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Nutrient Properties of Fresh and Composted Cotton Gin Byproducts and Cattle Manure for Soil Amendment
Femi Peter Alege1,*, Cody Daniel Blake1, Sean P. Donohoe1, Joseph W. Thomas1
Published in Journal of the ASABE 67(1): 151-159 (doi: 10.13031/ja.15766). 2024 American Society of Agricultural and Biological Engineers.
1 Cotton Ginning Research Unit, USDA ARS, Stoneville, Mississippi, USA.
* Correspondence: Femi.Alege@usda.gov
The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative commons.org/licenses/by-nc-nd/4.0/
Submitted for review on 2 August 2023 as manuscript number NRES 15766; approved for publication as a Research Brief by Associate Editor Dr. Ruth Book and Community Editor Dr. Kati Migliaccio of the Natural Resources & Environmental Systems Community of ASABE on 23 October 2023.
Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
Highlights
Abstract. Applications of fresh and composted Cotton Gin Byproducts (CGB) and cattle manure as soil amendments are very common practices in the industry. However, composting and direct application of the materials are often limited by factors such as bulkiness, varying nutrient composition and application rates, and handling/transportation costs. This study was conducted to characterize fresh and composted CGB and beef manure (BM) for subsequent co-pelletization and utilization as soil amendments, and to investigate the effects of composting on the nutrient composition and agronomic values. Raw and composted samples of CGB and BM at different storage periods and composting ages were sourced from two commercial gins and a beef farm. Nutrient content, acidity, and compost maturity indices such as carbon/nitrogen (C/N) and ammonium/nitrate (NH4-N/NO3-N) ratios were determined and compared. All samples were obtained at four sampling points on the respective storage or composting piles. The results showed that composted CGB generally contained at least 55% more total-N and phosphorus oxide (P2O5), but approximately 35% less potassium oxide (K2O) than fresh CGB. Composted BM had approximately 3% less total-N, but at least 25% more P2O5 than raw BM. The nutrient compositions affirm the potential for co-pelletizing both forms of CGB and BM to improve the agronomic values and enhance the utilization as soil amendments. In addition, establishing the various properties of CGB and BM is crucial for determining product formulations and selecting process parameters for co-pelletization.
The top four cotton-producing countries (including the USA) produce approximately 70% of global cotton production (Meyer and Dew, 2022; USDA-ERS, 2022). The United States' cotton production was estimated at approximately $7 billion in the marketing year 2019 (USDA-ERS, 2022). While the versatility of cotton fiber is well-documented, the total mass of cotton gin byproducts (CGB, commonly called “gin trash”) is estimated to be several million tons annually across the industry. Depending on the harvesting method and other factors, previous studies have estimated that up to 30% of harvested seed cotton is recovered as byproducts other than cleaned lint/fiber and cotton seeds (Agblevor et al., 2003).
CGB is comprised of motes, burs, leaves, stems, sand, and other materials, and the relative proportions of the materials vary significantly with the harvesting and ginning methods (Abuhay et al., 2021; Conner and Richardson, 1987). In addition to the broad composition and variability in the properties of CGB, the low density requires significant efforts to handle and transport the large volumes generated at the gins. As a result, gins traditionally spread the materials directly on fields or compost the materials in large windrows/piles for later use on fields or other applications (Kim et al., 2004; Norsworthy et al., 2017). Previous studies have investigated the effects of composting and compost age on the nutrient composition and contaminants of CGB (Gordon et al., 2006; Norsworthy et al., 2017). However, while windrowing is mostly adopted by gins, most of the studies on CGB composting were conducted in either containerized experimental setups or compost piles set up mainly to control the composting parameters—such as turning/mixing at a set frequency (Hills et al., 1981; Pessarakli, 1990; Tsatiris et al., 1998). Since such control measures and practices are not typical in traditional CGB composting, industry-wide scaling of new CGB-based products (e.g., in pelletized products for soil amendment) would depend on its compatibility with the traditional methods. Therefore, there is a need to investigate the effect of windrow composting (as commonly practiced by the gins) on the nutrient composition of CGB.
