Effects of composting technologies on the chemical and physicochemical properties of humic acids

Composting and vermicomposting processes are useful methods for producing a stabilized and mature organic material; rich in humic substances, which may change some properties in the soil solution. Through both processes the final products are stabilized and mature organic matter, rich in humic acids (HA), which are the main and more stable component of the organic matter, and contribute to the essential functions of global soil fertility and health. The quality assessment of the compost and/or vermicompost is not always sufficiently clear; many researchers are still exploring different methodologies, in particular from the point of view of their nutrient and microbial activities. In recent times, the amount and the chemical and physicochemical properties of HA in compost and vermicompost are considered as important indicator of their biological maturity and chemical stability and warranty for safe impact and successful performance in soil. Humic acid (HA) samples used were isolated from composted, vermicomposted materials and from soil. Their chemical and physicochemical properties were studied through different analytical techniques, elemental and functional group composition, FT-IR and potentiometric titrations. The aim of this research was to report a comparison, from chemical and physicochemical point of view, between the HA extracted from stabilized organic matter obtained through composting and vermicomposting and native soil HA. According to the obtained results, the HA derived from the less evolved organic materials (compost and vermicompost) showed different humification levels (higher in composting than in vermicomosting process), higher aliphatic nature, higher nitrogen compounds content, lower oxidation degree, lower charge development, differences in the acidity strength of the acidic functional groups and differences in the heterogeneity than those extracted from more evolved materials (soil). The acidic functional groups of the HA from composted materials are stronger than those of the HA from vermicomposted materials and the former have similar acidic strength in comparison with soil HA. The contribution of HA from composted materials to soil buffer capacity and cation exchange capacity (CEC) seems to be larger than those isolated from vermicomposting treatments. These data are very important from chemical and physicochemical point of view of HA isolated from mature organic matter obtained through different technologies, which can be an instrument for predicting the biological maturity, chemical stability and their contribution to some important soil properties (CEC, buffer capacity, among others) of the organic amendment, and in this way, it could be considered as an indicator for a successful agronomic performance of compost and vermicompost in soil.