After your search, the results page will show how many hits you received from the initial search. As you can see the original search for "South America" AND agriculture returned over 24,000 results.
With over 24,000 hits you will want to narrow the results. An easy way to do this is with the Filters on the left side of the page. Click the box next to the filter you want to use and the database will automatically update.
Use the Subject: Thesaurus Term to narrow even further. Click on the Show More link to open a window with more options.
Did you narrow your results too much? You can always go back by clicking an X next to a filter to undo it, (this box is at the top of the limiters column).