CHEMICAL ANALYSIS OF SUBSTRATES WITH CONTROLLED RELEASE FERTILIZER

Water-soluble fertilizer added to media containing controlled release fertilizer cannot be analysed with the 1:1.5 volume water extract, because the latter increases the element content in the extract. During storage and stirring or mixing the substrate with the extractant, part of the controlled release fertilizer is already released. To quantify this increase, a laboratory trial has been conducted. Osmocote granules with release characteristics of 3-4, 5-6, 8-9, 12-14 months have been mixed in different quantities to a peaty substrate and stored at 11 and 21 °C. Samples have been analysed at day 3 and 10. The increase of the element contents in water extracts was greater when a) the duration of the release was shorter (more release of the 3-4 months than the 12-14 months), b) the time between the mixing and the analysis was longer and c) the addition was higher. It was possible to predict the effect of a certain type and amount of Osmocote and number of days between mixing the fertilizer and the analysis on the element contents in the 1:1.5 volume extract. Substrates containing slow release fertilizer should not be stored.