Spent coffee grounds (SCGs) are a widely available organic by-product of coffee preparation. They contain organic matter, nutrients, polysaccharides, lipids, and phenolic compounds, which makes them promising for reuse as horticultural substrate amendments [1] [2]. However, untreated SCGs can inhibit plant growth because of phytotoxic compounds, high biological activity, salinity, and possible nitrogen immobilization [3] [4]. Recent studies show that plant responses depend strongly on SCG processing method and application rate, with lower rates usually considered to be safer than higher rates [5]. Therefore, biological stabilization use is important to reduce phytotoxicity, prevent plant burns, prevent environmental degradation, break down organic compounds, eradicate plant pathogens, reduce pollutants, and improve reliability of SCG derived amendments in horticultural substrates.

Composting can reduce phytotoxicity, lower microbial instability, and improve the agronomic value of organic amendments [6] [7]. In SCG-based materials, stabilization is especially important because plant response depends on both the processing method and the incorporation rate [2] [8]. Several studies show that composted or co-composted SCGs can support plant growth when used at low to moderate rates, but excessive additions may increase EC and restrict growth [4] [9]. Therefore, SCG derived amendments should be evaluated not only for nutrient content, but also for maturity, salinity, and crop response.

Compost maturity is a key criterion for using organic residues in horticultural substrates. Immature materials may continue to respire, release unstable compounds, or cause phytotoxic effects after incorporation into the growing medium [6] [7]. For this reason, maturity should be assessed with more than one indicator. Common criteria include C/N ratio, pH, EC, respiration activity, and plant-based responses [6] [10]. The Solvita^® CO₂ burst test is useful in this context because it provides a rapid estimate of biological activity and has shown good agreement with established CO₂ respiration methods [11] [12]. These indicators are especially relevant for rapidly processed SCG amendments, where short processing time may not always guarantee biological stability.

Accelerated household composters may offer a practical way to process SCGs at a small scale. These systems use heat, aeration, and mixing to reduce moisture and promote rapid organic matter transformation. Similar in-vessel and rotary systems have shortened organic waste processing times under controlled conditions [13] [14]. However, rapid processing does not always mean that the final material is mature or suitable for plant production. This is especially important for SCGs because their phytotoxicity and salinity can limit plant growth when they are used without adequate stabilization [2] [4]. Therefore, the horticultural value of SCG derived amendments from household composters should be tested through both maturity indicators and greenhouse plant responses.

Although the relative concentrations of individual compounds may differ between Coffea arabica and Coffea canephora var. robusta, SCGs generally share similar major chemical classes, including polysaccharides, lignin, proteins, lipids, minerals, and phenolic compounds [2]. This composition supports their potential use as organic amendments but also explains why untreated SCGs may require stabilization before incorporation into horticultural substrates. Despite increasing interest in SCG valorization, limited information is available on the horticultural suitability of SCG derived amendments produced in accelerated household composters. Most previous studies have focused on conventional composting systems, co-composting strategies, or direct soil application of SCGs [2] [3]. Information remains scarce regarding whether short-duration processing can produce biologically stable materials suitable for container substrates and greenhouse crop production. The second study’s objective was to evaluate the effects of accelerated household composting duration on the maturity, physicochemical properties, and horticultural performance of SCG derived amendments. Plant responses of lettuce (Lactuca sativa) and Petunia (Petunia × hybrida) grown in PB-based substrates amended with processed SCGs were also assessed. These two species of plants were selected for their intense leaf and flower production. We hypothesized that increasing processing duration would improve biological stability and enhance plant growth performance at low incorporation rates.