Despite the limitations associated with the composition of CGB, the highly cellulosic constituents of CGB contribute to its versatility and potential suitability for a wide range of applications, including as sources of bioenergy and bio-based materials (Abuhay et al., 2021; Haque et al., 2021; Sharma-Shivappa and Chen, 2008), and nutrients in animal feeds and crop production (Alege et al., 2023; Jackson et al., 2005; Pordesimo et al., 2005). CGB is rich in nitrogen (N), phosphorus (P), potassium (K), and mineral contents, making it suitable as a source of nutrient in crop production (Gordon et al., 2006; Qurat ul et al., 2021). Depending on the relative proportions of the constituent materials, CGB is also used for mulching and erosion control (Holt et al., 2005a; Holt et al., 2005b). In attempts to explore the unique properties of CGB, previous studies have applied several treatment technologies to CGB handling, such as thermochemical conversion (Aquino et al., 2010; Capareda and Aquino, 2007; Zabaniotou et al., 2000). Other studies explored the co-treatment of CGB and other materials with target properties (Kaur and Kommalapati, 2021; Pessarakli, 1990). Funk et al. (2005) anaerobically co-digested CGB and cattle (dairy) manure and reported that, in addition to the derived biogas, the residue was safer for food crop production than manure digested alone. In addition, the study found that the co-digestion of both materials resulted in residues that contained twice the nitrogen content of aerobically composted material. A similar study compared the nutrient and mineral composition of residues from aerobically composted manure and anaerobically co-digested manure and CGB (Macias-Corral et al., 2017). The study also reported that co-digestion of CGB and dairy manure resulted in considerably more nitrogen, and lower sodium contents than aerobically composted manure. Entry et al. (1997) reported that CGB (with poultry litter additives) resulted in higher cellulose degradation, net N-mineralization, and a higher rate of increase in soil nutrient contents than other combinations of poultry litter and agricultural residues, including woodchips or yard trimmings.
To address the bulkiness of CGB while harnessing its energy properties, previous studies (Holt et al., 2003; Holt et al., 2004; Kim et al., 2004; Simonton et al., 2008) also explored the pelletization of CGB to produce fuel pellets. However, the need for binding agents to produce durable pellets that meet fuel pellet standards makes the economics of scale a challenge (Holt et al., 2006). Therefore, research on economical, sustainable, and scalable processes for handling CGB remains important for the cotton (ginning) industry.
Previous studies have also established the potential of pelletizing animal manure with or without binding agents (Hao and He, 2020). Finding effective use for CGB and other agricultural byproducts as feedstock in making new products is economically and environmentally beneficial to both the gins and the producers of the additive materials in terms of circularity and reduced costs of handling/disposal (Haque et al., 2021). Additionally, incorporating two or more agricultural residues is particularly beneficial, especially from large industries such as cotton and cattle production, since CGB and beef manure (BM) are generated in very high quantities. Furthermore, both materials are traditionally applied as soil nutrient amendments, but prolonged application of agricultural residues of unknown or inaccurately determined nutrient composition as soil amendments can lead to over-fertilization of agricultural lands (Innes, 2013). Given the adverse effects of excess nutrients on plants and the environment, it is important to investigate the properties and composition of the CGB and BM to be used as feedstocks in co-treatment technologies.
One of the most important distinctions between chemical (commercial) and organic fertilizers is the ability to deliver known quantities precisely and quickly (Innes, 2013). Therefore, an understanding of the materials’ composition will inform the future accurate formulation of CGB-based products incorporating beef manure as an additive and, perhaps, generate a trend to establish some level of predictability of the materials’ properties. As a preliminary effort towards investigating the effect of windrow composting and co-pelletization on the nutrient composition of CGB and BM, the current study characterizes the fresh and composted forms of both materials from commercial-scale facilities. The main objectives are to establish the nutrient composition and compost stability/maturity indices that are relevant to economically and sustainably formulating pelletized products from CGB and BM for applications as fertilizers in crop production. This study is focused on CGB as the main feedstock; however, brief information is provided on BM as a target additive.
Materials and Methods
Sampling and Composting
Cotton gin byproduct samples were obtained from commercial operations spanning two states in the US cotton-belt (Mississippi and Tennessee). The two locations were approximately 130 km (80 miles) apart by road transportation. The Fresh Cotton Gin Byproduct samples (FCGB) were collected during the ginning season (in November) from piles generated within 30 days of ginning, while composted CGB samples (CCGB) were collected from approximately 1m high compost piles that were at least eight (8) months old. The sampled compost piles were exposed to ambient air temperatures and precipitation without turning. Each location received over 1050 mm (43 in.) of precipitation and 2–29°C (36–84°F) average monthly temperature in the first nine months of the year before sample collection (Weather Underground Incorporated, 2023).
