<P> The internal combustion engine; first the petrol engine, and later diesel engines; became the main source of power for the next generation of tractors . These engines also contributed to the development of the self - propelled, combined harvester and thresher, or combine harvester (also shortened to' combine'). Instead of cutting the grain stalks and transporting them to a stationary threshing machine, these combines cut, threshed, and separated the grain while moving continuously through the field . </P> <P> Combines might have taken the harvesting job away from tractors, but tractors still do the majority of work on a modern farm . They are used to push implements--machines that till the ground, plant seed, and perform other tasks . </P> <P> Tillage implements prepare the soil for planting by loosening the soil and killing weeds or competing plants . The best - known is the plow, the ancient implement that was upgraded in 1838 by John Deere . Plows are now used less frequently in the U.S. than formerly, with offset disks used instead to turn over the soil, and chisels used to gain the depth needed to retain moisture . </P> <P> The most common type of seeder is called a planter, and spaces seeds out equally in long rows, which are usually two to three feet apart . Some crops are planted by drills, which put out much more seed in rows less than a foot apart, blanketing the field with crops . Transplanters automate the task of transplanting seedlings to the field . With the widespread use of plastic mulch, plastic mulch layers, transplanters, and seeders lay down long rows of plastic, and plant through them automatically . </P>

Why are machines used for farming in usa