Influence of SOM Composition, Clay Minerals, and pH on 2,4-D and MCPA Retention in Peri-Urban Soils

The use of ionic herbicides in urban and peri-urban areas has serious environmental and health consequences due to their common overapplication and mobility in the soil profile. The specific objective of this study was to evaluate the influence of soil organic matter (SOM) fractions and clay minerals on the retention of 2,4-D and MCPA on loamy sand and loam topsoil materials under the pH range of 3–7. The results obtained indicate their weak, unfavorable, and physical sorption, presumably governed by partitioning. 2,4-D exhibited high affinity for polar SOM fractions as well as to kaolinite and montmorillonite, both present in the studied peri-urban soils. MCPA sorption was mainly related to soil fulvic (FA) and humic acid (HA) content; however, the pesticide was sorbed to a lesser extent than 2,4-D due to its great water solubility. This was reflected in MCPA’s low Koc values (41.33 and 84.21), indicating its very high mobility and leachability in the studied soils. Meanwhile, 2,4-D was moderately mobile in sandy soil while classified as a non-leacher in the loam topsoil material. Both herbicides were preferably retained at a low soil pH (3–4), which, together with soil amendment with exogenous, well-humified organic matter, could minimize the potential health and environmental risks of their application.