EFFECT OF CONTROLLED-RELEASE FERTILIZERS ON CHEMICAL PARAMETERS OF TWO GROWING MEDIA DURING 12 MONTHS STORAGE

A one year long incubation experiment was carried out using a peat-based growing media (GM) and a peat and pumice GM (80% and 20% v/v), in combination with two controlled-release fertilizers (CRFs) and stored in two conditions. Samples with 60% water content were kept at 21°C throughout the experiment (correct storage condition) or at 40°C for 15 days and then at 21°C until the end of the experiment (improper storage condition). Three replicates of each treatment were analyzed monthly for pH, EC, water soluble potassium, phosphorus and mineral N (1:5 vol/vol extraction method). The strong acidification induced by CRFs reduced pH of the peat based growing media below the permitted tolerance of pH (± 1 unit of the declared value) after 60 days, and peat-pumice GM after 30 days correct storage. Soluble salts buildup during two month storage led both GM to exceed admitted tolerance (25% of the declared value) for Italy. Biological oxidation of ammonium was more efficient in the most aerated substrate (peat-pumice GM) based on measured ammonium versus nitrate in the soil solution. Potassium concentrations were lower in peat-pumice based GM for possible interaction between potassium and pumice. No changes in phos¬phorus extracted by water were detected. The intensity and pattern of nutrient release from CRFs were influenced both by storage conditions and by GM type. During the first 3 months of incubation improper storage increased ammonium build up in both GM.