The first part of the question could be thought of as "What are the benefits you get from rotating crops?"
If you are a student with library access, you might have access to a scholarly database that can help you find high-quality, peer-reviewed sources that are both primary and secondary.
Primary sources are sources that were made from people who lived and witnessed an event or time period being covered. So, Anne Frank's journal is a primary source. For this particular question, a primary source would have to be made by someone who has rotated crops themselves, or conducted a study on it, for example.
Secondary sources are made "second-hand" based on primary sources. So, if you told me that you saw bigfoot last weekend, and then I wrote an article about what happened, my article would be secondary because I wasn't PRIMARILY the one who witnessed it. This is what I tell my students; primary is original, secondary is second hand.
Secondary sources exist in a wide variety. A secondary source might be if farmer Joe writes an article that says farmer Sally rotates her crops for X, Y, and Z reasons. Another example is if there is an anthology on crops and a sub-section references a famous study on crop rotation. An encyclopedia would be considered a secondary source.