Potassium and magnesium are macronutrients necessary for plant growth and development. The contents of these elements in plant and soil should be monitored, because during their uptake from the soil by the plant root system ionic antagonism can occur. A deficiency or excess of these elements reduces the quality of the resulting yield and feed. The aim of the study was to determine changes in the content of magnesium and potassium in  pea (Pisum sativum L.) and in soil under the influence of different potassium fertilisation under conditions of very high level of available phosphorus in the soil. The univariate field experiment conducted in 2010-2012 at the experimental plots of the University of Natural Sciences and Humanities in Siedlce included seven levels of fertilisation: no fertilisation; N; NK₁; NK₂; NK₃; NK₄; NK₅. Nitrogen was applied at 20 kg ha^-1, potassium in doses of K₁ – 41.5; K₂ –83; K₃ –124; K₄ –166 and K₅ –207.5 kg ha^-1. Based on the study, significantly the highest contents of potassium (11.71 g kg^-1 d.m.) and magnesium (1.28 g kg^-1 d.m.) were determined in pea seeds harvested from plots fertilised with N₂₀K₈₃. The stems of plants fertilised with N₂₀K₈₃ accumulated the largest amounts of magnesium (2.88 g kg^-1 d.m.), while the straw of plants fertilised with N₂₀K₁₆₆ accumulated the highest levels of potassium (16.44 g kg^-1 d.m.). Potassium and magnesium in pods and roots was ambiguously differentiated under the influence of nitrogen-potassium fertilisation. Average potassium content (in g kg^-1 d.m.) in the test plant can be presented in the descending sequence: seeds (10.81) > stem (8.45) > pods (7.59  > roots (3.07), and that of magnesium: pods (2.93) > stem (2.29) > roots (1.22) > seed (1.20). The total content of potassium and magnesium in the soil remained at a similar level (K – 0.64-0.67 and Mg – 0.74-7.90 g kg^-1 soil).