Agriculture is a major industry in the United States, and the country is a net exporter of food. As of the last census of agriculture in 2007, there were 2.2 million farms, covering an area of 922 million acres (3,730,000 km2), an average of 418 acres (1.69 km2) per farm. Although agricultural activity occurs in most states, it is particularly concentrated in the vast expanse of flat, arable land known as the Great Plains, which encompasses the central region of the nation. Corn, turkeys, tomatoes, potatoes, peanuts, and sunflower seeds constitute some of the major holdovers from the agricultural endowment of the Americas. European agricultural practices greatly affected the New England landscape, leaving behind many physical foot prints. Colonists brought livestock over from Europe which caused many changes to the land. Grazing animals required a lot of land and food to sustain them and due to grazing, native grasses were destroyed and European species began to replace them. New species of weeds were introduced and began to thrive as they were capable of withstanding the grazing of animals, whereas native species could not. The practices associated with keeping livestock also contributed to the deterioration of the forests and fields. Colonists would cut down the trees and then allow their cattle and livestock to graze freely in the forest and never plant more trees. The animals trampled and tore up the ground so much as to cause long-term destruction and damage. Soil exhaustion was a huge problem in New England agriculture. Farming with oxen did allow the colonist to farm more land but it increased erosion and decreased soil fertility. This was due to deeper plow cuts in the soil that allowed the soil more contact with oxygen causing nutrient depletion. In grazing fields, the large number of cattle in the New England, the soil was being compacted by the cattle and this didn't give the soil enough oxygen to sustain life. In the U.S., farms spread from the colonies westward along with the settlers. In cooler regions, wheat was often the crop of choice when lands were newly settled, leading to a "wheat frontier" that moved westward over the course of years. Also very common in the antebellum Midwest was farming corn while raising hogs, complementing each other especially since it was difficult to get grain to market before the canals and railroads. After the "wheat frontier" had passed through an area, more diversified farms including dairy cattle generally took its place. Warmer regions saw plantings of cotton and herds of beef cattle. In the early colonial south, raising tobacco and cotton was common, especially through the use of slave labor until the Civil War. In the northeast, slaves were used in agriculture until the early 19th century. In the Midwest, slavery was prohibited by the Freedom Ordinance of 1787. The introduction and broad adoption of scientific agriculture since the mid nineteenth century has made a large improvement in the USA's economic growth. This development was facilitated by the Morrill Act and the Hatch Act of 1887 which established in each state a land-grant university (with a mission to teach and study agriculture) and a federally funded system of agricultural experiment stations and cooperative extension networks which place extension agents in each state. Soybeans were not widely cultivated in the United States until the 1950s, when soybeans began to replace oats and wheat. Significant areas of farmland were abandoned during the Great Depression and incorporated into nascent national forests. Later, "Sodbuster" and "Swampbuster" restrictions written into federal farm programs starting in the 1970s reversed a decades-long trend of habitat destruction that began in 1942 when farmers were encouraged to plant all possible land in support of the war effort. In the United States, federal programs administered through local Soil and Water Conservation Districts provide technical assistance and partial funding to farmers who wish to implement management practices to conserve soil and limit erosion and floods.