Soil salinity can affect the absorption and transportation of mineral nutrition of plants, and the nutritional retaining of the soil. To investigate the nutritional supply-demand relationship between soil and plants under soil salinity conditions, we performed irrigation experiments using one-year old of Populus × euramericana ‘Neva’ seedlings, and the contents of microelements, carbon, nitrogen and phosphorus and ecological stoichiometry were analyzed. The results showed that: (1) Under non-magnetized brackish water (NMBW) irrigation, the Fe content in both leaves and roots was decreased, whereas the Cu content was increased, and the Zn and Mn contents were increased in leaves and decreased in roots. The total carbon content of leaves and roots was promoted by NMBW irrigation, whereas the total nitrogen and phosphorus contents were reduced; the C/N ratio decreased in leaves was decreased, while the C/N and C/P ratios were elevated in roots. (2) Under magnetized brackish water (MBW) irrigation, the contents of Fe, Zn and Cu were improved in both leaves and roots, and Mn contents was decreased. The total carbon and phosphorus contents and C/N ratio of leaves were increased, while the N content was decreased; and the total carbon content, and C/N and C/P ratios of roots were promoted. (3) Under NMBW irrigation, the Fe, Mn, Zn and Cu contents of the soil were decreased, whereas the content of available microelements were increased; and the organic carbon and total nitrogen contents, and the C/P and N/P ratios were decreased, but the total phosphorus content and the C/N ratio were increased. (4) The total content of Fe, Mn and Zn of the soil were improved by MBW irrigation, as were the available content of Fe, Mn, Zn and Cu, whereas the total Cu content decreased. The organic carbon and total nitrogen contents, and the C/P and N/P ratios in soil were all increased. Therefore, the magnetization improved the absorption and distribution of microelements in plants and could also improve the N sequestration capacity of soil and the C supply of plants. Moreover, the magnetization could increase the Fe content and C/N ratio of plants under salinity conditions, which could maintain normal photosynthesis rates and improve plant growth and development under salt-stress.