Background and aim: Phosphorus (P) is an essential macronutrient with major impacts on global crop productivity. Recent work showed that chlorophyll a fluorescence analysis can be used as a sensitive indicator of latent P deficiency across different plant species. Here, we demonstrate that chlorophyll a fluorescence OJIP transients are a powerful tool for early detection of P deficiency directly in the field. Methods: Barley was grown in a P responsive field. One treatment received 30 kg P ha⁻¹ at sowing, four treatments were fertilized with P at 26, 35, 46 or 56 days after sowing (DAS), respectively, and the final treatment did not receive any P throughout the experiment. Chlorophyll a fluorescence measurements, multi-elemental leaf analysis, and growth stage evaluation were performed 26, 35, 46, 56, and 69 DAS. Results: Phosphorus deficiency during early vegetative growth irreversibly affected plant development including tiller outgrowth and grain yields. However, in the present study, yield reduction could be avoided if short-term P deficiency was corrected by application of P fertilizer no later than 35 days after sowing, when plants had not yet entered the tillering stage. The chlorophyll a fluorescence OJIP transients were able to detect latent P deficiency in this critical phase, thereby providing an opportunity for avoiding a potential yield reduction of up to 27 hkg ha⁻¹. It was further noted, that chlorophyll a fluorescence analysis and P leaf tissue analysis should be performed during early vegetative growth, as probable remobilization of P within the plant during tillering and shoot differentiation masks the effects of P deficiency at the single leaf level. Conclusions: It is concluded that chlorophyll a fluorescence analysis provides a unique opportunity for a timely detection and correction of P deficiency under field conditions to prevent yield reductions.