Song. Unlike the Song, however, the new industrial centres were located in the south, rather than in North China, and did not have ready access to coal, a factor that may have contributed to the Great Divergence. Iron output increased to triple Song levels, at well over 300,000 tons. Further innovations helped improve industrial capability to above Song levels. Many Ming-era innovations were recorded in the Tiangong Kaiwu, an encyclopaedia compiled in 1637. Ming agriculture Under the Ming, some rural areas were reserved exclusively for the production of cash crops. Agricultural tools and carts, some water-powered, allowed the production of a sizable agricultural surplus, which formed the basis of the rural economy. Alongside other crops, rice was grown on a large scale. The population growth and the decrease in fertile land made it necessary that farmers produce cash crops to earn a living. Landed, managed estates grew substantially; while in 1379 only 14,241 households had over 700 mou, by the end of the Ming some large landowners had over 70,000 mou. Increasing commercialisation caused huge advances in productivity during the Ming dynasty, allowing a greater population. Many markets, with three main types, were established in the countryside. In the simplest markets, goods were exchanged or bartered. In 'urban-rural' markets, rural goods were sold to urban dwellers; landlords residing in cities used income from rural landholdings to facilitate exchange in the cities. Merchants bought rural goods in large quantities and sold them in these markets. The 'national market' developed during the Song dynasty became more important during the Ming. In addition to the merchants and barter, this type of market involved products produced directly for the market. Many Ming peasants no longer relied upon subsistence farming; they produced products for the market, which they sold for a profit. The Cambridge history states about the Ming that: "The commercialisation of Ming society within the context of expanding communications may be regarded as a distinguishing aspect of the history of this dynasty. In the matter of commodity production and circulation, the Ming marked a turning point in Chinese history, both in the scale at which goods were being produced for the market, and in the nature of the economic relations that governed commercial exchange." Ming collapse At the end of the Ming dynasty, the Little Ice Age severely curtailed Chinese agriculture in the northern provinces. From 1626, famine, drought, and other disasters befell