The Raw Beef Manure (RBM) samples were collected from the confinement facility of a commercial beef farm with approximately 2000 head of beef cattle, mostly including weaned calves and yearling steers/heifers. Two categories of RBM were collected within the facility—samples generated within two days prior to collection, and samples generated more than two days but within 30 days. The latter category was stored in small piles in the respective pens within the confinement facility until the materials were transferred to the compost pile or to other locations for different applications. Each category was sampled at four points in the facility, with no distinction by the animals’ ages and feed types. Two categories of composted manure samples (CBM) were also collected from different compost piles that were approximately 1 m high. The samples were categorized by compost age at the time of collection as CBM-01 (at least eight months old) and CBM-02 (less than eight months). The CBM piles were turned at least once every two weeks, and the location received approximately 1230 mm (48.4 in.) of precipitation and 7.5–27 °C (45–81°F) average monthly temperature in the nine months before sample collection (Weather Underground Incorporated, 2023). Table 1 provides a description of the various samples included in this study.
| Table 1. Description, Sources, and Composting Age of Cotton Gin Byproducts and Beef Manure Samples. | ||
|---|---|---|
| Sample[a] | Description | Composting/ Storage Age |
| FCGB-01 | Fresh Cotton Gin Byproducts | Less than 30 days |
| FCGB-02 | Fresh Cotton Gin Byproducts | Less than 30 days |
| CCGB-01 | Composted Cotton Gin Byproducts | Greater than 8 months |
| CCGB-02 | Composted Cotton Gin Byproducts | Greater than 8 months |
| RBM-01 | Raw Beef Manure | Less than 2 days |
| RBM-02 | Raw Beef Manure | Less than 30 days |
| CBM-01 | Composted Beef Manure | Greater than 8 months |
| CBM-02 | Composted Beef Manure | Less than 8 months |
| [a] FCGB = Fresh Cotton Gin Byproducts; CCGB = Composted Cotton Gin Byproducts; RBM = Raw Beef Manure; CBM = Composted Beef Manure; 01 and 02 are the respective sample collection points at each gin or beef farm. | ||
The samples were collected at the respective locations ‘as-is,’ in an attempt to work as closely as possible with the stakeholders and the prevailing composting methods/processes in the local industries. All CGB and BM samples were sampled at four points, at least 5 m apart, along the respective pens or piles. The shapes and sizes of the piles differed between the locations and type of materials.
Nutrient Composition
The collected samples were stored in 5-gallon plastic buckets for transportation to the USDA-ARS Cotton Ginning Research Unit in Stoneville, MS. At the USDA site, approximately 500g of representative samples were bagged and stored in a chest freezer at approximately -18? (0?) until shipment to Brookside Laboratories (New Bremen, OH) for analysis. The samples were analyzed for macronutrient composition, including nitrogen (total-N, organic-N, ammonium-N, and nitrate-N), phosphorus (P), and potassium (K), as well as micronutrients and pH. The values obtained from the analyses were used to compute the potassium oxide (K2O) and phosphate (P2O5) (Clay et al., 2016; Uddin et al., 2020) and carbon/nitrogen and ammonium-N/nitrate-N ratios. The analyses were conducted according to standard procedures of the United States Environmental Protection Agency (US EPA) and the Test Methods for the Examination of Composting and Compost. The nitrogen and carbon contents of the CGB and BM samples were determined by combustion using an Elementar Vario MAX CN analyzer (Elementar Inc., USA). Inorganic-N components (NH4-N and NO3-N) were determined by KCL extraction using a flow injection autoanalyzer (FIAlab Instruments Inc., USA). Using established methods (Peters et al., 2003; US Composting Council, 2002), other elements/minerals composition were determined by nitric acid digestion using the CEM MARS microwave system (CEM Corporation, USA) and analyzed by inductively coupled plasma-optical emission spectrometry (iCAP 6500 Duo ICP, Thermo Fisher Scientific, USA) (Brookside Laboratories, 2023; Peters et al., 2003; US Composting Council, 2002). The respective nutrient compositions of the fresh and composted samples were determined, but direct comparison is limited because of the differences in the sources and insufficient information on the transformations during composting processes.
Statistical Analyses
The experimental design consisted of two main byproducts (CGB and BM) that were obtained in two forms/levels (fresh/raw and composted). This combination gives a total of four ‘treatment’ types for nutrient content comparison—FCGB, CCGB, RBM, and CBM. Two piles were sampled for each ‘treatment,’ giving a total of eight ‘piles.’ Samples were collected at four points on each pile. The CGB samples were obtained at two locations (01 and 02), and the distance between the sampling points ranged between 5 m and 20 m, depending on the pile sizes. For comparison of the different treatment types, the sampling points on individual piles were considered repeated measures, while the locations were considered replicates.
