Composting is a process of transforming waste into beneficial and valuable agricultural products, making it reusable ([12]). Composting is carried out in modified used gallon containers. The used gallon containers need to be modified by adding air ventilation to facilitate optimal air circulation aimed at enhancing the semi-aerobic process within the composting. Next, raw materials such as coffee grounds and cattle manure are collected and mixed into the modified used gallon containers with ratios of K10 (90% spent coffee grounds + 10% cattle manure), K20 (80% spent coffee grounds + 20% cattle manure), and K30 (70% spent coffee grounds + 30% cattle manure). To produce high-quality compost, several determining factors must be considered, such as oxygen, temperature, moisture, and chemical composition ([10]). The composting process lasts for 2 weeks, with observations of temperature, humidity, and pH parameters. This process is conducted using a soil meter and thermometer, with the materials being turned at least once a week to ensure even heat distribution and aeration. After 2 weeks, N, P, and K tests are performed to evaluate the quality of the compost using the Kjeldahl method with a spectrophotometer. Three independent composting units were prepared for each treatment (K10, K20 and K30), giving a total of nine composting units. Each composting unit was considered one experimental replicate.

Compost enrichment is carried out after the composting is terminated, after stabilization of temperature and pH. The resulting compost quality was subsequently evaluated against SNI 19-7030-2004 prior to enrichment. The finished compost is soaked in a 0.5 M calcium hydroxide solution. The compost is soaked in calcium hydroxide with a ratio of 160 mL to 2 grams of coffee grounds for 24 hours for each composition (K10, K20, K30) while slowly stirring with a flocculation machine. After soaking, the compost is oven-dried for 24 hours at 60˚C. Next, the compost is soaked in a calcium hydroxide-treated compost and subsequently immersed in 0.5 M phosphoric acid (H₃PO₄) solution for 240 minutes while being slowly stirred with a flocculator, then oven-dried for 24 hours at 60˚C. The results of this compost enrichment will be further processed into compost blocks.

The purpose of compost blocking is to facilitate the application of compost to seeds and reduce the use of plastic polybags that can pollute the environment. Thirty compost blocks were produced for each treatment. The compost-blocking process involves combining soil from Sumbersari, which has a loamy texture, with a tapioca flour adhesive mixed with water. The tool used for the blocking process is a modified balloon pump. The adhesive is prepared by dissolving 250 grams of tapioca flour in 300 mL of water, then mixing it with 700 mL of water. For compost blocks, the following combinations are used: K10M (0.3 grams of K10 compost + 100 grams of soil), K20M (0.3 grams of K20 compost + 100 grams of soil), K30M (0.3 grams of K30 compost + 100 grams of soil), with T (100% soil) and (100% commercial compost) as comparisons. Adding 0.3% of the compost to coffee grounds follows the research on coffee grounds modification by [8]. Using the modified balloon pump, compost blocking can produce ready-to-use compost without the need for plastic polybags that can pollute the environment. The first step in making compost blocks is dissolving tapioca flour in hot water while continuously stirring. Next, weigh and mix the soil with the compost, then add the tapioca flour solution while stirring until evenly mixed. Finally, place the mixture into the molding tube until it forms a cylinder, then remove it. The blocked compost will have a diameter of approximately 5 cm and a height of 7 cm.

During the early growth phase, or vegetative stage 1, observations will be made on several parameters, including stem height, number of leaves, leaf width, and leaf length from 7 days after planting until two weeks later. The seeds used are chili pepper seeds. The seeds are planted by making a small hole at the top of the compost block, placing the seed into the hole, and then covering it. Regular watering should be done, and exposure to intense sunlight should be avoided until the seeds begin to sprout.

After the research procedures are carried out, the data obtained will include compost quality, compost enrichment, analysis of N, P, and K content in the compost, and the growth rate of chili plants. These data will then be analyzed using a one-way ANOVA method at a 5% significance level in the SPSS application. Further testing will be conducted using the Duncan Multiple Range Test (DMRT) at a 5% significance level to identify differences in the effects among treatments.

3.1.1. pH Compost

Changes in the organic matter and pH levels occur during the composting process. Monitoring pH is crucial because the pH of composting affects the growth of decomposer microorganisms. Here are the observed pH results.

During the 14-day composting process (Figure 1) with compositions K10, K20,

Figure 1. pH observations of coffee grounds compost over 14 days.

and K30, the most acidic pH occurred on the sixth and eighth days. These pH changes are influenced by the decomposition of cattle manure. High decomposition activity makes the compost more acidic. On the 14th day, composting can be stopped because the pH of all three compositions stabilized following SNI 19-7030-2004, with compost pH ranging between 6.80 and 7.49. At the beginning of composting, the pH is alkaline due to the release of acids but becomes more acidic due to increased bacterial activity that converts nitrogen-containing ammonia compounds, which lowers the pH. Afterward, the pH becomes alkaline again as bacterial decomposition activity decreases. The final pH of the compost is essential because extreme pH can affect compost quality. A pH that is too high can cause the release of ammonia gas, increase oxygen consumption, and harm the environment. Conversely, a pH that is too low can cause the death of essential microorganisms in the compost. However, ammonia can also be an essential factor because it can kill harmful bacteria that can infect plants and use them as hosts ([7]).

