The effects of irrigation and fertilization on the migration and transformation processes of main chemical components in the soil profile

Understanding the changes in chemical composition of soil plays an important role in effective control of irrigation and fertilization in agricultural productions, which further protects the groundwater quality and predicts its evolution. Field trials were conducted from 2014 to 2016 to investigate the impacts of irrigation and fertilization on mineral composition transformation in the soil profile. Based on HYDRUS-HP1 and Visual MINTEQ, this paper simulated and computed the migration and transformation of chemical components during the irrigation and fertilization in the vadose zone soil of Jinghuiqu district. The results showed that when the nitrogen fertilizer entered the soil, the urea was hydrolyzed to NH4+ and it was nitrified as NO2-, which caused pH value to drop around the first 4 days after irrigation, and rise slightly on the 12th day. Due to the fact that soil belongs to calcareous soil, concentration of CaCO3 and other carbonates (Mg or Na in sodic soils) could buffer the soil pH well above 8.5. Thus, on the 30th day of the post-irrigation the pH reached the same level as it was before irrigation. The change in pH resulted in the main ions reacting, dissolving and precipitating simultaneously in the soil profile. The concentrations of Ca2+, Mg2+ and HCO3- had significant correlations with the increasing ammonia nitrogen hydrolyzed from urea, and this process is accompanied with the saturation index of minerals and the main ion content changing. At the same time, the varying temperature action on pH of the soil was higher in summer than that in winter. Thus, the irrigation, fertilization and temperature had affected pH and main chemical components in the soil.