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Problems often found in cactus crops in the semi-arid region of Brazil include diminished soil fertility and low plant density. Despite its importance, root development of cactus is not often measured, particularly at the field and associated to different agronomic practices. Thus, this research evaluated the effect of four organic fertilization levels (0, 20, 40, and 80 mg/ha of manure applied on a dry matter basis) and four plant population (20,000; 40,000; 80,000; and 160,000 plants/ha) on the shoot and root biomass yield of cactus (Opuntia ficus-indica Mill. 'IPA 20'). A split-plot arrangement in a complete randomized block design was applied; main plots were formed by organic fertilization levels and split-plot formed by plant populations, with four replications per treatment. The harvest was performed after two years of regrowth. Response variables measured included root biomass, biomass of planted cladode and shoot biomass (not including planted cladode). Shoot biomass increased from 5.4 to 14.2 mg of dry matter (DM) per ha from 0 to 80 mg/ha of manure, respectively. Biomass of planted cladodes (DM basis) reduced as plant population increased (870 g/plant at 20,000 plants/ha vs. 568 g/plant at 160,000 plants/ha). A similar pattern was observed for the roots, with less root biomass per plant at higher population density (136 g/plant at 20,000 plants/ha vs. 77 g/plant at 160,000 plants/ha). Root biomass per ha was negatively affected by increasing plant population in the absence of organic fertilization (4.3 vs. 1.9 mg/ha of root DM at 20,000 and 160,000 plants/ha, respectively). When manure was added the fluctuation in root biomass per area was lower. These results have implications on plant persistence and drought tolerance. Regions more prone to drought and lower average rainfall should not have dense populations in order to maximize root development per plant.