Specify a domain:
After entering search terms in Google, try specifying a domain where you'd like to limit your search. Adding "site:.gov" to your search (without the quotes), for instance, will limit your search to sites in the .gov domain. Adding "site:usda.gov" will only find results on the USDA site.
Include subject keywords and site keywords:
Searching for "sorghum nutrient deficiency" is a good first step. Adding "horticulture" or "cooperative extension" or "fact sheet" will also help.