Particle size distribution differed substantially among initial materials and SCG derived amendments (Table 1). PB and peat were dominated by coarse fractions (3/8-inch and #6), whereas raw SCGs consisted primarily of medium and fine particles, with the highest proportion retained in the #25 fraction (55.3%). Accelerated composting decreased particle size, increasing the #25 fraction in both systems. In VC, this fraction increased from 37.3% after 1 day to 59.3% after 10 days; in the Lomi^® Composter (LC), it remained consistently high throughout processing, reaching 65.7% at 10 days. Coarser fractions (3/8-inch and #6) were reduced to negligible levels in all composted treatments from day 1. The #40 mesh screen fraction decreased modestly with increased composting duration, and the pan fraction showed no significant differences among treatments. Significant treatment effects were detected for the 3/8-inch, #6, #14, and #25 mesh screen fractions (p < 0.05; Table 1).

Bulk density increased significantly with processing duration in both systems (p < 0.001; Table 1), however, also resulted in significant differences between both composters. PB and peat had the lowest values (0.215 and 0.211 g·cm⁻³, respectively). SCGs had a higher bulk density (0.338 g·cm⁻³), which increased further during composting. In VC, bulk density rose progressively from 0.359 g·cm⁻³ at 1 day to 0.463 g·cm⁻³ at 10 days, the highest value recorded. In LC, bulk density increased from 0.443 g·cm⁻³ at 1 day to 0.451 g·cm⁻³ at 5 days, followed by a slight decline to 0.426 g·cm⁻³ at 10 days.

Table 1. Particle size distribution of composts produced.

^zMeans within each column having similar letters are not significantly different at the α = 0.05 level.

Both electric composters substantially reduced the mass and volume of SCGs during processing (Figure 2). In LC, mass declined rapidly during the first three days, decreasing from 100% of the initial value on day 0 to approximately 37% on day 2 and approximately 33% by day 3, after which it remained nearly unchanged through day 10. Volumes in LC followed a similar pattern, decreasing to approximately 55% of the initial value by day 2 and approximately 40% by day 3, with only minor additional reduction thereafter. In VC, mass decreased more gradually, from 100% on day 0 to approximately 53% on day 4, 45% on day 5, and approximately 36% by day 6, after which it stabilized through day 10. Volume reduction in VC was also progressive, declining from 100% on day 0 to approximately 50% on day 4 and approximately 39% on day 6, with final values near 35% by day 10. These results indicate that LC achieved rapid mass and volume reduction within the first three days of processing, whereas VC exhibited a more gradual reduction pattern, reaching similar final remaining values after approximately six to ten days.

The quality of compost health was dependent upon processing times and on the type of accelerated electric composter (Figure 3). The vertical axis shows the compost health index (Solvita^®), and the horizontal axis displays the experimental variants. This indicator test was utilized for its inexpensiveness and universal acceptance. Results are presented as mean values ± standard deviation. Horizontal lines represent interpretive Solvita^® test ranges: compost of practical quality (6.0 - 6.9) and mature compost (≥7.0). The lowest Solvita^® values were recorded after 1 day of composting in both systems, indicating that the material was still immature. Extending the processing time to 5 and 10 days resulted in a clear improvement in compost quality, with samples Lomi^® Composter 5 days (LC5), Lomi^® Composter 10 days (LC10), Vego^® Composter 5 days (VC5), and Vego^® Composter 10 days (VC10) reaching the thresholds of practical or mature compost. These results highlight the essential role of processing time in achieving stable and mature compost in electric kitchen composters.

Changes in pH during SCG processing in the two electric composters are shown in Figure 4. The results are presented as mean values, and different letters above the bars indicate statistically significant differences among treatments (p < 0.05). Throughout the study period, pH remained within a narrow, slightly acidic range of approximately 4.8 to 5.7, with only small differences among composters and processing durations. This narrow pH range suggests relatively stable chemical conditions during processing. However, pH alone should not be interpreted as evidence of biological maturity, which was assessed separately using the Solvita^® CO₂-Burst response.

Changes in EC during SCG processing in the two electric composters are shown in Figure 5. The results are presented as mean values, and different letters above the bars indicate statistically significant differences among treatments (p < 0.05). In both systems, EC increased with processing duration, with the lowest values recorded after 1 day and the highest values after 10 days. No significant difference between composters was observed at the end of processing. These results indicate that EC increased during accelerated processing, likely reflecting moisture removal, concentration of soluble ions, and organic matter transformation. However, elevated EC may constrain higher incorporation rates of SCG derived amendments in horticultural substrates, particularly for salt-sensitive crops.

