sometimes involving slaves. Hoofed animals could also be used to till soil by trampling, in addition to pigs, whose natural instincts are to root the ground regularly if allowed to. The wooden plow was then invented. It could be pulled with human labor, or by mule, ox, elephant, water buffalo, or a similar sturdy animal. Horses are generally unsuitable, though breeds such as the Clydesdale were bred as draft animals. Tilling could at times be very labor-intensive. This aspect is discussed in the 16th-century French agronomic text written by Charles Estienne: The popularity of tillage as an agricultural technique in early modern times had to do with theories about plant biology proposed by European thinkers. In 1731, English writer Jethro Tull published the book "Horse-Hoeing Husbandry: An Essay on the Principles of Vegetation and Tillage," which argued that soil needed to be pulverized into fine powder for plants to make use of it. Tull believed that, since water, air, and heat were clearly not the primary substance of a plant, plants were made of earth, and thus had to consume very small pieces of earth as food. Tull wrote that each subsequent tillage of the soil would increase its fertility, and that it was impossible to till the soil too much. However, scientific observation has shown that the opposite is true; tillage causes soil to lose structural qualities that allow plant roots, water, and nutrients to penetrate it, accelerates soil loss by erosion, and results in soil compaction. The steel plow allowed farming in the American Midwest, where tough prairie grasses and rocks caused trouble. Soon after 1900, the farm tractor was introduced, which made modern large-scale agriculture possible. However, the destruction of the prairie grasses and tillage of the fertile topsoil of the American Midwest caused the Dust Bowl, in which the soil was blown away and stirred up into dust storms that blackened the sky. This prompted re-consideration of tillage techniques, but in the United States, as of 2019 3 trillion pounds of soil are still lost to erosion and adoption of improved techniques is still not widespread. Types Primary and secondary tillage Primary tillage is usually conducted after the last harvest, when the soil is wet enough to allow plowing but also allows good traction. Some soil types can be plowed dry. The objective of primary tillage is to attain a reasonable depth of soft soil, incorporate