Characterizations of compost-based growing media. A Chemical, Thermal, Spectroscopic and Isotopic Approach.

Peat is the best substrate in growing media, but its rapid depletion needs at least a partial substitution. Composting of urban green wastes and agro-industrial by-products can produce good compost (Cm) for this purpose. Cm-based substrates are increasing in the market. However, physical and chemical characterization of Cm-based growing media is the first step for their proper use. This paper reports the results of the fractionation of the bulk organic matter (OM) of four growing media, containing increasing amount (v/v) of Cm [100% Peat; 80% Peat + 20% Cm (CP20); 40% Peat + 60% Cm (CP60); 100% Cm) into well-defined fractions: humic acid-like (HAL), non-humic fraction (NH) and dissolved OM (DOM) and their characterization by a combination of chemical, thermogravimetric (TG), differential thermal analysis (DTA), spectroscopic (Diffuse Reflectance Infrared Fourier Transform-DRIFT) and isotopic (δ 15N) techniques. TG-DTA of bulk growing media, HAL, NH and DOM showed significant differences among increasing Cm in Cm-based growing media. DRIFT spectra confirmed these differences. The N isotope ratio (δ 15N) of all the substrates increased with the amount of Cm in the substrate. The combination of chemical, TG-DTA, DRIFT and δ 15N techniques can be successfully applied for the chemical characterization of Cm based growing media with a good identification of different matrices.