Creosotebush (Larrea tridentata) and tarbush (Flourensia cernua) are two of the major shrub invaders of grassland in many desert areas of southern New Mexico. Soils and root systems associated with these two shrubs were studied at three sites on an alluvial fan piedmont. The soils have formed in alluvium derived from monzonite, rhyolite, and andesite, in deposits ranging in age from late Holocene to middle Pleistocene. Soil age, carbonate morphology, particle size, and landscape position were found to be major factors associated with root variability. The stage I carbonate that occurs in youngest soils has relatively little influence on root distribution because the carbonate consists only of thin coatings on sand grains and pebbles. But the increasing carbonate that occurs in stages II, III, and IV results in denser zones of carbonate that control the routes for movement of both soil water and roots. Not only do individual nodules and cemented pebbly zones grow and eventually merge, they also represent zones of restricted hydraulic conductivity, funneling soil water and roots to as yet uncemented parts of the horizon. Continued carbonate accumulation leads to a plugged horizon and an overlying stage IV laminar horizon that is a barrier to roots. Particle size controls the time and amount of carbonate required for formation of these horizons. Calciargids of late Pleistocene age that averaged 4% and 32% by volume of gravel and contained 415 and 317 kg m⁻² of pedogenic carbonate had only stage III horizons. In contrast, a Petrocalcid of the same age and averaging 63% by volume of gravel required only 205 kg m⁻² of carbonate to form the stage IV horizon. Roots penetrated occasional openings in the stage IV horizon. Runoff from soils sloping 2% reduces the number of days with available soil water at various depths as compared to 1% slopes below. At the 2% slope, roots did not extend below 2 m depth, whereas at the 1% slope roots extended to as much as 5 m depth.