After the plough comes the harrow, from Han dynasty rakes to concrete-weighted giant discs
A plough leaves a field of furrows and big clods, which is useless for planting small seeds. The harrow follows to crumble that rough ground into fine, even tilth. Chinese farmers had turned the humble rake into two distinct kinds of harrow by the end of the Han dynasty, one for dry fields and one for paddies.
Unlike a plough, which slices the top 12 to 25 centimetres of soil and leaves parallel trenches, a harrow works the entire surface. That also sets it apart from a cultivator, which only stirs narrow strips between crop rows to kill weeds. Coarse harrowing can itself uproot weeds and bury freshly sown seed.
Han farmers knew a flat harrow called the pa, used across China in wet and dry fields alike, and an upright type called the chao, found only in the rice paddies of the south. Murals at Jiayuguan from the Wei-Jin period show the pa three times, and the oldest known image of the chao is a pottery model from a Guangdong grave made between 310 and 312. The Northern Wei official Jia Sixie described the tool in his farming manual Qimin Yaoshu as a rake with iron teeth, shaped as a strip, a Y or a square, used to break clods and fight weeds, pests and disease. Some versions swapped the iron teeth for canes and brambles. Europe used harrows too, through antiquity and the Middle Ages.
Modern types fall into four families: disc, tine, chain and chain-disc. Once pulled by horses, mules, oxen or people, they now ride behind tractors. A common routine runs a disc harrow first to chop the clods left by the mouldboard plough, with a spring-tooth harrow hitched directly behind it, leaving powdery soil in a single pass. Where the result is fine enough to blow away, a roller brings up the rear.
Chain harrows also tidy pastures by spreading dung and tearing out dead thatch, and groundskeepers drag them over sports fields to erase boot marks. In rough southern hemisphere country full of stumps and rocks, giant scalloped discs weighted with concrete or steel blocks replace the plough entirely. Power harrows spin sets of vertical tines, moving soil sideways without turning it over, so buried weed seeds stay buried and no hardpan forms.
Source: Harrow (tool)
