Leaves of bilberry (V. myrtillus), bog bilberry (V. uliginosum) and downy birch (B. pubescens) were collected five times during a growing season from 18 plots in a drainage area, and needles of Scots pine (Pinus sylvestris L.) once during the following winter from the same plots at Parkano, southern Finland. The aim was to study the potassium nutrition of the test plants and relationships between the foliar potassium concentrations of Scots pine and of the test plants. The estimation of the potassium nutrition of test plants was based on the accumulation of putrescine in foliage. Apart from single observations, elevated putrescine concentrations were found when the potassium concentrations were < 5 mg g–1 in bilberry, < 4 mg g–1 in bog bilberry and < 6 mg g–1 in downy birch, and the highest concentrations below the potassium levels of 3.0–3.5 mg g–1. At the concentrations of 2–3 mg g–1 the accumulation increased pronouncedly in bog bilberry but less in downy birch and only slightly in bilberry. The foliar potassium concentrations in test plants correlated closely with the concentrations in pine needles. The concentrations of all species in August were quite stable at the levels of the severe and slight potassium deficiency of pine (3.5 and 4.5 mg g–1 respectively) indicating that August would be suitable for collecting foliage. The concentrations in bog bilberry were very close to the concentrations in pine at the severe and those in bilberry at the slight deficiency level of pine. All test plants could be used for predicting the potassium nutrition of Scots pine, but additional research is needed for the practical application of the method.