# Methods for Obtaining Humus Substances: Advantages and Disadvantages
## 1 Introduction
The key differences are generally the age and location of the raw source material, the levels of weathering, heavy metal content, and, of course, humic and fulvic acids concentration levels [27]. Various humus acids (humic acids, fulvic acids) are produced by, e.g., HGS BioScience, Biochar Now, and Humintech GmbH [27; 28; 29]. Raw materials such as lignite, peat, recycled waste, cellulose, agricultural waste, wood, etc., are used to produce humus acids (HsA). The production of HS from lignite, brown coal, and peat includes crushing and grinding the raw material, followed by alkaline extraction to dissolve HsAs. The extracted HsA is purified from residues to obtain a high-quality concentrate. Various products, such as liquid fertilizers, powders, and granules, are produced from concentrated HsAs, and may be directly used in agriculture. It is possible to enrich the composition of produced humic acids with biogenic elements NKP due to the demand of plants for organic soil components and minerals during their growth. This process is carried out at the stage of chemical extraction of HA from natural materials such as peat by replacing the reagents recommended by IHSS in the extraction with those that contain biogenic elements, such as HNO\({}_{3}\), NH\({}_{3}\) H\({}_{2}\)O, KOH, H\({}_{3}\)PO\({}_{4}\)[30]. The production of HS from agricultural waste, wood, and other organic materials starts with drying and grinding. Then, the biomass is subjected to pyrolytic anaerobic decomposition (in high water), forming biochar, which is cooled, sorted, and packed into bags or containers. Hydrothermal carbonization technology involves heating biomass in water, thus producing hydrothermal coal, which is rich in humus substances. This process is carried out at a temperature range of 180-250 \({}^{\circ}\)C and a pressure range of 2-10 MPa. A mixture of humic and fulvic acids is formed due to the hydrolytic-oxidative decomposition of lignocellulosic. Lignosulfonate is a product of decomposition of cellulose and wood pulp. Additionally, filtration, sulfonation, thickening, and spray drying are used. The final product is a dark brown, loose powder, soluble in water [31; 32]. The digested sludge method has been described in the patent application [33]. It consists of the alkalization of digested sludge with NH3(aq). Next, the dissolved humus substances are centrifuged in a sedimentation centrifuge and neutralized with orthophosphoric acid H3 PO4. The resulting effluent is a solution of humic substances that can be used in liquid or solid form (after water evaporation), e.g., as a powder or granules. Depending on the raw material and technology type, the obtained HsA product is a stable solution, even at high concentrations or a gel-forming solution at increased concentrations. The gel is difficult to use in practice and hardly effective after dilution. Therefore, the authors decided to investigate why humus acids take different forms and what factors determine their solubility and gelation. Explaining this issue allows the selection of the right raw material and the appropriate method of HsA extraction in practice. The main objective was to analyze the methods of HS extraction, their advantages and disadvantages, and the modification procedures for their better use. Three different methods of HS extraction were compared in terms of molecular composition, inorganic impurities, IR spectrum and gravimetric analysis, and the percentage contribution of fulvic and humic acids was determined. The paper evaluates the properties of the HS obtained and highlights the role of the tested substances in improving soil quality and increasing the soil's ability to retain water and nutrients. It was also pointed out that the substances discussed are the least understood components of soil and one of its most essential elements.