Sun, Q.; Liu, J.; Huo, L.; Li, Y. C.; Li, X.; Xia, L.; Zhou, Z.; Zhang, M.; Li, B. Humic Acids Derived from Leonardite to Improve Enzymatic Activities and Bioavailability of Nutrients in a Calcareous Soil. Int J Agric & Biol Eng 2020, 13, 200-205.