The chemical composition of composts obtained from SCG using LC and VC over a processing period of up to 10 days is presented in Table 2.

Concentrations of P, K, Ca, Mg, and S generally increased with processing duration in both systems, consistent with progressive mass loss and concentration of mineral components. Potassium was the dominant mineral nutrient across all composted treatments, ranging from 1134 parts per million (ppm) (VC1) to 2015 ppm (LC10), compared with 46 ppm in PB and 3089 ppm in raw SCGs. Phosphorus reached the highest concentrations at 10 d: 457 ppm in LC and 434 ppm in VC. Calcium concentrations were variable and did not follow a consistent directional trend with processing duration. Total carbon remained stable across all composted treatments (48.6% to 50.3%), as did total nitrogen (2.1% to 2.3%). The C/N ratio of all composted SCG materials ranged narrowly from 22:1 to 24:1, substantially below that of PB (103:1) and peat (30:1), and below the 25:1 threshold commonly associated with compost maturity. Significant treatment effects were detected for all chemical parameters (p < 0.001; Table 2).

Table 2. Chemical composition of horticultural standards and coffee ground treatment groups.

Table 3. Lettuce plant growth characteristics and biomass.

^zMeans having similar letters are not significantly different at the 0.05 level. ** = Significant at the 0.01 level; ***Significant at the 0.001 level.

SPAD readings and biomass (g) of lettuce (Lactuca sativa) and petunias (Petunia × hybrida) grown in substrates enriched with compost produced using LC and VC electric composters are shown in Table 3 and Table 4 respectively.

Table 4.Petunia plant growth characteristics and biomass.

^zMeans having similar letters are not significantly different at the 0.05 level. ** = Significant at the 0.01 level; ***Significant at the 0.001 level.

Compost additions of 0% (control-PB), 5%, and 10% (v/v) were used. The vertical axis shows plant biomass (g), while the horizontal axis shows the percentage of compost in the substrate (% v/v). The results are presented as mean values, and different letters above the bars indicate statistically significant differences between variants (p < 0.05). Compost addition significantly increased the biomass of both species compared to the control (Table 3 and Table 4). In the case of petunias, the highest biomass was obtained with 10% LC compost addition, while VC compost showed a comparable effect at 5% addition. In lettuce, maximum biomass was observed at 5% compost addition in both systems, while 10% LC compost addition led to a decrease in biomass compared to the 5% variant. The results indicate that a moderate addition of 5% to 10% (v/v) compost can significantly improve plant growth, with the optimal level depending on both the plant species and the type of composter used. Direct application of SCGs to soil in concentrations above 20% (v/v) has caused phytotoxic effects due to the presence of caffeine, tannins, and polyphenols [5] [17]. Composting with other organic matter can mitigate negative effects and enhance the agronomic value of SCGs. Previous research established that applying 10% (v/v) SCGs to lettuce cultivation improved chlorophyll content by up to 37%, with only a minor decrease in organic nitrogen (4.4%) [2]. Pre-treated SCGs significantly improved soil pH, C:N ratio, and microbial activity compared to raw SCGs, which tended to reduce seedling growth and nutrient uptake in early stages of growth [5]. The blend of SCGs with other organic sources could unlock the potential of soils while reducing plant phytotoxicity by itself [18]. This statement lends itself to further research, especially horticulture, where most substrates are organically based. SPAD readings with different letters behind their values indicate significant differences among treatments at p < 0.05. In both species, SPAD values increased with compost addition relative to the PB control, indicating improved leaf greenness. In Petunia, the highest SPAD reading was observed at 10% LC compost, whereas VC compost produced comparable values at 5% and 10%. In lettuce, both LC and VC compost significantly increased SPAD readings at 5% and 10%, with the highest values observed at 10% compost addition. Overall, the results indicate that moderate incorporation of composted SCGs improved leaf chlorophyll status in both species.

