Harrow, farm implement used to pulverize soil, break up crop residues, uproot weeds, and cover seed. Harrows comprise several varieties. Disk harrows mount concave disks and are frequently referred to simply as disks. One type, the single-action two gang, has two groups of disks, more or less horizontally aligned, with opposing concavities, that throw the soil in opposite directions. The tandem harrow has two to four gangs in tandem, and the offset has two to three gangs in tandem on one side of the tractor, used particularly under low-hanging fruit trees. The horse-drawn or tractor-drawn spike-tooth harrow, or drag, developed in the early 19th century, has sections 1 to 1.5 metres (3 to 5 feet) wide with long spike teeth mounted nearly vertically on horizontal bars. It is used chiefly for pulverizing soil and for early cultivation. Spring-tooth harrows (developed in the 1860s) have curved, springy teeth designed for use in rough, stony ground and around roots. Knife-tooth harrows, with twisted blades spaced several inches apart, are driven in a rotary motion by a small gasoline motor. They are used chiefly by gardeners and are often referred to as rototillers or rotary plows.
