Cassava is a staple food mainly produced with low management inputs, causing soil depletion and low yields. The use of organic inputs, such as humic substances (HSs), represents a sustainable alternative to increase cassava growth and production, mainly in semi-arid regions such as the Brazilian Northeast. Thus, the objective was to evaluate the foliar application of a biostimulant based on humic substances on the morphophysiology, production, and mineral nutrient contents of cassava. The biofortified cultivar BRS Dourada was grown under field conditions and foliar application of a biostimulant based on humic substances (BHSs, treated plants) or water (untreated, control). The experiment was conducted in a randomized block design with four repetitions. At 225 days after planting, the growth, productivity, and mineral nutrient contents of soil, roots, and leaves were determined. No differences between treated and untreated plants were found for growth and productivity (average 15.2 t ha−1). On the other hand, BHS treatment reduced net carbon assimilation, water use efficiency, and carboxylation efficiency by 34%, 24%, and 47%, respectively. Moreover, BHS treatment reduced nutrient uptake from soil, and Na and K contents in roots and leaves, respectively. A foliar BHS application is not recommended for cassava production in the conditions evaluated here.