Twenty-five species of Leguminosae, planted in randomized replications on three southern Iowa sites which were distinct in soil types and previous treatment and which differed somewhat also in microclimate, were studied in 1939, 1940 and 1941 for variations in response to habitat;The experimental species included selected strains of several crop plants and certain other native and introduced plants which were considered to be of promise in soil conservation and improvement;Two of the sites were upland areas of nearly equal elevation, both of which had southeast exposure, and were otherwise comparable except for soil. The third was a nearly level lowland site whose lower elevation was presumed to be its only major cause of microclimatic difference. The three sites were separated by a maximum distance of about 660 yards;In soil factors, two of the sites were higher in general fertility levels and in pH than the other because of previous liming and fertilizer treatments not given the third site. The only major soil property in which the unlimed site was considered more favorable to plant growth than the other sites was its greater availability of soil moisture;In microclimate, the three sites differed chiefly in atmospheric moisture, the distinction in other climatic factors being less great. Maximum air temperature, minimum air temperature, maximum soil temperature, relative humidity and wind velocity also were measured. The upland limed site was most xeric and had the highest mean air temperature;Best average growth response of plants was obtained in the lowland site, poorest on the upland limed site and intermediate on the upland unlimed site. Rank in growth response was in the same order as was the presumed microclimatic favorability of the sites, but contrary to the general fertility levels of the upland sites, in which, except for soil moisture, the limed site was more favorable than the unlimed;Highly significant correlations were shown within sites between average plant growth response and soil organic matter as measured by carbon and nitrogen contents, as well as between plant growth and exchangeable calcium; and total exchangeable bases. Negative but non-significant correlations between average plant growth and pH and percentage base saturation were found;Of the experimental species, Lotus corniculatus was shown to be best adapted to the wide range of soil and climatic conditions found in the three habitats. Its excellent response was partially attributed to its marked ability to absorb and utilize calcium and phosphorus from the low-quality soils.