Statistical analysis of the data was completed in R (R Core Team, 2023) using the ‘lme4’ and ‘emmeans’ packages (Bates et al., 2015; Lenth, 2023). An analysis of variance (ANOVA) was used to assess the effects of the treatment types on the nutrient properties. The linear mixed effects models (‘lmer’ function, lme4 package) were used to fit the effects of treatment, pile, and location on the respective nutrient properties (total-N, P2O5, and K2O). The mixed effects models included ‘pile’ and ‘location’ as random effects, which allowed accounting for repeated measurements of byproduct samples from the same pile. Assumptions of normality of the residuals of the models were assessed by examining q-q plots. Pairwise comparisons (Tukey’s HSD) between all treatments (FCGB, CCGB, RBM, and CBM) were conducted using the ‘emmeans’ and ‘contrast’ functions (emmeans package).
Results and Discussion
Total Nitrogen
The Total Nitrogen (total-N) contents of the CGB and BM piles analyzed in this study are shown in table 2, along with the pH, moisture, and C/N ratio. The coefficient of variation is provided after the mean value for each pile as an indication of the variability of the material properties within the pile.
Table 3 shows that there was insufficient evidence to conclude that the mean total-N of FCGB, CBM, RBM, and CBM were significantly different. However, the total-N of CCGB was statistically higher than FCGB and RBM. The mean total-N content of CCGB was approximately 59%, 45%, and 51% higher than FCGB (p = 0.027), RBM (p = 0.048), and CBM (p = 0.037), respectively.
| Table 2. Macronutrients composition, Ammonium-Nitrate Ratio, Carbon-Nitrogen Ratio, pH, and Moisture Content; Reported as Mean (Coefficient of Variation,%). | |||||||||||||
| Pile[a] | Nutrient Concentration (% dry basis) | NH4/NO3 Ratio | C/N Ratio | pH | Moisture | ||||||||
| Total-N | NH4-N | NO3-N | P2O5 | K2O | |||||||||
| FCGB-01 | 2.29 (7.9) | 0.03 (18.2) | 0.01 (0.0) | 0.84 (15.9) | 2.56 (5.2) | 3.1 (18.2) | 19.8 (7.0) | 6.85 (5.4) | 22.62 (12.7) | ||||
| FCGB-02 | 1.99 (11.9) | 0.06 (29.7) | 0.01 (0.0) | 0.84 (20.7) | 2.73 (17.9) | 6.4 (29.7) | 22.4 (13.8) | 6.65 (12.3) | 41.44 (21.4) | ||||
| CCGB-01 | 3.54 (11.1) | 0.04 (20.4) | 0.14 (17.5) | 1.49 (42.3) | 2.34 (39.5) | 0.3 (17.7) | 13.4 (15.9) | 7.01 (4.2) | 60.22 (9.1) | ||||
| CCGB-02 | 3.27 (26.4) | 0.04 (11.8) | 0.17 (0.0) | 1.14 (18.8) | 1.12 (21.7) | 0.3 (11.8) | 15.0 (27.4) | 7.50 (12.3) | 70.91 (3.5) | ||||
| RBM-01 | 2.29 (21.6) | 0.20 (30.3) | 0.01 (0.0) | 2.41 (18.3) | 2.62 (20.8) | 22.2 (30.3) | 11.3 (18.3) | 8.85 (2.5) | 59.00 (5.8) | ||||
| RBM-02 | 2.38 (34.0) | 0.22 (35.5) | 0.01 (0.0) | 2.48 (28.1) | 2.93 (38.3) | 24.7 (35.5) | 10.2 (12.1) | 9.02 (0.9) | 57.14 (5.4) | ||||
| CBM-01 | 1.96 (9.9) | 0.06 (30.3) | 0.05 (0.0) | 3.66 (12.1) | 2.54 (18.3) | 1.3 (30.3) | 14.3 (11.4) | 8.74 (3.8) | 60.27 (8.0) | ||||
| CBM-02 | 2.56 (15.9) | 0.15 (54.7) | 0.01 (0.0) | 2.73 (13.4) | 2.01 (18.4) | 16.7 (54.7) | 11.3 (11.2) | 8.53 (2.2) | 62.67 (3.6) | ||||
[a] FCGB = Fresh Cotton Gin Byproducts; CCGB = Composted Cotton Gin Byproducts; RBM = Raw Beef Manure; CBM = Composted Beef Manure; 01 and 02 are the respective sample collection points at each gin or beef farm. | |||||||||||||
| Table 3. Mean N, P, and K composition of the byproducts. | |||
| Sample[a] | Nutrient Concentration (% dry basis)[b] | ||
| N | P | K | |
| FCGB | 2.14a | 0.37a | 2.19a |
| CCGB | 3.40b | 0.57ab | 1.43a |
| RBM | 2.34a | 1.07bc | 2.30a |
| CBM | 2.26a | 1.40c | 1.89a |
[a] FCGB = Fresh Cotton Gin Byproducts; CCGB = Composted Cotton Gin Byproducts; RBM = Raw Beef Manure; CBM = Composted Beef Manure. [b] Different superscript letters within a column indicate significantly different means according to Tuckey’s HSD at p = 0.05. | |||
