With labor in short supply, workers' bargaining power increased dramatically. Various inventions that reduced the cost of labor, saved time, and raised productivity–such as the three-field crop rotation system, the iron plow, the use of manure to fertilize the soil, and the water pumps–were widely adopted. Many former serfs, now free from feudal obligations, relocated to the cities and changed profession to crafts and trades. The more successful became the new middle class. Trade flourished as demands for a myriad of consumer goods rose. Society became wealthier and could afford to fund the arts and the sciences. The Black Death marked the end of the Middle Ages in Europe; the Renaissance had begun.