The relationship between soil and vegetation was studied in a salt marsh dominated by Sporobolus virginicus (L.) Kunth (Poaceae). We used 42 sampling units placed on a 66 m long transect; in each soil sample we measured the bulk density (g/cm³), organic matter (%), inorganic carbon (%), pH, texture, soil salinity (‰) and interstitial salinity (‰). Also, the relative cover of each species (%), total vegetation cover (%), total biomass of the vegetation (g/m²), and the relative composition (%) of each species was measured. The community consisted of 7 species and was dominated by 2 S. virginicus morphotypes: a large morphotype (probably virginicus variety) was associated with soils characterized by higher pH, higher bulk density, lower salinity and lower organic matter (%), in comparison with the small morphotype (probably minor L. M. Bailey variety). The pH, organic matter, salinity and bulk density determined significantly (p<0.001) 26% of explained variance of this community. The transect showed three zones with different edaphic characteristics and different herbaceous communities; nevertheless, those communities had some common species. The water dynamic of the soil may respond to small topographic variations, which could in turn determine differences on local flood and evaporation processes.