The research conducted revealed the presence of the following groups of phytohormones: auxins (AXs), brassinosteroids (BRs), cytokinins (CKs), gibberellins (GAs), abscisic acid (ABA), and salicylic acid (SA). Table 1 displays the content of phytohormones in composts. The average phytohormone content ranged from 0.13 to 2039.18 ng g⁻¹ dry weight (dw). Brassinosteroids and AXs were the most prevalent, while GA₃ was the least abundant.

Auxins (AXs) and brassinosteroids (BRs) were identified as the prevailing types of phytohormones, as evidenced in Fig. 1.

The utmost concentration of BRs was found in organic compost-pellets, at 99.87%, and in biohumus extract, at 96.01%. Additionally, considerable quantities of brassinosteroids were present in garden compost and compost with buckwheat husk, registering 69.37% and 51.97%, respectively. Notably, compost hemp chaff and apple pomace, or purely organic sources did not exhibit detectable levels of BRs. Nevertheless, BRs have proven to be effective even in scant. They are recognized for encouraging cell division and elongation, influencing the growth of stems and roots, and playing a role in the initiation of floral structures and fruit development²¹. Moreover, Divi and Krishna²³ highlighted the ability of BRs to safeguard plants against various stresses, both abiotic and biotic. Conversely, AXs exhibited their highest concentration in compost with buckwheat husk, at 38.24%, and their lowest in organic compost-pellets, at a mere 0.001%. Both AXs and BRs are integral to numerous physiological processes within plants^24,25. Aremu et al.²⁶ found a notably high AXs concentration, between 0.55 and 0.77 pmol mL⁻¹, in vermicompost derived from garden waste. Additionally, Façanha et al.²⁷ observed that substances from earthworm compost could spur lateral root development in maize plants.

Abscisic acid and GA₃ show the lowest content in the evaluated waste, both less than 0.20% (Fig. 1). A peak ABA level of 0.20% was found in compost with buckwheat husk, while four types of compost, i.e., garden, hemp chaff and apple pomace, organic compost, and organic compost-pellets, exhibited no detectable levels of ABA. A study by Stirk et al.²⁸ also reported low ABA concentration. Regarding GA₃, the highest level was present in organic compost (0.06%), whereas the smallest was in compost from hemp chaff and apple pomace (< 0.001%). GAs, vital throughout the plant life cycle, promote stages like germination, hypocotyl elongation, and the development of organs, flowers, and seeds²⁹. Over 90% CKs were found in compost hemp chaff and apple pomace, and organic sources. Salicylic acid levels peaked in compost with buckwheat husk at 8.58%, and plumbed to their lowest in organic compost-pellets at 0.001%.

An analysis using liquid chromatography–mass spectrometry (LC–MS) quantified the presence of up to 35 phytohormones in the compost samples. The various composts displayed a range of 14–35 distinct phytohormone types, i.e., compost with buckwheat husk and biohumus extract contained 35 compounds; compost hemp chaff and apple pomace, 14; organic compost, 15; organic compost-pellets, 21; and garden compost, 25 (Table 2).

Biohumus extract exhibited the highest concentration of total phytohormones at 2026.42 ng g⁻¹ dw, while organic compost had the lowest at 0.18 ng g⁻¹ dw (Table 3). The compost with buckwheat husk showcased the highest content of SA (4.02 ng g⁻¹ dw), whereas GA₃ was the least concentrated hormones across all composts (Fig. 2).

Every type of phytohormone was detectable in both compost with buckwheat husk and biohumus extract. Dominant phytohormones in composts derived from agri-food industry waste included IAA (Fig. 2), BL, HBL, epiCS, epiBL, and norBL (Fig. 3).

cZ-type of CKs were not detected in organic compost and aromatic CKs were not noted in organic compost-pellets (Table 4). Dominant type of CK group in composts was cZ-type (Fig. 4). Schäfer et al.³⁰ summarized studies that investigate the role of this type of CK in regulating plant development and defense responses to pathogen and herbivore attack. It can be concluded that the diversity in phytohormone content across composts is influenced by the type of substrates utilized during the composting process^13,31.

Our findings suggest a notably elevated content of plant hormones in the leachate derived from composted hemp chaff and grass. The overall content of the plant hormones analyzed spanned from 1456.32 to 4094.34 ng g⁻¹ dw, averaging at 462.56 ng g⁻¹ dw. This leachate demonstrated the highest concentrations of BRs (ranging from 661.66 to 3368.31 ng g⁻¹ dw), AXs (324.75–336.73 ng g⁻¹ dw), and SA (300.31–350.55 ng g⁻¹ dw). Arthur et al.³² and Aremu et al.²⁵ have noted that leachate from thoroughly decomposed compost contains CK-like substances, originating from the hydrolysis of CK glucosides by β-glucosidase, an enzyme produced by microbes. Subsequent research should, therefore, prioritize evaluating the content of plant hormones in leachates, specifically those derived from composts produced from the organic fraction of agri-food industry waste³³.

Hierarchical Cluster Analysis effectively delineated the assessed composts into two discrete clusters, denominated as A and B, based on the constituent phytohormone content (Fig. 5). Cluster A, encompassing garden compost, compost derived from hemp chaff and apple pomace, compost with an inclusion of buckwheat husk, organic compost, and analogous organic compost in a pelletized configuration, exhibits a phytohormone composition that is discernibly lower in concentrations of ABA, AXs, BRs, CKs, and GA₃ relative to the mean of the group; notwithstanding, it is punctuated by the pinnacle of SA concentration within this cluster. Such a phytohormone profile implicates potential limitations of these compost variants for agricultural applications. Conversely, cluster B, exclusively represented by biohumus extract, albeit exhibiting diminished SA content, is distinctly characterized by superlative concentrations of ABA, AXs, BRs (i.e., BL, HBL, epiCS, epiBL, and norBL), CKs, and GA₃, thereby positing it as a potentially preeminent stimulator of plant growth.

The analysis of plant hormone content in the examined composts revealed two distinctly dissimilar groups of objects when objects and features were simultaneously grouped (Fig. 6). In the upper part of the map, an area was obtained where the garden compost, organic compost, organic compost in the form of pellets, compost from hemp chaff and apple pomace, and compost with buckwheat husk were grouped (green color). Conversely, in the lower part of the map, an area was designated where the biohumus extract was grouped (red color).

PCA analysis enabled the categorization of analyzed composts, maintaining a significant degree of explained variance. During this analysis, the variable count was condensed to two principal components (designated as PC1 and PC2), which intimates that the initial dataset of 35 plant hormones is notably correlated and therefore reducible (Fig. 7). All variables, with the exception of DHZ7G and SA, presented high negative loadings, ranging from − 0.9996 to − 0.8995, in association with the first component. Contrastingly, DHZ7G was associated with a high positive loading (0.6334), whereas SA exhibited a high negative loading (− 0.8768) with the second component. The values of factor loadings for most variables were proximate, resulting in a superimposition of points on a singular graph.

Comparison of case positions on the graph, considering component forms and factor loadings, revealed distinct characteristics among them. Specifically, Case 1, exhibiting negative coordinate values on the first axis, was identified as having a higher content of plant hormones, excluding DHZ7G and SA, according to the relevant factor loadings. Conversely, Case 2, with a positive coordinate value for the second axis, was associated with a higher DHZ7G content, based on the factor loading with the second axis. In contrast, Case 3, showing a negative coordinate value on the second axis, was characterized by a higher SA content, based on its factor loading with the second axis (Fig. 7). The interplay between the content of plant hormones and the two principal components (PC1 and PC2) is visually represented in a three-dimensional surface plot (Fig. 8).