Soil and plant analyses are useful diagnostic tools only if they supply adequate information about the current state and can predict effects of different types of management. The prerequisites for reproducibility and consistency may not be met with boron which is not distributed homogeneously in plants. It accumulates in marginal areas and between veins, is transported with the transpiration stream and accumulates at terminal places of transpiration. The boron concentration within the same leaf may vary 100‐fold and foliar analyses represent an average value only. Moreover, the boron content increases with the age of leaves, and there is no indication of a homeostatic control of foliar boron contents. Accurate, representative sampling may thus be difficult and critical. An extreme case is shown where toxicity occurs in old leaves and deficiency in new growth. The rate of transpiration greatly affects boron transport into leaves and its distribution. Predictions must thus take water relations into account.