by allowed farmers to harvest rice twice a year, was a key innovation of the Song era. Per-mou agricultural output doubled for South China, while increasing only slightly in the north. Scholars offer "conservative estimates" which suggest that agricultural yields rose at least 20% during the Song. Other scholars note that although taxable land rose only 5% during the Song, the actual amount of grain revenues taken in increased by 46%. Agricultural organisation also changed. Unlike the Han and Tang, in which agriculture was dominated by self-sufficient farmers, or the pre-Warring States period and era of division (period between Han and Tang), which was dominated by aristocratic landowners, during the Song agriculture was dominated by non-aristocratic landowners. The majority of farmers no longer owned their land; they became tenants of these landowners, who developed the rural economy through investment. This system of agriculture was to continue until the establishment of the People's Republic of China under Mao. Song commerce During the Song dynasty, the merchant class became more sophisticated, well-respected, and organised. The accumulated wealth of the merchant class often rivalled that of the scholar-officials who administered the affairs of government. For their organisational skills, Ebrey, Walthall, and Palais state that Song dynasty merchants: ... set up partnerships and joint stock companies, with a separation of owners (shareholders) and managers. In large cities, merchants were organised into guilds according to the type of product sold; they periodically set prices and arranged sales from wholesalers to shop owners. When the government requisitioned goods or assessed taxes, it dealt with the guild heads. Unfortunately, like their counterparts in Europe, these guilds restricted economic growth through collaboration with government to restrict competition. Large privately owned enterprises dominated the market system of urban Song China. There was a large black market, which grew after the Jur'chen conquest of North China in 1127. Around 1160, black marketeers smuggled some 70 to 80 thousand cattle. There were many successful small kilns and pottery shops owned by local families, along with oil presses, wine-making shops, and paper-making businesses. The "... inn keeper, the petty diviner, the drug seller, the cloth trader," also experienced increased economic success. Song abolition of trade restrictions greatly aided the economy. Commerce increased in frequency and could be conducted anywhere, in contrast to earlier periods where trade was restricted to the 'Fang' and 'Shi' areas. In all the major cities of the Song