B. Malcheva

Department Silviculture, Faculty of Forestry, University of Forestry, 1000 Sofia, Bulgaria
(Manuscript received 13 May 2025; accepted for publication 28 July 2025)
Abstract. The main groups of soil microorganisms were determined in the experimental combination of ammonium fertilizers, microbial fertilizer containing Bacillus subtilis and molasses. In the cultivation of a nutrient medium for non-spore-forming bacteria and bacilli, the best variant in terms of the quantity of bacilli is the combination of ammonium nitrate (45%), ammonium polyphosphate (45%), ammonium thiosulfate (5%), molasses (5%) and 0.15% Bacillus subtilis, followed by the variant with 0.30% Bacillus subtilis and the lowest quantity of bacilli is in the variant with 0.50% Bacillus subtilis, with the same amounts of other additives. After fertilization with the variants, the quantity of bacilli and the biogenicity of the soil are the highest in the variant with 0.30% Bacillus subtilis, followed by the variant with 0.15% Bacillus subtilis and the lowest biogenicity of the soil is in the variant with 0.50% Bacillus subtilis. While in the first case the development of microorganisms is in a controlled environment, in the second case a complex of environmental factors exerts influence. The highest percentage share in the composition of the total microflora is occupied by non-spore-forming bacteria, followed by bacilli and mold fungi, and the least represented are actinomycetes. In the variant: liquid nitrogen fertilizer (45%) + liquid nitrogen-phosphorus fertilizer, ammonium polyphosphate (45%) + liquid nitrogen-sulfur fertilizer, ammonium thiosulfate (5%) + molasses (5%) + microbial fertilizer containing Bacillus subtilis (0.30%), a rearrangement in the composition of the total microflora was established – mold fungi were more numerous than bacilli. In the same variant, the values of catalase, cellulase and urease are the highest, following the trends for the total quantity of microorganisms and are strongly dependent on it.