During the last fifteen years, the impressive increase in the area planted with soybeans in Argentina, since the commercial release of glyphosate-tolerant varieties in 1996, has sparked a heated debate about its implications. There is wide concern about the detrimental effects of this process, especially on organic matter content (its main competitor for land, maize, provides a significant amount of organic matter, which is not the case with soybeans). A comparative analysis of the evolution of the area planted with both soybeans and maize was made between five countries in the Americas: The United States, Argentina, Brazil, Paraguay and Bolivia, for the crop seasons 1980/81 through 2011/12. An index was constructed to reflect deviations from an assumed equilibrium allocation of land, based on the ratio: area with soybeans / (area with soybeans + area with maize). This “relative land allocation index” (RLAI) was calculated for the countries included in the study for the period under analysis and the resulting trend lines were compared. Argentina was the country with the highest RLAI in 2011/12, with a deviation of 67% above the assumed equilibrium value for the RLAI (1.0), followed by Paraguay (that was already in “disequilibrium” at the beginning of the period of analysis), Bolivia and Brazil. The RLAI for the USA remained very close to the assumed equilibrium point for the entire period. Bolivia showed the highest positive rate for the RLAI series trend line (5%/yr), followed by Argentina (3%) and Brazil (2%). Both USA and Paraguay show a flat trend line (0%) but the casual factors are considered to be of a completely different nature. To estimate the effect of the commercial release of genetically modified (GM) glyphosate-tolerant soybeans on the RLAI, a simple linear regression model was constructed, using a dummy variable to control for the year in which glyphosate-tolerant varieties were made available to farmers. This variable turned out to be significant for Brazil, Argentina and Paraguay. The adjusted R2 for Brazil resulted to be the highest (0.8), followed by Bolivia (0.6) and Argentina (0.5), suggesting that the availability of GM soybeans played a bigger role in the expansion of this crop in Brazil than it did in both Bolivia and Argentina.