China, from Yan in the north to Chu in the south. Large feudal estates were broken up, a process hastened when Lu changed its taxation system in 594 BCE. Under the new laws, grain producers were taxed by the amount of land under cultivation rather than an equal amount being levied upon every noble. Other states followed their example. Free peasants became the majority of the population and provided a tax base for the centralising states. Warring States period (475–221 BCE) The Warring States period saw rapid technological advances and philosophical developments. As rulers competed to take control of one another's lands, they implemented various reforms, greatly changing China's economic system. Common-born landowners and merchants prospered and the remaining aristocracy lost influence. Some merchants, such as Lü Buwei, may have been as wealthy as minor states. State-sponsored foundries made iron tools ubiquitous; the iron plough, draft oxen, row cultivation, and intensive hoeing were introduced. Cast iron was invented in China during the 4th century BCE. Governments, which controlled the largest ironworks, developed a monopoly on military equipment, strengthening the states at the expense of the feudal lords. Iron agricultural tools allowed a massive increase in surplus farm goods. After Shang Yang's reforms in the 3rd century BCE, land could be bought and sold, stimulating economic progress in agriculture and an increase in productivity. The newly powerful states undertook large-scale irrigation projects, such as the Zhengguo Canal and the Dujiangyan Irrigation System. Large amounts of previously desolate land were cultivated and integrated into the Qin economy. The agricultural boom allowed larger armies. In this era, the growing power of the state strengthened the monarchy, allowing it to undertake reforms to strengthen the monarch's authority. The most extensive of these reforms was carried out in Qin by Shang Yang, including the abolition of the feudal nobility, redistribution of nobles' land based on military merit, and allowing private ownership of land. He encouraged the cultivation of unsettled lands, gave noble ranks to soldiers who performed well in battle, and established an efficient and strict legal code. Absolute monarchy persisted in China until its gradual weakening under the Song and Ming dynasties. Early imperial era The early imperial era was marked by a strong, unified and centralised monarchy, though local officials still maintained limited autonomy. During the early imperial era, self-sufficient peasant farmers and artisans dominated the economy and largely operated independently of the