more crops and therefore to increase production and legumes have the ability to fix nitrogen and so fertilize the soil. With more crops available to sell, this also helped the economy in general to thrive.
The three field system required more ploughing of land and its introduction coincided with the adoption of the moldboard plow. These parallel developments worked hand in hand to increase the productivity of the land. The legume crop needed summer rain to succeed, and so the three field system was less successful around the Mediterranean. Oats for horse food could also be planted in the spring, which, combined with the adoption of horse collars and horseshoes, led to the replacement of oxen by horses for many farming tasks, with a concomitant increase in agricultural productivity and the nutrition available to the population.
One of the first Germans to question this system, and new ways of expanding beyond this medieval system was Johann Friedrich Mayer, in his 1769 work Lehre vom Gyps als vorzueglich guten Dung zu allen Erd-Gewaechsen auf Aeckern und Wiesen, Hopfen- und Weinbergen.