The recently isolated root-hairless mutant of barley (Hordeum vulgare L), bald root barley, brb offers a unique possibility to quantify the importance of root hairs in phosphorus (P) uptake from soil. In the present study the ability of brb and the wild-type, barley genotype Pallas producing normal root hairs to deplete P in the rhizosphere soil was investigated and the theory of diffusion and mass flow applied to compare the predicted and measured depletion profiles of diffusible P. Pallas depleted twice as much P from the rhizosphere soil as brb. The P depletion profile of Pallas uniformly extended to 0.8 mm from the root surface, which was equal to the root hair length (RHL). The model based on the theory of diffusion and mass flow explained the observed P-depletion profile of brb, and the P depletion outside the root-hair zone of Pallas, suggesting that the model is valid only for P movement in rhizosphere soil outside the root-hair zone. In low-P soil (P in soil solution 3 µm) brb did not survive after 30 d, whereas Pallas continued to grow, confirming the importance of root hairs in plant growth in a P-limiting environment. In high-P soil (P in soil solution 10 µm) both brb and Pallas maintained their growth, and they were able to produce seeds. At the high-P concentration, RHL of the Pallas was reduced from 0.80 ± 0.2 to 0.68 ± 0.14 mm. In low-P soil, P-uptake rate into the roots of Pallas was 4.0 × 10−7 g mm−1 d−1 and that of brb was 1.9 × 10−7 g mm−1 d−1, which agreed well with the double amount of P depleted from the rhizosphere soil of Pallas in comparison with that of brb. In high-P soil, the P uptake rates into the roots of brb and Pallas were 3.3 and 5.5 × 10−7 g mm−1 d−1, respectively. The results unequivocally confirmed that in a low-P environment, root hairs are of immense importance in P acquisition and plants survival, but under high-P conditions they may be dispensable. The characterization of phenotypes brb and Pallas and the ability to reproduce seeds offers a unique possibility of molecular mapping of QTLs and candidate genes conferring root-hair formation and growth of barley.