1. The USDA data is most easily searched using Google (not Google Scholar) or the site's own search software, which is borrowed from Google.
Use site:usda.gov keywords or site:usda.gov filetype keywords
to get data on the topic of the keywords, or data in, for example a spreadsheet file, about the topic of the keyword search.
site:usda.gov .xls coffee, for example, will return links to spreadsheet files of data about coffee.
2. Each of the portals will have different data and be searched somewhat differently. See the tabs on this page for more information on each portal.
3. USDA data is useful for lots of disciplines, including business, and sustainability but also history, sociology,economics, and more.
4. Much of the USDA data goes back decades, or even close to a century. For example, the Census of Agriculture goes back to 1925.
5. The USDA includes trade and commodity data for many countries, both from the USDA and from the World Agriculture Outlook Board.
Animal Health (including veterinary topics), Biotechnology, Employee Services, Energy, Environment and Natural Resources (water, soil and air quality among other topics), Emergency Preparedness and Response, material about the current Farm Bill and related legislation, Food and Nutrition, Food Safety, Forestry, Homeland Security, Laws and Regulations, Marketing and Trade, Outreach, Plant Health, Research and Science, Rural and Community Development, Travel and Recreation, and a section called USDA for Kids which includes materials suitable for children and for school lessons related to agriculture.
Under Assisting Rural Communities, we find programs for: broadband provision, grants and loans, disaster assistance, and insurance programs
Under Food and Nutrition: information about SNAP, WIC, food security, child nutrition programs, and the National Organic Program
Under Conservation: find restoration and conservation, environmental markets, water resources, and wildfire prevention
Under Marketing and Trade: importing, exporting, and data about imports and exports
Under Education and Research: links to Economic Research, Agricultural Research, Agricultural Statistics
Each of the portals is slightly different in its search parameters, but if you start at the USDA's page, they have adopted Google as their internal search engine. You may search the whole array of data or specify the portal that you'd like to cover. You may also specify either English or Spanish language materials.
The USDA's data spans such a long time and is so large a set of files that some files are in older file types, such as *.wk1, which is a Lotus 1 2 3 file. ApacheOpenOffice will open these. Some other files may or may not be convertible to current file standards.