For most farmers until the 1930s, the choice for tilling, sowing, and reaping crops was human power or animal power. Gas engines had existed since the early 19th century, but they were limited to stationary farm and household chores such as grinding grain, churning butter, splitting logs, and washing clothes. In 1910, American farms had more than 24 million horses and mules, about three of four animals per farm. Early tractors were steam-powered, large, difficult to operate, and dangerous. The first gas-powered tractor was introduced in 1892. In 1917, Henry Ford designed the first mass-produced farm tractor, the Fordson, and it spread quickly. By 1920, ten percent of California farmers were using tractors, putting the state well ahead of the rest of the nation. Tractor wheels, however, sank in soft soil until Benjamin Holt solved the problem with caterpillar tractor treads that rolled over soggy ground.