Partial substitution of polymeric methylene diphenyl diisocyanate (pMDI) resin by 10 to 15 percent soy flour for the manufacture of strand board improves board properties while decreasing cost. For particleboard and medium-density fiberboard the soy-substituted resin performs as well as the control pMDI. The reaction of soy flour with pMDI occurs over several hours as tracked by CO2 evolution. The soy-amended resin must be used within about 30 minutes of formulation. Uniform mixing of soy flour with pMDI is critical because unreacted soy flour tends to retain water, which degrades the wet properties of the board. The soy flour increases the tack of pMDI resin, which increases the surface coverage and the relative bonded area at the glue line.