_____________________________________________________________________________________________________ *Corresponding author: E-mail: jfleme@vt.edu; International Journal of Plant & Soil Science 32(7): 56-75, 2020; Article no.IJPSS.57887 ISSN: 2320-7035 The Synergistic Effects of Humic Substances and Biofertilizers on Plant Development and Microbial Activity: A Review Jose Franco Da Cunha Leme Filho 1* , Wade E. Thomason 1 , Gregory K. Evanylo 1 , Xunzhong Zhang 1 , Michael S. Strickland 2 , Bee K. Chim 3 and Andre A. Diatta 1 1 School of Plant and Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, USA. 2 Department of Soil and Water Systems, University of Idaho, Moscow, USA. 3 United States Department of Agriculture, Agricultural Research Service, North Central Agriculture Research Lab, Brookings, USA. Authors’ contributions This work was carried out in collaboration among all authors. All authors read and approved the final manuscript. Article Information DOI: 10.9734/IJPSS/2020/v32i730306 Editor(s): (1) Prof. Alejandro Hurtado Salazar, Universidad de Caldas, Colombia. Reviewers: (1) Soumahoro Gueu, National Polytechnic Institute Félix Houphouet Boigny, Ivory Coast. (2) Rek Kurniawan, University of Jenderal Soedirman, Indonesia. (3) Sara Dantas Rosa, Universidade de Brasília, Brazil. Complete Peer review History: http://www.sdiarticle4.com/review-history/57887 Received 10 April 2020 Accepted 16 June 2020 Published 24 June 2020 ABSTRACT Agroecosystem and ecological cycling loops are open when considering the reutilization of inputs applied in farming areas. Non-renewable resources have been transformed or relocated from the air, water and land into the system and are flowing out as wastes rather than reusable, recyclable resources. This current environmental situation is promoting the development of methods able to optimize nutrient cycling, minimize use of external inputs, and maximize input use efficiency. Some humic products are derived from lignin found in wheat straw and biofertilizers as compost and manure teas can be made using residues. Also, these biostimulants might decrease the necessity of synthetic inputs. This review strives to enhance our understanding of the conjunctive use of humic substances (HS) and biofertilizers. The biostimulant effects of each of these compounds are Review Article