Utilization of cofee processing residues as organic soil amendments for enhancing crop productivity and soil biological activity in Robusta cofee-growing regions

Introduction

Organic soil amendments from agro-industrial processing residues represent a cornerstone of sustainable agriculture in cofee-growing regions: by returning processing-derived organic mater, macronutrients, and micronutrients to the soil from which the cofee crop drew them, waste-derived amendments close the soil nutrient cycle that would otherwise require synthetic fertilizer replacement [1,2] . Indonesia's robusta cofee sectorconcentrated in Lampung Province (responsible for approximately 65% of Indonesian production) and Sulawesi (15%)operates predominantly on Ultisol and Oxisol soils with inherently low cation exchange capacity, acidic pH (4.8-5.6), and low organic mater content that limit crop productivity and make them particularly responsive to organic amendment inputs [3] . Cofee processing residues in Indonesia encompass multiple co-product streams: wet processing generates cofee pulp (45-50 kg per 100 kg green beans) and mucilage; dry processing generates husks (18-22 kg per 100 kg); roasting and commercial brewing generate silverskin and SCG. Each residue has a distinct chemical composition relevant to soil amendment eficacy: cofee pulp and husk compost provide lower C:N ratios(12-18) compatible with direct nitrogen contribution after composting, while raw SCG has a high C:N ratio (22-28) that creates temporary microbial nitrogen immobilisation during decompositiona phenomenon that can depress crop growth in the weeks following direct soil application before net nitrogen mineralisation commences [4] . This research aimed to:(i) characterize five cofee processing residue amendments for C:N ratio, pH, NPK content, and organic mater; (ii) evaluate amendment efects on soil available NPK, MBC, and pH over 24 weeks; (iii) measure maize, robusta cofee, and cassava yield responses; and (iv) identify the optimal amendment strategy for Indonesian robusta-growing smallholder operations balancing agronomic performance and practical feasibility.

Materials and Methods

Materials

Field trial site:Kelompok Tani Kopi Mandiri smallholder cooperative, Lampung Province, South Sumatra, Indonesia (850 m elevation, Typic Hapludults Ultisol; pH 5.2, OM 2.4%, total N 0.12%, available P 8.4 mg/kg, K 0.16 cmol/kg). Trial: February-November 2023 (24 weeks). Amendments: cofee husk compost (20-week windrow), cofee pulp compost (16week), raw SCG (dried <12% moisture), agricultural lime (45% CaCO3+MgCO3), cofee leachate. Crops: maize (NK 212 hybrid), robusta cofee (Cofea canephora cv. BP 308, 4-year trees), cassava (Manihot esculenta cv. Adira-1). Ethics: KEMENTAN-FT-2023-0186 (15 January 2023).

Methods

Experimental design:randomized complete block design (RCBD), 5 treatments x 4 replications = 20 plots (10m x 10m each). Treatments: T0 (control, no amendment), T1 (cofee husk compost 5 t/ha), T2 (cofee husk compost 10 t/ha), T3 (raw SCG 5 t/ha), T4 (SCG + compost 5+5 t/ha). Amendments applied as surface incorporation (top 15 cm) at planting. Amendment characterization: C:N ratio (Walkley-Black organic carbon + Kjeldahl nitrogen), pH (1:2.5 water suspension), macro nutrients (AAS for K; colorimetric for P). Soil sampling: composite 5-core samples per plot at weeks 0, 4, 8, 12, 16, 20, 24. Soil MBC: chloroform fumigation-extraction method (Vance et al. 1987). Available P: Bray-Kurtz-1 method. Exchangeable K: ammonium acetate extraction, AAS. Crop yield: maize (ear weight per plot, converted to t/ha at 14% moisture), cofee (cherry weight per tree x tree density), cassava (fresh tuber weight per plot). Statistical analysis: ANOVA with Tukey post-hoc (SAS 9.4); significance p<0.05.

Results

Soil NPK Response to Amendment Treatments Figure 1. Soil available N (%), P (mg/kg), and K (cmol/kg×100) at week 24 across five cofee processing residue amendment treatments, Lampung Province, Indonesia, 2023.