Previous studies have reported different effects of composting on total-N composition for different agricultural residues, as the type of raw material significantly influences the humification process during composting (Chefetz et al., 1996). Composting is a biochemical process in which microorganisms partially decompose/mineralize organic carbon (C) and nitrogen (N) while consuming oxygen and generating carbon dioxide (CO2), heat, and water vapor (Irshad et al., 2013; Larney et al., 2006). Therefore, reductions in the organic matter and N compositions, as well as the moisture and dry matter contents, are often observed (Irshad et al., 2013). C is mostly lost in gaseous forms as CO2, and N loss is mostly as ammonia (NH3), nitrous oxide (N2O), nitrogen (N2) gas, etc., via volatilization. However, increasing or constant concentrations of total-N during the composting of the agricultural byproducts have also been reported (table 4). Larney et al. (2006) explained that total-N concentrations remain fairly constant during manure composting when N loss approximately equals dry matter loss but decrease when N loss is greater than dry matter loss. On the other hand, an increase in total-N concentrations is obtained when N losses are lower than dry matter losses. The latter situation is more desirable since it shows that compost management practices with respect to environmental conditions and other factors result in a higher nutrient density of the finished product.
Therefore, the relatively small differences in mean total-N contents of the FCGB and CCGB samples from different sources, as well as the higher concentrations in CCGB relative to FCGB, are desirable. These results suggest that the differences in cotton production/ginning practices, cotton cultivars, weather, and other factors attributable to the two gin locations used in this study may not significantly impact the nitrogen composition of the gin byproducts.
| Table 4. Macronutrients (NPK) Comparison with Values Previously Established in Literature (CGB = Cotton gin Byproducts). | ||||
| Material | Total-N (%) | P (%) | K (%) | Reference |
| Fresh CGB | 2.14 | 0.37 | 2.19 | Current Study |
| 1.25 | 0.60 | 1.25 | Anthony et al. (1992) | |
| 1.16 | - | - | Sadaka (2013) | |
| 2.48 | 0.26 | 1.61 | Buser (2001) | |
| 1.23 | 0.21 | 3.01 | Hills (1982) | |
| 1.39 | 0.22 | 1.03 | LSU Ag. Center (n.d.) | |
| Composted CGB | 3.40 | 0.57 | 1.43 | Current Study |
| 2.14 – 2.29 | 0.37 – 0.52 | 1.25 – 2.90 | Hills (1982) | |
| 2.18 | 0.56 | 0.20 | Grant (2021) | |
| - | 0.45[a] | 2.41[a] | Papafotiou and Vagena (2012) | |
| 1.52 | 0.10 | 0.32 | LSU Ag. Center (n.d.) | |
| 2.18 | 0.20 | 0.56 | ||
| Composted CGB (Anaerobic) | 2.40 | 0.34 | 4.03 | Hills (1982) |
| Raw Beef/ Cattle Manure | 2.34 | 1.07 | 2.30 | Current Study |
| 1.70 – 1.96[b] | 0.41 – 0.73[b] | - | Parkinson et al. (2004) | |
| 1.93 | 1.57 | 1.54 | Zhen et al. (2021) | |
| - | - | 0.92[b] | Irshad et al. (2013) | |
| 0.93 | 0.21 | 0.17 | Khater (2015) | |
| 0.81 – 1.52[b] | 0.31 – 0.68[b] | 0.84 – 1.11[b] | Eghball et al. (1997) | |
| Composted Beef/ Cattle Manure | 2.26 | 1.40 | 1.89 | Current Study |
| 2.74 – 3.03[b] | 0.85 – 1.07[b] | - | Parkinson et al. (2004) | |
| 1.78 | 1.45 | 1.48 | Zhen et al. (2021) | |
| 0.95 | 0.31 | 0.27 | Khater (2015) | |
| - | - | 0.76[b] | Irshad et al. (2013) | |
| 0.76 – 1.10[b] | 0.32 – 0.87[b] | 0.93 – 1.18[b] | Eghball et al. (1997) | |
[a] Values were originally presented in ppm on dry weight basis. [b] Values were originally presented in g kg-1. | ||||
During storage of RBM and CGB in windrows and compost piles, turning frequency and weather elements such as high rainfall often result in the loss of materials and nitrogen through erosion and runoff (Karlen et al., 2021; Vagstad et al., 1997). The results in this study suggest that the composting process and other handling practices in relation to the prevailing weather conditions at the sampling location resulted in fairly constant total-N concentrations. It should be noted, however, that this study only considered CGB samples sourced from two gins, and there is a need to investigate the nutrients trend across more gins or sample sizes. In addition, the fresh vs composted samples are not directly comparable since the materials were not from the same source or monitored throughout the composting process.
