<P> Modern archaeologists have unearthed Han iron farming tools throughout China, from Inner Mongolia in the north to Yunnan in the south . The spade, shovel, pick, and plow were used for tillage, the hoe for weeding, the rake for loosening the soil, and the sickle for harvesting crops . Depending on their size, Han plows were driven by either one ox or two oxen . Oxen were also used to pull the three - legged iron seed drill (invented in Han China by the 2nd century BCE), which enabled farmers to plant seeds in precise rows instead of casting them out by hand . While artwork of the Wei (220--265 CE) and Jin (265--420) periods show use of the harrow for breaking up chunks of soil after plowing, it perhaps first appeared in China during the Eastern Han (25--220 CE). Irrigation works for agriculture included the use of water wells, artificial ponds and embankments, dams, canals, and sluice gates . </P> <P> During Emperor Wu's (r . 141--87 BCE) reign, the Grain Intendant Zhao Guo (趙 過) invented the alternating fields system (daitianfa 代田 法). For every mou of land--i.e. a thin but elongated strip of land measuring 1.38 m (4.5 ft) wide and 331 m (1,086 ft) long, or an area of roughly 457 m (0.113 acres)--three low - lying furrows (quan 甽) that were each 0.23 m (0.75 ft) wide were sowed in straight lines with crop seed . While weeding in the summer, the loose soil of the ridges (long 壟) on either side of the furrows would gradually fall into the furrows, covering the sprouting crops and protecting them from wind and drought . Since the position of the furrows and ridges were reversed by the next year, this process was called the alternating fields system . </P> <P> This system allowed crops to grow in straight lines from sowing to harvest, conserved moisture in the soil, and provided a stable annual yield for harvested crops . Zhao Guo first experimented with this system right outside the capital Chang'an, and once it proved successful, he sent out instructions for it to every commandery administrator, who were then responsible for disseminating these to the heads of every county, district, and hamlet in their commanderies . Sadao Nishijima speculates that the Imperial Counselor Sang Hongyang (d . 80 BCE) perhaps had a role in promoting this new system . </P> <P> Rich families who owned oxen and large heavy moldboard iron plows greatly benefited from this new system . However, poorer farmers who did not own oxen resorted to using teams of men to move a single plow, which was exhausting work . The author Cui Shi (催 寔) (d . 170 CE) wrote in his Simin yueling (四 民 月 令) that by the Eastern Han Era (25--220 CE) an improved plow was invented which needed only one man to control it, two oxen to pull it, had three plowshares, a seed box for the drills, a tool which turned down the soil, and could sow roughly 45,730 m (11.30 acres) of land in a single day . </P>

Which culture used natural gas as a fuel source in the 3rd century bc