We used broad beans ( Vicia faba L.) as a case study to characterise the development of magnesium (Mg) deficiency symptoms in plants and make a comparative evaluation of the suitability of various physiological characteristics as prospective tools for early diagnosis of Mg deficiency. Growth characteristics were measured at monthly intervals from plants grown in soil solution with a wide range of Mg concentrations (from 1 to 200 ppm). The data were then correlated with plant yield responses, pigment composition and nutrient content in leaves, as well as with visual deficiency symptoms. At the age of 4 weeks, no visual symptoms of deficiency were evident even for plants grown at 1 ppm (severe Mg deficiency). Shoot growth characteristics were very similar for a wide range of treatments, although a pronounced difference in plant yield was observed at the end of the experiment. It appears that neither plant biomass nor leaf area are good indicators for use as diagnostic tools for detection of Mg deficiency in broad beans. Although pigment analysis revealed some difference between treatments, at no age was it possible to distinguish between moderately Mg-deficient (10 or 20 ppm) and sufficient (50–80 ppm) treatments. Leaf elemental analysis for Mg content remained the most sensitive and accurate indicator of Mg deficiency in broad beans. However, it is unsuitable for screening purposes as it is both costly and time-consuming. There is a need for less expensive but effective, rapid screening tools of Mg deficiency in crops at early stages of plant ontogeny.