The onion is one of the most cultivated and consumed vegetables worldwide. In Colombia its seed is concentrated in the departments of Boyacá and Cundinamarca. However, this crop currently presents problems of plagues, diseases and low mineral nutrition. The good supply of macro and micronutrients is essential for the plant to develop its metabolic processes properly. Within the macronutrients, phosphorus (P) plays an important role since it is part of the transfer of energy in the metabolic processes and synthesis of the cellular structures of plants. For this reason the aim of the research was to evaluate an unconventional source of phosphorus in onion plants (Allium cepa L.) under field conditions, for which a completely randomized design was used with four treatments that consisted in the application of a thermal phosphate in different percentages The physiological and growth variables evaluated were: fresh and dry foliar, and bulb mass, leaf area, phosphorus, calcium and magnesium foliar content, and yield. There were significant differences (P ≤ 0.05) in the variables foliar and bulb fresh mass, bulb dry mass, leaf area, magnesium leaf content and yield versus the application of a conventional source, this converts the thermal phosphate into an alternative to take into account within the fertilization plans of semi-annual crops within a mineral nutrition scheme under the study conditions.