What factors have enabled American farmers to grow huge amounts of crops for sale in cities throughout this country and in foreign countries?
Highly mechanized machinery that uses fossil fuels + large tracts of land, and heavy use of fertilizers all make this an economic proposition for farmers.
North American agriculture evolved from small plots of land wherein the soil was tilled mostly by animals or even human power. This is called subsistence farming as a single family survives on the output of their farm. Around the turn of the last century, fossil fuel tractors were introduced and a farmer could farm larger tracts of land, have a surplus beyond what his family needed and they could sell the rest.
Now in the 2000, the economics of farming favour very large tracts of land, and highly mechanized machinery to produce large surpluses for sale in the local and global markets. They also must make heavy use of fertilizers to keep crop production high and they have to make extensive use of pesticide and herbicides to control insects and weeds. In this sense, the small family farm is getting squeezed out of existence. Many farmers now have semi-automated tractors or combines that drive themselves mostly, by GPS and laptop technology. One farmer can now essentially feed hundreds of city dwellers!
There is also a well established infrastructure of grain elevators, roads, railways and shipping ports to get surpluses to overseas markets.
But this farming approach has a number of environmental concerns and is probably not sustainable in the long run. Water pollution, soil depletion/erosion and contributing to greenhouse gas emissions are all related problems that are growing. People are also demanding more organically grown and sustainable food and so farmers need to adapt.