Influence of pumice on ammonium dynamics in soilless culture

Pumice is widely used in Italy as an alternative to other mineral components e.g., perlite to produce growing media in order to improve physical properties. Since less attention is paid to effects exerted by pumice on chemical environment in substrates, incubation experiments were conducted focusing on the mobility of ammonium nitrogen. Apart from ammonium dynamics pH, EC, bulk density, maximum water holding capacity, CEC and exchangeable cations were determined on a commercial pumice collected from a volcanic deposit in central Italy. Three experiments were performed to study the interactions of ammonium and pumice. Ammonium adsorption capacity at equilibrium was defined at 21°C in the range 0-2000 mg L-1 of N-NH4, (adsorption isotherm): the calculated Qmax (maximum adsorption, 2.24 mg N-NH4 g-1 pumice) was congruent with the cation exchange capacity of pumice. Ammonium retention was tested on N-NH4 charged pumice by a three-step sequential extraction procedure using ethanol, distilled water and 0.1 M CaCl2 (1:5 v/v): only 50% of the nitrogen supplied was recovered in ready available or exchangeable pool. Lastly the aptitude of pumice to prevent leaching of ammonium added with fertilization was studied throughout a 15 days trial simulating greenhouse irrigation management in comparison to a fine white peat and a peat/pumice mixture. The results indicated that the studied pumice can be usefully employed as an ingredient in growing media particularly when ammonium preservation is needed.