Is soybeans a common noun

Yes, the Word 'soybeans' is a common noun, the plural form for the noun soybean; a Word for any soybeans anywhere.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• American Soybean Association, St. Louis, MO
• North Carolina Soybean Producers Association, Inc., Raleigh, NC
• Soybean Lane, Duncannon, PA
• Soybean Asian Grille, Wilmington, DE
• "Sex, Lies, and Soybeans", a novel by Rick Goeld