Additional Factors
In addition to the factors previously discussed, the properties of composts are also influenced by the pH, carbon-to-nitrogen ratio, inorganic nitrogen, and ammonium-nitrate ratio. (Bernal et al., 2009). Plants absorb nitrogen in its inorganic forms as nitrate or ammonium, and their availabilities are largely influenced by mineralization and mobilization (An et al., 2022; Larney et al., 2006; Sullivan, 2008). Therefore, both forms of N are important components of N-transformation by microorganisms, and their ratio is an important measure of composted material’s stability (An et al., 2022; Sullivan et al., 2018). Nitrate (NO3) concentration is generally lower in fresh feedstock than in composts because most of the inorganic N in the fresh materials exists in the ammonium-N (NH4-N) form, which is converted to NO3-N due to nitrification during the latter stages of composting (Caceres et al., 2018). In this study, the mean NH4-NO3 ratios were lower (approximately 94% for CGB and 62% for BM) in the composts than the respective fresh samples (table 2). For co-pelletization, the results suggest that a formulation comprising of CCGB with either RBM or CBM may provide the highest inorganic-N in the feedstock. However, when formulating feedstock based on CGB and BM, it is crucial to consider the potential effects of the pelleting process on the inorganic forms of nutrients in relation to the specific nutrient requirements of different crops and growing media.
The C/N ratio controls the evolution of the microbial community and, consequently, how much nitrogen is available and how quickly nitrogen will become available from the material (Wang et al., 2019). In the current study, the mean C/N ratios were approximately 32% less in CCGB and 19% higher in CBM than in FCGB and RBM, respectively (table 2). The observed trend for the C/N ratio was consistent for CGB samples from both locations. In addition, storage and composting periods did not appear to significantly influence the C/N ratios of the BM at the sampling location, except when composting extended longer than eight months. Lower C/N ratios generally imply faster N-mineralization, while the rate of decomposition is reduced when the ratio is greater than 30:1 (Hodge et al., Robinson and Fitter, 2000; Wang et al., 2019). As feedstocks in co-pelletization, all CGB and BM samples in the current study had C/N ratios less than 25, which suggests a potential for fast decomposition of pelletized product after field application.
The pH of the CGB samples from both locations did not differ significantly, although the FCGB samples were slightly more acidic than the CCGB samples (table 2). Similarly, all the RBM and CBM samples were slightly alkaline, but storage and composting periods did not appear to significantly affect the pH at the location in the current study. The pH influences the oxidation of organic nitrogen and ammonium to nitrates during composting. Zhang et al. (2011) showed that pH effects on nitrification may not be significant in acidic media (pH < 5) in forest soils, but nitrification was faster when pH was greater than five (pH >5). The pH of CGB obtained in this study agrees with the previously reported value of about 7.5 for CCGB (Papafotiou and Vagena, 2012), suggesting a potential for a fast nitrification rate when co-pelletized with BM and a more stable release and availability for plants.
