Soil humic precursors could be considered the most active and mobile fraction and are highly significant to a series of biochemical processes in all types of soil. The microbial biosynthesized humic precursors attracted increasing attention on green synthesis of nanocomposite compounds realized between biopolymers and metal nanoparticles. Silver nanoparticles are the most used engineered nanocomposite serving as antimicrobial agents. In the present study we used aqueous solution of humic precursors synthesized by four microbial consortia (C1-C4) and selected on the basis of high quantities of exometabolites with structural similarities to soil humic acid fraction. The humic precursors were used as capping agents of silver nanoparticles in the nanocompozite synthesis. Biosynthesized humic precursors act as reductive and stabilizative agents of nanoparticles which are found between 5-300nm in size and with spherical preponderant shape. The presence of humus precursors and the biosynthesized silver nanoparticles was confirmed by FTIR and UV-Vis. At a given precursor concentration, the efficiency of nanocomposite synthesis increased with particle concentration and time of reaction, property which can be attributed to the high reduction capacity of humic precursors. The induced antimicrobial effect of exposure to nanocomposites differs due to the size, time of preparation and stability. Stabilization of nanocomposite by specific metal-ligand bonds was obtained in the solution for three months without any precipitate. The antimicrobial effect of nanocomposites was estimated under laboratory agar well diffusion tests against mycotoxigenic soil fungal isolate Aspergillus niger (A27). The green synthesis of nanocomposite material with the best antimicrobial effect against test fungus was realized by microbial consortium C3and C4.