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 phosphorus 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.