Phosphorus and Potassium
The mean phosphorus and potassium concentrations in the CGB and BM samples are reported in tables 2 and 3. Although there is insufficient evidence to conclude that the differences are statistically significant, the average phosphorus contents of CCGB and CBM were 56% and 31% higher than FCGB and RBM, respectively. The values obtained for both nutrients in the current study agree with values previously reported in the literature (table 4). For co-pelletization and product formulation, CBM samples contained significantly higher phosphorus (P2O5) than all the CGB samples (p < 0.05). The RBM samples were statistically higher in P content than FCGB (p = 0.035) but not CCGB (p = 0.103). The results imply that BM addition will potentially boost the phosphate composition of the pelletized products.
Composting, as commonly practiced by cotton ginners, tends to increase the phosphate contents of CGB (table 2). Previous studies have also reported an increase in the phosphorus contents of CGB (Hills et al., 1982). In addition, Jakubus (2016) reported that composting organic wastes resulted in a higher Total-P composition regardless of the material composition or the applied composting process. In a study on the effects of different carbon-nitrogen ratios, Wang et al. (2019) also reported an increase in the proportions of inorganic-P to total-P during composting. However, the extent of the phosphorus increase depends on several other factors, including composting period, compost feedstock composition, pH, organic matter content, reaction/precipitation with iron, aluminum, calcium, etc. (Jakubus, 2016; Khan and Joergensen, 2009; Wang et al., 2019).
Given the complexity of phosphorus, in terms of supplementing plants’ phosphorus requirements and its interaction with other nutrients, a more in-depth analysis of the various forms of phosphorus in the samples must be considered in the formulation of feedstock for pelletization, the estimation of quantities of pelletized product required to meet the individual crop’s phosphorus needs, and during the composting process. The need for further characterization is especially necessary for CGB given the relatively smaller documentation of the composting compared to other agricultural residues.
Potassium composition was approximately 35% lower in CCGB than FCGB, and 18% lower in CBM than RBM. However, there is insufficient evidence to conclude that these differences are statistically significant. The coefficients of variation reported in table 2 show that CCGB-01 samples have the highest variability in K content (approximately 39.5%), while FCGB samples from the same location have the least variability (approximately 5.2%). Comparison with the previously reported values of potassium concentrations in CGB also suggests wide variabilities (table 4). Hills (1982) reported that the potassium content of whole fresh gin waste was 3.01 (% d.b.), while the fine fractions of fresh gin waste comprised 2.04 (% d.b.). These values suggest that the proportions of fine materials, often associated with the harvesting methods or the ginning processes, may affect potassium composition. The same study also reported a decrease in potassium concentrations during composting, which was associated with the existence of potassium in ionic form and leaching during watering (comparable to precipitation in outdoor windrow composting). Overall, the current study suggests that there is potential for losses in potassium during composting, and there is a need for more sampling and monitoring of the composting processes to ensure that the nutrients are accurately accounted for during formulations and co-treatments for soil amendment.
Other Nutrients
The respective composition of calcium, magnesium, sodium, and sulfur in the CGB and BM used in the current study are reported in table 5. While these nutrients are required by most crops in relatively smaller quantities than NPK, and the amounts needed are often provided by most agricultural soils, their complex interactions with other macronutrients and elements continue to draw attention from agronomists and fertilizer formulators (Aftab and Hakeem, 2020; Fageria et al., 2002).
