Environmental Safety for Chernozem Soil Fertilized with Phosphogypsum and Ash for Spring Wheat Cultivation in North KazakhstanAinur Muhanbet, Abilzhan Khusainov and Sagintai ElubaevMATEC Web Conf., 73 (2016) 03010DOI: https://doi.org/10.1051/matecconf/20167303010