3.1.2. Temperature Compost

Temperature is a crucial physical parameter for assessing compost maturity. Additionally, temperature is used to evaluate the effectiveness of the composting system and measure the ongoing decomposition rate.

Based on the data in Figure 2, temperature analysis over 14 days with cattle manure compositions of 10%, 20%, and 30% shows that the average highest mean composting temperature sequence is 30%, 20%, and 10%. Microbial decomposition processes in compost can generate heat, leading to temperature increases. Adding cattle manure to the composting process of coffee grounds significantly affects temperature changes due to the decomposition process. Once all components are decomposed, the temperature will decrease. According to Standard National Indonesia19-7030-2004, compost has no minimum temperature limit, but its maximum limit is based on groundwater temperature.

Figure 2.Temperature observations of coffee grounds compost over 14 days.

Factors influencing temperature fluctuations include microbial processes within the compost. Compost with 30% cattle manure content has the highest mean composting temperature because it contains more decomposer microorganisms. Certain bacteria can convert organic waste into simpler substances that plants easily absorb. Bacteria survive and produce CO₂, H₂O, nutrients, humus, and energy through organic matter changes or decomposition processes. During this process, an exothermic reaction occurs, a chemical reaction that releases heat. Therefore, higher temperatures lead to increased oxygen consumption, accelerating waste decomposition. Conversely, temperatures decrease as the decomposed organic matter in the compost diminishes.

3.1.3. Humidity Compost

Moisture plays a crucial role in microbial metabolism and indirectly affects the oxygen microbes required for survival. Therefore, in the research on developing compost from modified coffee grounds using calcium hydroxide, moisture testing was conducted over a 2-week composting period. Here are the moisture results for coffee grounds and cattle manure composting (Figure 3).

20% and 30% show the highest to lowest relative humidity sequence as 30%, 20%, and 10%. This is influenced not only by the local climate and weather conditions around the composting site but also by microbial activity that transforms organic materials into vapor, thereby increasing the relative humidity of the compost. Additionally, insufficient or decreasing oxygen levels can increase water content and lead to excessively humid conditions. The optimal moisture range for aerobic composting is typically between 50% and 60%. If moisture is less than 50%, the composting process may slow down; if it exceeds 60%, the air volume within the compost decreases. Based on the results of the three treatments, the moisture levels still need to meet the standards set by SNI 19-7030-2004. Moisture in compost significantly impacts the activity of microorganisms involved in the composting process. Excessive moisture can create anaerobic conditions, lower the temperature, and reduce the quality of decomposition because it hinders the breakdown activity ([6]).

Figure 3.Humidity observations of coffee grounds compost over 14 days.

3.1.4. Nutrient Content and C/N Ratio Compost

Nitrogen (N), phosphorus (P), and potassium (K) are nutrients contained in compost, each having a specific function and serving as parameters to indicate certain symptoms when their availability in the soil is reduced or insufficient to meet plant needs. Therefore, the balance of nutrients in the compost must be maintained and observed to determine whether the compost we produce is balanced and meets the standards. Additionally, the analysis of the C/N ratio is vital in the composting process to assess the effectiveness of organic matter decomposition.

Table 1. Composting nutrients.

Note: Description: SNI (Indonesian National Standard) 19-7030-2004 on the standard of compost from organic waste.

Based on Table 1, the highest nitrogen (N) content was found in the K20 treatment, while K30 had the highest phosphorus (P) content. The high moisture condition in K30 caused nitrogen to react with water, forming soluble nitrate, thereby reducing the N content in the compost. Only K10 complied with the SNI requirement for organic carbon, whereas K20 and K30 exceeded the maximum allowable limit. Nitrogen concentrations in all treatments exceeded the SNI minimum requirement (0.40%). In contrast, potassium content in K10 (0.17%) remained slightly below the minimum standard of 0.20%, resulting in insufficient oxygen circulation and the potential death of decomposer bacteria. The highest mean composting temperature occurred in K20 on the eighth day, which was unstable and had the potential to damage P-decomposing microbes.

The highest potassium (K) content was in K30 and the lowest in K10. All N, P, and K contents were above the SNI standards, except P in K10, which was slightly below the standard due to the addition of cattle manure containing essential nutrients ([9]). The high moisture in K10 caused the potassium-containing water solution to dissolve quickly in the compost. The C/N ratio is the comparison of carbon (C) and nitrogen (N) in organic matter.

C/N analysis after 14 days of composting with 10%, 20%, and 30% cattle manure showed that the highest to lowest ratios were 30%, 20%, and 10%, with respective values of (35.06/1.80), (37.51/2.11), and (29.00/2.00). These results comply with SNI 19-7030-2004, which sets the C/N ratio between 10 and 20. A too-high C/N ratio reduces microorganisms, while a too-low ratio indicates immature compost, resulting in nitrogen loss as ammonia or through denitrification ([5]). Immature decomposition processes increase soil temperature around plant roots due to microbial activity generating heat, which can damage roots, especially those sensitive to temperature changes.

