Trace elements of organic wastes with a potential use for improving soils

There are a lot of materials, i.e. organic and inorganic wastes that can be use to improve soil properties, and also can be part of Anthrosols and Technosols, both defined in the World Reference Base for Soil Resources (2015) as soils with strong human influence. The first one, associated to intensive agriculture and comprise soils that have been modified profoundly through human activities, such as addition of organic or mineral material. The second one combine soils whose properties and pedogenesis are dominated by their technical origin. Technosols contain a significant amount of artefacts (something in the soil recognizably made or strongly altered by humans or extracted from greater depths) or are sealed by technic hard material (hard material created by humans, having properties unlike natural rock) or contain a geomembrane. The use of organic wastes to improve their properties or being part of the components forming a Technosol can increment the presence of trace elements which an alteration of their composition. These trace elements could suppose an environmental risk as a source of pollution affecting soil, water and biodiversity. The objective of this work was to determine the trace element composition of five organic wastes (pine bark, palm leaf, hay straw, almond pruning, pomegranate fruit skin) in order to know their composition and possible effects when they will be used for Technosol making or to improve soil properties. The composition of them varied. Five samples from each waste were digested by using H₂O₂+HNO₃ in microwave furnace and after that, elemental composition was measured by Inductively Coupled Plasma Optical Emission Spectroscopy. As, Cd, Co, Ni, Pb and V were under the limit of detection (we consider for all the elements a general limit of detection to compare all of them of 2 micrograms per litter; for this technique the limit varies from 0.03 to 1.5 micrograms per litter depending on the element). The elemental composition of the wastes showed that Ba, Cr, Cu, Li, Mn, Mo, Se and Zn were below 20 mg/kg dw in all of them. The rest of the elements presented concentrations over 20 mg/kg dw. In general, major differences were found in some trace and major elements: B and Ca in pine bark, K in pomegranate skin, Mg and Na in palm leaf and Sr in almond pruning. As a conclusion, the elemental composition of these wastes would be considered when a Technosol will be prepared including these materials or when they will be used as soil amendments.