Accelerated processing altered the physical structure of SCGs in both electric household composters. Bulk density increased from 0.338 g·cm⁻³ in raw SCGs to 0.463 g·cm⁻³ in Vego^® after 10 days and reached 0.451 g·cm⁻³ in Lomi^® after 5 days, followed by a slight decline to 0.426 g·cm⁻³ after 10 days, while particle size shifted toward finer fractions, particularly the #25 sieve fraction (Table 1). These changes are consistent with moisture removal, grinding, and progressive organic matter transformation reported for other in-vessel composting systems [13] [14]. Lomi^® produced rapid particle size reduction from the first day, whereas Vego^® showed a more gradual shift over time. This physical homogenization is relevant for substrate use because fine SCG derived amendments can be more uniformly incorporated into coarser PB substrates.

Both composters substantially reduced SCG mass and volume during processing (Figure 2). In the Lomi^® system, the most reduction occurred within the first three days, whereas Vego^® showed a more gradual reduction pattern over the 10 day period. These results indicate that both systems can reduce SCG waste volume and produce more concentrated material for horticultural use, although the temporal pattern of reduction differed between devices.

Biological stability was strongly influenced by processing duration. Materials processed for 1 day remained below the Solvita^® practical compost health threshold, indicating continued biological activity, whereas 5 and 10 day treatments reached values consistent with practical stability or mature compost (Figure 3). This pattern agrees with the established relationship between composting duration and respiration-based maturity indicators [6] [10] [11]. Importantly, rapid dehydration and particle size reduction alone were insufficient to indicate maturity. SCGs contain readily available carbohydrates, caffeine, and phenolic compounds that may sustain microbial activity after physical processing [1] [2]. Therefore, visual dryness or volume reduction should not be interpreted as evidence of biological stability. Under the conditions of this study, at least 5 days of processing was required for SCG derived amendments to approach practical stability.

Chemical indicators further supported the maturity assessment. The C/N ratio of processed SCG amendments remained narrow, ranging from 22:1 to 24:1, and was below the commonly used 25:1 threshold associated with compost maturity [6]. Total C and N contents remained relatively stable across processing durations, suggesting proportional mass loss without substantial N depletion. In contrast, EC increased with processing duration, reaching the highest values after 10 days (Figure 5). This increase was likely caused by moisture loss and concentration of soluble ions, as well as mineralization of organic matter, consistent with previous SCG-based amendment studies [3] [4]. The pH of processed SCGs remained slightly acidic, which is relevant for PB-based substrates but may require adjustment for crops with higher pH requirements.

The nutrient profile of SCG derived amendments also differed markedly from the PB and peat reference materials. Processed SCGs contained higher macronutrient concentrations, particularly K, P, and N (Table 2). Potassium was the dominant mineral nutrient, and P and K generally increased with processing duration as material mass decreased. These results support the potential of processed SCGs as supplemental nutrient sources in container substrate systems. However, the simultaneous increase in EC indicates that nutrient value must be balanced against salinity risk, particularly when using higher incorporation rates or salt-sensitive crops.

Because all greenhouse substrate treatments received the same baseline controlled-release fertilizer, lime, and micronutrient amendment, plant growth responses should be interpreted as the effect of SCG derived amendments under a common fertility regime rather than as a response to fertilizer addition alone. No additional fertilizer was applied after transplanting. Therefore, the improved biomass and SPAD responses observed in amended substrates likely reflect combined effects of SCG derived nutrients, nutrient retention and release within the PB matrix, and changes in substrate physicochemical properties.

Both lettuce and petunia responded positively to selected SCG derived amendments incorporated at 5% to 10% (v/v), with biomass and SPAD values generally exceeding those of the unamended PB control. Lettuce biomass increased most strongly to a 5% amendment, whereas Petunia continued to respond positively at 10% LC incorporation. These species-specific responses likely reflect differences in salinity tolerance. Lettuce (Lactuca sativa) is moderately salt-sensitive, and elevated EC at higher incorporation rates may have partially offset the nutrient benefit. Petunia (Petunia × hybrida) appears comparatively more tolerant of variable substrate conditions [19]. Therefore, SCG amendment rates should be adjusted to crop-specific salinity thresholds rather than applied uniformly.