| Table 5. Other nutrients and Trace Elements Composition of the sampled Cotton Gin Byproducts and Beef Manure (Mean ± Standard Error of Mean). | ||||||
| Sample[a] | Ca (% d.b.) | Mg (% d.b.) | Na (% d.b.) | S (% d.b.) | Fe (ppm) | Mn (ppm) |
| FCGB-01 | 1.67 ± 0.22 | 0.22 ± 0.00 | 0.04 ± 0.00 | 0.38 ± 0.05 | 188 ±10 | 120 ± 20 |
| FCGB-02 | 2.29 ± 0.22 | 0.34 ± 0.03 | 0.04 ± 0.01 | 0.46 ± 0.03 | 251 ± 42 | 100 ± 22 |
| CCGB-01 | 3.01 ± 0.38 | 0.43 ± 0.06 | 0.03 ± 0.00 | 0.47 ± 0.07 | 367 ± 30 | 327 ± 73 |
| CCGB-02 | 2.69 ± 0.15 | 0.37 ± 0.04 | 0.03 ± 0.00 | 0.49 ± 0.05 | 1447 ± 98 | 187 ± 7 |
| RBM-01 | 1.25 ± 0.13 | 0.53 ± 0.04 | 0.48 ± 0.06 | 0.57 ± 0.05 | 8537 ± 1098 | 405 ± 22 |
| RBM-02 | 1.26 ± 0.21 | 0.53 ± 0.05 | 0.52 ± 0.09 | 0.63 ± 0.10 | 9140 ± 1988 | 424 ± 34 |
| CBM-01 | 3.19 ± 0.16 | 0.75 ± 0.03 | 0.41 ± 0.04 | 0.70 ± 0.05 | 5755 ± 848 | 549 ± 25 |
| CBM-02 | 1.69 ± 0.10 | 0.58 ± 0.02 | 0.35 ± 0.04 | 0.57 ± 0.05 | 6435 ± 1073 | 409 ± 30 |
[a] FCGB = Fresh Cotton Gin Byproducts; CCGB = Composted Cotton Gin Byproducts; RBM = Raw Beef Manure; CBM = Composted Beef Manure; 01 and 02 are the respective sample collection points at each gin or beef farm. | ||||||
As with macronutrients, there is a fine line between micronutrient toxicity and sufficiency to plants (Karlen et al., 2021), and their concentrations/availability in soil solutions are affected by several factors, including organic matter content, soil water, pH, temperature, carbonate contents, etc. (Aftab and Hakeem, 2020; Karlen et al., 2021). One of the most important aspects in the consideration of micronutrients in the formulation and co-pelletization of materials for soil amendment is the interaction of the micronutrients with other chemicals and the resultant influence on the availability of those nutrients. For instance, Mg is essential for chlorophyll formation in plants and significantly influences phosphorus uptake, while Ca is essential for soil pH control, which influences microbial activities, the solubility of nutrients, and plants’ nutrient uptake (McKibben, 2012). In addition, McKibben (2012) highlighted that Ca deficiency might also result in Mn toxicity, and trace elements are significantly reduced at pH = 7 and toxic at pH <5.
Therefore, while the co-treatment and utilization of agricultural residues (such as CGB and BM) to produce materials for soil amendment typically focus on their macronutrient composition, an understanding of the micronutrient contents also provides an insight into the potential interactions between microorganisms and other nutrients (Fageria et al., 2002). In addition, given the variabilities in the properties of CGB, there is a sustained need to monitor the effects of the compositing processes on the materials’ properties that are relevant to improving the sustainable utilization of CGB and other agricultural byproducts such as BM with more predictable properties. Monitoring the composting process for CGB at multiple geographic locations having different environmental conditions is critical to an in-depth understanding of the nutrients’ transformations and formulation of the pelletized products. From sustainability and circular bioeconomy perspectives, economic and environmental benefits may result from the successful application of established techniques (or a combination of such techniques), like the co-pelletization of fresh and composted byproducts from crop and animal production systems for reutilization in crop production.
Conclusions
Fresh and Composted Cotton Gin Byproducts (CGB) and Beef Manure (BM) were characterized for potential utilization as soil nutrient amendments. Rather than containerized composting, windrow composted byproducts, as typical in the industry, were investigated, along with the effects of differences in CGB sources on the materials’ nutrient composition. The study determined the suitability of the materials for further treatment, such as co-pelletization. The results showed that the NPK values obtained in the present study agree with previously reported values in the literature. Composting resulted in a significant increase in the total-N and phosphate contents of CGB, but the effect was not statistically significant for the total-N content of BM. Inorganic nitrogen (estimated as the sum of NH4-N and NO3-N) was significantly higher in BM than in the CGB samples. In addition to potentially improving the strength of the pellets, this finding suggests that co-pelletizing CGB with BM as additives may also boost nutrient composition and nitrogen availability when applied as soil amendments. Similarly, the higher average pH of the BM samples (8.8) than the CGB samples (7.0) suggests a potential for reduced alkalinity or neutralization as feedstocks for co-pelletization. Overall, the nutrient composition of the fresh and composted CGB and BM samples supports the potential for co-pelletization of both materials for soil amendment. Additional sampling and characterization/analyses of CGB from more gins/locations are necessary to establish the variabilities of the properties and aid in a more accurate prediction of the effects of composting on the properties of CGB. Further studies are necessary to monitor the composting process and investigate the potential effects of pelleting and composting on various forms of phosphorus and other nutrients.
Acknowledgments
This research was partially funded by Cotton Inc. grant number 23-215. The authors thank the anonymous gins and beef farm that donated the samples used in this study. The authors also acknowledge the various support provided by the staff and technicians at the USDA-ARS Cotton Ginning Research Unit, located in Stoneville, MS.
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