Pot experiment was carried out with the growing of pepper (Capsicum annuum L.). Before vegetation, controlled release fertilizer Osmocote Plus (10-11-18) was placed on the bottom of the pot that was then filled with peat substrate. The nutritive components embodied in this type of fertilizer are slowly released during 5-6 months. The dynamics of the macroelement contents in the peat substrate was analysed as the effect of Osmocote Plus (10-11-18) fertilizer application. It was shown that the use of localized placement of Osmocote Plus (10-11-18) fertilizer does not sufficiently enrich the substrate with nitrogen (N-NH4 and N-NO3), phosphorus and potassium. The maximal content of these components in relation to the applied rates showed the following values: (N-NH4+N-NO3) – 15.5%; P – 8.0% and K – 15.1%. The values were revealed in the final period of growing (September-October). Optimal content of magnesium in the substrate which was maintained throughout the whole growing period was not only the result of the application of Osmocote Plus (10-11-18), but it was also caused by the presence of magnesium in the lime fertilizer used for the liming of peat and by the water used for irrigation. It was found that localized placement of fertilizer in the substrate caused not only some problems in maintaining optimal nutrient resource level in the substrate for plants, but it also created many methodological problems. It refers to the method of sampling substrate for analyses, to the release of components from the fertilizer granules during extraction, and to the interpretation of results. The problem of substrate analyses with the use of slow-release fertilizers requires further methodological studies.