# Methods for Obtaining Humus Substances: Advantages and Disadvantages
## 4 Conclusions  
The results of the FT-IR analysis show that HSs extracted from clean raw materials are not contaminated with alcohols, phenols, ethers, carboxylic acids, and acid anhydrides. Gravimetric analysis shows that they contain a few simple aromatic compounds, hydrocarbons, and carbohydrates--HS(A) 25%, HS(B) 35%. So, they might be considered ecological.  
On the other hand, HS(C) is obtained from waste raw material (digested sewage sludge). It contains 63% aromatic compounds, hydrocarbons, and carbohydrates, including alcohols, phenols, ethers, carboxylic acids, and acid anhydrides. HS(C) may also carry other toxic pollutants. Therefore, this product requires additional toxicity tests to be used in agriculture. However, it can be used for remediation or non-agricultural purposes.  
HS(A) and HS(B) are examples of ecological products. However, they contain high amounts of ash, an inorganic HS admixture. Mineral substances are pollutants for HS, while these elements are beneficial to the soil (e.g., P, K, Si, Al, and others). HS(A) has good properties, but it is expensive. HS(B)--the result of the alkaline extraction of HS from hard brown coal, a colloidal solution, which easily coagulates, forming a gel. Such a product contains small amounts of FA and significant amounts of HA, which precipitate at pH < 2. Hard brown coals contain high amounts of C and small quantities of O. They are hydrophobic and poorly soluble. Therefore, they are not a good raw material for obtaining soluble humic substances. Leonardite, a lignite brown coal with a well-preserved wood structure, an earthy coal with a loose to solid consistency, or a shale brown coal should be used instead of brown coal to obtain a good product that does not gel with increasing solution concentrations.  
Conceptualization, A.M.A.; methodology, A.M.A. and R.S.-D.; software T.D.; formal analysis, A.M.A. and R.S.-D.; investigation, A.M.A., R.S.-D. and D.L.-P.; resources, A.M.A. and R.S.-D.; data curation, A.M.A. and R.S.-D.; writing--original draft preparation, A.M.A.; writing--review and editing, A.M.A., T.D. and D.L.-P.; visualization, A.M.A. and K.P.; supervision, A.M.A. and T.D.; project administration, A.M.A.; funding acquisition, A.M.A. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Center for Research and Development in Poland, grant number POIR.04.01.04-00-0039/17. The APC was funded by the EU.  
**Institutional Review Board Statement:** Not applicable.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{3}{*}{**Sample**} & \multicolumn{3}{c}{**HS**} & \multicolumn{3}{c}{**FA (After Precipitation**} & \multirow{3}{*}{**\%FA**} \\  & & **[mg C/L]** & & & **of HA with HCI at pH 1)** & \\  & \(\times\)**1** & \(\times\)**2** & \(\times\)**Average** & \(\times\)**1** & \(\times\)**2** & \(\times\)**Average** & \\ \hline HS (A) & 34.13 & 34.48 & 34.31 & 33.62 & 34.67 & 34.15 & 99.5\% \\ \hline HS (B) & 45.97 & 46.20 & 46.09 & 3.096 & 3.008 & 3.05 & 6.6\% \\ \hline \multicolumn{1}{c}{DM—dry matter.} & & & & & & & \\ \end{tabular}
\end{table}
Table 5: TOC content in aqueous solutions of HS(A) and HS(B) and at pH 1. The initial concentration of HS solution 0.1 mg\({}_{\text{DM}}\)/L.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
**Conflicts of Interest:** The authors declare no conflicts of interest.