Final answer:
The Song Dynasty introduced a new variety of rice that grew well in poor soil, had a shorter maturation period, and could be harvested twice a year, enabling the support of a larger population.
Step-by-step explanation:
The Song Dynasty introduced a new variety of rice that could grow well in poor soil. This innovation was part of the dynasty's focus on internal economic development and agricultural technology advancements. By implementing irrigation methods, such as chain-driven pumps, and introducing strains of rice that yielded more food per acre, the Song Dynasty was able to support a growing population.
The new varieties of rice not only produced more food per acre compared to wheat or millet but could also be harvested twice a year due to a shorter maturation period. Moreover, these rice strains were more resistant to disease and had other beneficial attributes. The Song government facilitated the distribution of these efficient seeds among farmers, which supported the sustenance of a larger population while allowing cultivators to pursue specialized farming and participate in the market economy.