This study investigated the root uptake and adsorption kinetics of Kiperin dihydrolyzed collagen (KDHC), an organic nitrogen source rich in amino acids and low-molecular-weight peptides, in radish (Raphanus sativus L.) seedlings grown in perlite. KDHC was applied at 0.1, 1.0, and 10 g L⁻¹ to 7-day-old seedlings. Directly measured solution depletion and tissue accumulation were distinguished from processes inferred from model fits, such as apoplastic adsorption and transport between organs. Amino acid depletion from the root medium increased with the applied concentration, and the Freundlich, Langmuir, Dubinin–Radushkevich, and Flory–Huggins isotherm models described amino acid binding to the root surface. Depletion from the root medium followed pseudo-first-order kinetics (R² = 0.94–0.99), reflecting rapid apoplastic surface adsorption within 60 min. In an ex vivo leaf disk assay, cellular accumulation in leaf tissue followed the pseudo-second-order model (R² = 0.9881), suggesting a carrier-mediated mechanism. Differences among amino acids were interpreted in relation to their charge states at pH 5.8 and their molecular size. Root-to-shoot translocation was not traced directly and should be confirmed using isotope-labelled amino acids. These findings indicate that KDHC-derived amino acids are readily available to radish roots and may serve as an organic nitrogen source.