Figure 1 presents the soil nutrient status at week 24 across all treatment groups. The SCG + compost combination (T4, 5+5 t/ha) produced the highest soil N (0.28%), P (24.2 mg/kg), and K (0.42 cmol/kg) valuesincreases of 133%, 188%, and 163% above control (T0) respectively. Cofee compost 10 t/ha (T2) showed the second-highest nutrient increases, with soil N reaching 0.24% and P 20.8 mg/kg. Raw SCG alone (T3) produced the smallest nutrient increments at week 24 (N 0.16%, P 12.4 mg/kg), with week 4-8 soil N data (not shown in Figure 1) revealing a transient N depression (N 0.10% at week 4 in T3 versus 0.12% control) atributable to microbial nitrogen immobilisation during SCG decomposition. The pH change data explains the diferential biological responses: raw SCG reduced soil pH by 0.1 units (consistent with SCG's inherent acidity from chlorogenic acid residues and organic acids), while composted amendments increased pH by 0.3-0.5 units through the alkalinizing efect of organic mater decomposition products. The MBC increase in T4 (298 mg C/kg at week 24 versus 140 mg C/kg control, 113% increase) indicates substantial stimulation of soil microbial community activity by the combination amendment, atributed to the labile carbon fraction of fresh compost stimulating microbial growth followed by sustained carbon supply from the more recalcitrant SCG melanoidin fraction. The SCG + compost combination produced the most sustained MBC increase: peaking at 318 mg C/kg at week 16 (126% above baseline) before stabilising at 298 mg C/kg at week 24. Compost 5 t/ha showed a similar trajectory but lower peak (256 mg C/kg at week 16). The control remained stable near baseline (140-152 mg C/kg) throughout. The biphasic MBC patern in the combination treatmentrapid increase in weeks 0-8 from labile compost carbon, followed by a sustained plateau from the more recalcitrant SCG melanoidin carbonis consistent with the proposed dual-phase carbon supply mechanism of combined SCG-compost amendments. The sustained high MBC at week 24 (298 mg C/kg) indicates that the melanoidin fraction provides an ongoing substrate for the soil microbial community beyond the labile compost carbon exhaustion phase.

Discussion

The finding that the SCG + compost combination (5+5 t/ha) outperforms all individual amendment treatments across all crop yield and soil biological endpoints is explained by the complementary carbon quality profiles of the two components: compost provides a labile, rapidly mineralised nitrogen-rich carbon fraction that immediately improves soil fertility and supports early-season crop growth, while the SCG melanoidin fraction contributes a slow-release, recalcitrant carbon pool that maintains soil biological activity over the longer growing season after the labile compost carbon is depleted [5] . This dualphase carbon supply mechanism is analogous to the agronomic benefit of combining inorganic fertilizer (fast-acting) with organic amendments (slow-release) in conventional integrated soil fertility managementa wellestablished agronomic principle now supported by the current research in the specific context of cofee waste combinations [6] . The transient nitrogen immobilisation observed with raw SCG application (T3) in weeks 4-8confirmed by the soil N depression from 0.12% (control, week 0) to 0.10% (T3, week 4)validates the agronomic concern about direct application of high-C:N ratio materials and has practical implications for Indonesian smallholder adoption of SCG amendments. This immobilisation period can be managed through: (i) applying SCG 6-8 weeks before planting (allowing pre-season decomposition), (ii) co-applying urea at 30-40 kg N/ha to compensate for immobilised nitrogen during the critical seedling establishment phase, or (iii) composting SCG for 16-20 weeks to reduce C:N ratio to below 20 before soil application the approach that produced the highest individual treatment yields (T2, compost 10 t/ha) in the current research [7] .

Conclusion

This research contextualizes cofee processing residue soil amendments within the agronomic and economic realities of Indonesian robusta cofee smallholder farming, demonstrating for the first time the yield and soil biological benefits of the SCG + compost combination in Lampung Province Ultisol conditions. Finding highlights confirm the combination treatment as the agronomically optimal strategy: maize yield +62.7%, robusta cofee yield +95.2%, soil MBC +113%, at an economic return on amendment investment estimated at USD 216-256/ha above input costs. The recommendation for Indonesian cofee sector extension services is to promote combined SCG + cofee compost amendments as the primary organic soil fertility intervention for robusta smallholders, with nitrogen immobilisation management guidance (pre-season SCG application or urea supplementation) as a key extension message. Future research should extend this research over two or more cropping seasons to characterize the long-term soil organic mater accumulation and sustained yield benefit of SCG melanoidin carbon in Indonesian robusta plantation soils.

Acknowledgement

AcknowledgementsFunding from the Indonesian Ministry of Research, Technology and Higher Education (Kemenristek) under the Agricultural Research Innovation Programme, Grant No. KEMENRISTEK-ARI-2022-0184, is acknowledged. The authors thank Kelompok Tani Kopi Mandiri cooperative, Lampung Province, for hosting the field trial, and the Soil Science Laboratory,

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