Plant families and species show great variabilities in sulfur concentrations. In general, gramineous species have lower sulfur levels than dicotyledonous crops. Within each genus, however, species producing S-containing secondary metabolites accumulate more sulfur than those without this capacity. The ratios of sulfur concentrations in photosynthetically active tissue of cereals, sugar beet, onion, and oilseed rape are approximately 1:1.5:2:3 (114,236). Thus plants with a higher tendency to accumulate sulfur, such as brassica species, are very suitable as monitor crops to evaluate differences between sites and environments, or for quick growing tests (176). Generative material is less suited for diagnostic purposes (237), because the sulfur concentration in seeds is determined much more by genetic factors (43,103,116). During plant growth, morphological changes occur and there is translocation of nutrients within the plant. Thus, changes in the nutrient concentration are not only related to fluctuations in its supply, but also to the plant part and plant age. These factors need to be taken into account when interpreting and comparing results of plant analysis (216,238–243). Basically, noting the time of sampling and analyzed plant part is simply a convention, but there are some practical reasons for it that should be considered: (a) photosynthetically active leaves show the highest sulfur concentrations of all plant organs, and as sulfur has a restricted mobility in plants sulfur concentrations in young tissues will respond first to changes in the sulfur supply; (b) sampling early in the vegetative state of a crop allows more time to correct sulfur deficiency by fertilization.

It is relevant in this context that plant analysis is a reliable tool to evaluate the sulfur nutritional status, but usually it is not applicable as a diagnostic tool on production fields because of the shortcomings mentioned above. In dicotyledonous crops, young, fully expanded leaves are the strongest sinks for sulfur, and they are available during vegetative growth. Therefore, they are preferable for tissue analysis (88,103,244). Oilseed rape, for instance, delivers suitable leaves for tissue analysis until 1 week after flowering, and sugar beet gives suitable leaves until the canopy covers the ground and the storage roots start to extend (103). For the analysis of gramineous crops, either whole plants (1 cm above the ground) after the appearance of the first and before the appearance of the second node, or flag leaves are best suited for providing samples for analysis (142,143,245–249).

In all cases, care has to be taken to avoid contamination of tissue samples with sulfur from foliar fertilizers or sulfur-containing pesticides. Care is also needed when cleaning samples, because water used for washing may contain significant amounts of sulfate. Paper used for sample drying and storage contains distinct amounts of sulfate, originating from the manufacturing process. As sulfate bound in paper is more or less insoluble, the risk of contamination when washing plants is low, but adherent paper particles may significantly influence the results obtained.