picture of the market economy of the preceding decades in his writing in the early 1690s. The situation only significantly improved after the peace was gradually recovered. By the 1680s the Qing had consolidated its control over the empire, and favourable economic developments resumed. Those developments resemble the late Ming expansion, but the early Qing market had more interregional trade, depended more on overseas markets, and had a larger population. Meanwhile, the relationship between the market and customary and command economies changed. Kang–Qian Age After crushing all Ming royalists in the south by 1683, the Kangxi Emperor revoked destructive measures, such as princely estates and the clearing of the shoreline. He quickly recovered the war-ravaged economy and initiated a period of great prosperity in history. Under his successors' reigns, the Qing dynasty reached its peak in the Kang–Qian Age between the reigns of the Kangxi Emperor and the Qianlong Emperor. This period is also known as "High Qing", during which Kangxi and his successors controlled the areas of Xinjiang and Tibet, conquered the Dzungar Khanate and exercised tighter control over these regions than the Ming dynasty had. This removed a major threat to China and incorporated the regions of Xinjiang and Mongolia into the Chinese economy. During the Qing dynasty, foreign food crops, like the potato, were introduced during the 18th century on a large scale. These crops, along with the general peace in the 18th century, encouraged a dramatic increase in population, from approximately 150–200 million during the Ming to over 400 million during the Qing. During the 18th century, markets continued to expand as in the late Ming period. To give people more incentives to participate in the market, they reduced the tax burden in comparison with the late Ming, and replaced the Corvée system with a head tax used to hire labourers. China continued to export tea, silk and manufactures, creating a large, favorable trade balance with the West. Reversing one of the Ming's trends, the Qing government interfered greatly in the economy. The monopoly on salt was restored and became one of the greatest sources of revenue for the state. Qing officials tried to discourage cultivation of cash crops in favor of grain. Wary of the power of wealthy merchants, Qing rulers limited their trading licences and usually refused them permission to open new mines, except in poor areas. Merchant guilds proliferated in all of the