SPAD values increased with amendment addition in both species, indicating improved leaf greenness and nutritional status. However, because all treatments received the same baseline fertilizer, the SPAD response cannot be attributed solely to N supplied by SCG derived amendments. Instead, it likely reflects the combined effects of supplemental nutrients from processed SCGs, nutrient retention in the PB matrix, and changes in substrate conditions.

Overall, both Lomi^® and Vego^® produced SCG derived amendments with Solvita^® values consistent with practical stability after 5 to 10 days of processing. Lomi^® achieved more rapid initial mass and volume reduction, whereas Vego^® showed a more gradual reduction pattern. The greenhouse trial demonstrated that selected processed SCG amendments can improve plant growth when incorporated at low rates into PB-based substrates. A primary limitation of the processed materials was an elevated EC, particularly after 10 days of processing, which may reduce incorporation rates and limit use with salt-sensitive crops.

These results support accelerated household processing as a small-scale strategy for converting SCGs into organic amendments for horticultural substrates. Processing for at least 5 days and incorporation at rates not exceeding 10% (v/v) appear appropriate under the conditions evaluated here. Future research should evaluate additional crop species with contrasting salt sensitivity, nutrient release and leaching behavior under irrigation, longer production cycles, and the performance of mixed household feedstocks in electric composters. Additionally, incorporating coffee into container compost heaps to finish the composting process should be explored in other scientific studies.

Accelerated processing altered the physical, chemical, and biological properties of spent coffee grounds (SCGs). Both Lomi^® and Vego^® reduced material mass and volume and produced finer, denser, and more uniform SCG derived amendments. Materials processed for 1 day remained biologically active, whereas 5 and 10 day treatments reached Solvita^® CO₂ values consistent with practical stability.

The processed SCG amendments were acidic and nutrient-rich, with C/N ratios below 25:1. However, EC increased with processing duration and remained an important limitation for horticultural use. Therefore, SCG derived amendments should be used at low incorporation rates, particularly for salt-sensitive crops.

Lettuce and Petunia responded positively to SCG derived amendments incorporated at 5% to 10% (v/v). Biomass and SPAD values increased compared with the PB control, indicating that processed SCGs can improve plant performance when used conservatively. Based on the combined maturity, physicochemical, and plant growth responses, incorporation rates not exceeding 10% (v/v) are recommended for PB-based substrates.

Overall, accelerated household processing can convert SCGs into practically stable, nutrient-rich organic amendments suitable for small-scale horticultural substrate use. Further research should evaluate additional crop species, longer production cycles, nutrient release and leaching behavior, and the performance of mixed household feedstocks in electric composters.

The authors thank the LSU AgCenter Soil Testing and Plant Analysis Laboratory, School of Plant, Environmental, and Soil Sciences, Louisiana State University Agricultural Center, Baton Rouge, Louisiana, USA, for providing analytical support.

Conceptualization, B.D. and E.W.B.; methodology, C.B., A.H., W.J.D. and E.W.B.; software, B.D. and E.W.B.; validation, B.D., E.W.B. and A.E.; formal analysis, B.D.; investigation, C.B., A.H. and W.J.D.; resources, E.W.B. and A.E.; data curation, B.D.; writing-original draft preparation, B.D.; writing-review and editing, B.D., A.H., E.W.B. and A.E.; visualization, B.D. and E.W.B.; supervision, E.W.B. and A.E.; project administration, E.W.B.; funding acquisition, E.W.B. and A.E. All authors have read and agreed to the published version of the manuscript.

This research was supported by USDA National Institute of Food and Agriculture (NIFA) Hatch funds through the Louisiana State University Agricultural Center (LSU AgCenter) Nursery Crops Program.

The data supporting the findings of this study are available from the corresponding author upon reasonable request.