Abstract This review focuses on the radioisotope methods used for studying the root activity patterns of woody perennials and root competition in multi-species cropping systems. 32P soil-injection is by far the most widely used method. The method is unique as it enables one to delineate the lateral and vertical spread of the absorbing roots. A system of classification of crop plants based on the concept of `effective foraging space’ has been developed. The scope and applicability of the radioisotope methods in different cropping situations as well as from the point of view of radiological safety were discussed.