The co-polymer soil subgrade binder relates to compositions utilizing ionic polyampholyte co-polymers in combination with soil to form improved subgrade soil binder compositions for supporting roads and pavement surfaces. The binders were aqueous solutions of three synthetic ionic variations of polyacrylamide: cationic poly(acrylamidopropyl trimethyl ammonium chloride), anionic hydrolyzed poly(acrylamide) and the ampholytic terpolymer poly(acrylamide- co-sodium acrylate-co-(3-acrylamidopropyl) trimethylammonium chloride). Mechanical testing of soil samples showed that a 50% aqueous solution of the ampholytic terpolymer (TPAM) gives the best properties in terms of strength and toughness. Also, the use of this ampholytic terpolymer gave better soil mechanical properties than the traditional standard, Portland cement under equivalent conditions.