3.1.5. Compost Phosphate Content

Phosphate content in compost is crucial as it serves as a medium for seed germination and fulfills the phosphorus nutrient requirements essential for plant physiological processes.

Figure 4.Compost phosphate content.

Based on Figure 4, the initial coffee grounds had a phosphate content of 0.06%, while the cattle manure used as an activator contained 0.34% phosphate. The composting process over two weeks using cattle manure in treatments K10, K20, and K30 showed that the highest phosphate content was in treatment K30, reaching 0.15% ([9]).

The modification process using calcium hydroxide (Ca(OH)₂) and phosphoric acid (H₃PO₄) produces calcium phosphate (Ca₃(PO₄)₂), which has been proven to enhance soil nutrient quality. Test results indicate that adding 0.3% modified compost in treatments K10M, K20M, and K30M also increased phosphate content in the compost, although it still falls below the SNI 19-7030-2004 standard, which specifies a minimum of 0.10% phosphate. Treatment K30M had the highest phosphate content among the three treatments at 0.015%. However, all treatments still have values below commercial compost, which typically has around 2.1% phosphate content.

3.2.1. Chili Plant Height

Chili plant height measurement results are used as the primary indicator to evaluate growth and potential yield. Here are the Duncan Multiple Range Test (DMRT) results for the various effects of compost treatment on chili height.

Based on Figure 5, numbers followed by the same letter in the column indicate no significant difference in the Duncan test at the 0.05 level. The research results show differences in chili seedling height growth among different treatments. Soil and K10M compost treatment show similar effects. Meanwhile, K20M and K30M compost treatments also exhibit similar effects. Commercial compost shows no significant difference with K30M but K30M exhibited the highest numerical value among the modified compost treatments, although it was not significantly different from K20M according to the DMRT test.

Figure 5.DMRT (Duncan Multiple Range Test) diagram for chili plant height.

Figure 6.Side view of the chili seeding.

Based on the ANOVA results and Figure 6, it is evident that commercial compost shows superior growth compared to all other treatments. However, the three treatments using K10M, K20M, and K30M compositions show growth improvements, with K30M recording the highest results. This factor is attributed to the high phosphorus content and the cattle manure’s ability to bind aluminum and iron, making phosphorus more available to plants. Specifically, K30M composition has the highest phosphorus content after commercial compost. Phosphorus is crucial for plants as it forms phospholipids in cell membranes, contributing to better plant growth. Further ANOVA analysis was conducted to evaluate the effects of various modified coffee grounds block compost compositions using calcium hydroxide on chili plant height.

3.2.2. Average of Chili Plant Leaves

The average number of leaves on chili plants is a crucial indicator to assess the extent of chili plant development during vegetative stage 1. Here are the results.

Figure 7.Chili leaf DMRT results.

Based on Figure 7, numbers followed by the same letter in the same column indicate no significant difference according to the Duncan test at the 0.05 significance level. The results show that the K10M compost treatment and soil do not differ significantly. On the other hand, K20M, K30M, and commercial compost provide similar results. Statistical tests indicate that commercial compost performs best, although not significantly different from K30M and K20M.

Figure 8.Top view of chili seeding.

Based on the ANOVA results and Figure 8, commercial compost remains the best, followed by K30M and K20M. This could be attributed to the commercial compost being enriched with complete NPK nutrients, whereas the coffee grounds compost enrichment focused solely on phosphate, using cattle manure as an activator. Nitrogen is crucial as it influences leaf formation, which can enhance carbohydrate production for vegetative growth.

3.2.3. Chili Stem Diameter

Measurement of stem diameter is essential in this study as it serves as an indicator of vegetative growth phase 1. Here are the results.

Figure 9. Duncan Multiple Range Test (DMRT) results for chili stem diameter.

Figure 10. Appearance of the diameter of the chili seedling stem.

Based on Figure 9, no significant differences were observed between soil and composts K10M, K20M, and K30M in the Duncan test at a significance level of 0.05. However, there was a significant difference between soil and commercial compost. Nevertheless, no significant difference was observed between commercial compost and K30M, K20M, and K10M. The analysis of variance (ANOVA) results confirm that commercial compost consistently provided the best results among all treatments tested. On the other hand, K30M compost showed the highest values among the three types of compost evaluated, indicating the potential of using cattle manure and phosphate enrichment to improve soil quality for chili plant growth.

Based on the ANOVA results and Figure 10, it is evident that commercial compost consistently showed the best performance, with K30M being the treatment that closely approached the performance of commercial compost compared to other treatments. The study results revealed that the addition of phosphate and cattle manure positively influenced stem diameter across all compost treatments, particularly in K30M, which nearly matched the performance of commercial compost. Phosphate is crucial in various plant metabolic processes, including cell division and enlargement. Sufficient phosphate availability supports the synthesis of molecules that regulate these processes and facilitates the absorption of water and essential nutrients crucial for plant growth. Phosphate is also essential for developing reproductive cells and overall plant growth. The analysis of variance (ANOVA) for the growth rate of chili plant stem diameter planted in modified coffee grounds compost blocks validates this significant improvement.