Reuse of dairy waste in sweet potato fertilization: impacts on productivityAbstract
There are few studies evaluating the effects of dairy waste application on sweet potato productivity, especially when combined with organic compost fertilization. In this context, the objective was to evaluate the effects of dairy waste fertilization, with or without organic compost, on the yield components and quality of sweet potato. The experiment was conducted under field conditions from April to August 2019 at the UFRPE experimental farm in Garanhuns, Pernambuco, using a randomized block design with different doses of dairy waste (0, 5, 10, and 15 m3 ha−1) in the presence and absence of organic compost, in addition to a control with mineral fertilization. The analyzed variables included ash and starch contents, total and marketable yield, as well as tuber quality parameters. For ash content in the absence of organic compost, the dose of maximum efficiency was 10.21 m3 ha−1, resulting in 0.55% ash, while the number of marketable tubers reached maximum efficiency at 8.05 m3 ha−1, providing 25.09 tubers per plot. The application of dairy waste, either alone or combined with organic compost, promoted increases in sweet potato yield (t ha−1) and quality, showing performance comparable to mineral fertilizer. The highest ash and starch contents were obtained with the exclusive use of dairy waste, highlighting its potential as a sustainable fertilization alternative.
Keywords:
Ipomoea batatas (L.) Lam; reuse; sustainability; recycling
Resumo
São escassos estudos que avaliam os efeitos da aplicação de resíduos de laticínio sobre a produtividade da batata-doce, sobretudo quando associados à adubação com composto orgânico. Nesse contexto, objetivou-se avaliar os efeitos da adubação com resíduos de laticínio associados ou não ao composto orgânico nos componentes de rendimento e qualidade da batata-doce. O experimento foi conduzido em campo, de Abril a Agosto de 2019, na fazenda experimental da UFRPE em Garanhuns-PE, utilizando um delineamento em blocos casualizados, com diferentes doses de resíduo de laticínio (0, 5, 10 e 15 m3 ha−1) na presença e ausência de composto orgânico, além de uma testemunha com fertilização mineral. As variáveis analisadas incluíram teores de cinzas e amido, produtividade total e comercial, além de parâmetros de qualidade das túberas. Para os teores cinzas na ausência do composto orgânico, a dose de máxima eficiência foi de 10,21 m3 ha-1, proporcionando 0,55% de cinzas, o número de tubérculos comerciais teve dose de máxima eficiência de 8,05 m3 ha-1 proporcionando 25,09 tubérculos por parcela. A aplicação de resíduos de laticínio, isolados ou combinados ao composto orgânico, promoveu aumento na produtividade (t ha-1) e qualidade da batata-doce, apresentando desempenho semelhante ao fertilizante mineral. Os maiores teores de cinza e amido foram obtidos com o uso exclusivo do resíduo de laticínio, evidenciando seu potencial como alternativa sustentável de adubação.
Palavras-chave:
Ipomoeabatatas (L.) Lam; reutilização; sustentabilidade; reciclagem
1. Introduction
Sweet potato (Ipomoea batatas (L.) Lam.) is an important source of fiber, carbohydrates, antioxidants, and minerals, with 12 elements identified in its composition, in addition to proteins and essential amino acids (Nascimento et al., 2025). Due to its high nutritional value, it has been widely used in the food industry (Zhang et al., 2022). Furthermore, its hardiness and adaptability to different climatic conditions, combined with high productivity in a short period, make it essential for small farmers, especially in low-technology agricultural systems (Tirado-Malaver and Tirado-Lara, 2025).
However, achieving high yields requires a substantial supply of nutrients, which often increases production costs, particularly for small and medium-sized farmers with limited investment capacity (Andrade et al., 2025). In this context, the use of alternative nutrient sources, such as organic compost and dairy waste, emerges as a promising solution, as it can increase productivity and improve tuber quality while reducing dependence on costly inputs (Shabitha and Rajeswari, 2021).
In addition to crop benefits, composting improves the chemical, physical, and biological properties of the soil, contributing to its regeneration and the mitigation of environmental impacts (Jain and Kalamdhad, 2020; Lima et al., 2023; Wang et al., 2024; Cabral et al., 2025). On the other hand, residues from the dairy agroindustry, rich in organic matter and nutrients such as nitrogen (N), phosphorus (P), and potassium (K), have shown great potential to increase soil fertility, enhance carbon sequestration, and improve crop productivity (Wei et al., 2021).
Beyond agricultural benefits, the reuse of these residues aligns with Sustainable Development Goal (SDG) 12, as it promotes the recycling of organic materials from production chains. In agriculture, especially in organic sweet potato management, these practices represent a key pillar in achieving the SDGs, particularly SDG 2 (Zero Hunger and Sustainable Agriculture), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action).
Unsustainable agricultural practices, such as the excessive use of mineral fertilizers, have caused significant environmental impacts, including soil acidification, contamination of water resources, and greenhouse gas emissions (Zhu et al., 2018). Therefore, the transition toward more sustainable systems requires the adoption of alternatives that integrate production efficiency with environmental preservation.
Studies have demonstrated the potential of dairy residues in the production of nitrogen-rich fertilizers with greater bioavailability, capable of improving crop yield and soil physical and chemical properties. Moreover, the use of these residues contributes to the advancement of organic agriculture and sustainability (Shamsuddoha et al., 2024). Nevertheless, studies evaluating the effects of dairy waste application on sweet potato productivity remain scarce, especially when combined with organic compost fertilization (Hu et al., 2021; Esan et al., 2021; Fan et al., 2023; Tirado-Malaver et al., 2025).
Therefore, the objective of this study was to evaluate the effects of dairy waste fertilization, with or without organic compost, on the yield components and quality of sweet potato.
2. Materials and Methods
2.1. Experimental site
The experiment was carried out under field conditions at the Experimental Farm of the Federal Rural University of Pernambuco (UFRPE), located in the municipality of Garanhuns, Pernambuco, Brazil, from April to August 2019. The experimental area is situated at 08°58’28” S latitude, 36°27’11” W longitude, and an altitude of 736 meters. The regional climate is classified as tropical rainy with a dry summer (As) according to the Köppen-Geiger classification, with an average annual temperature ranging from 20.1 °C to 22.0 °C and annual rainfall between 751 and 1000 mm (Barbosa et al., 2016). Climatic data recorded during the experimental period are shown in Figure 1.
Daily mean values of relative air humidity (RH, %), rainfall (Prec., mm), and mean temperature (T. mean, °C) from April to August 2019. Garanhuns, PE. Source: INMET (2025).
The soil used in the experiment was previously collected from the 0-20 cm layer and taken to the laboratory for chemical characterization. The results are presented in Table 1.
2.2. Seedling production
The cuttings of the cultivar BRS Rubissol were obtained from the company Frutplan Mudas Ltda., located in Pelotas, Rio Grande do Sul, Brazil. Propagation was carried out in manually prepared ridges (leirões) using hoes, each 12 meters long. The cuttings were planted with a spacing of 80 × 25 cm, irrigated according to weather conditions, and weeded three times during a 90-day period. When sufficient cuttings were obtained for the experiment, they were trimmed to 30 cm in length with 5 to 8 buds each, and three buds were buried per cutting at planting.
2.3. Soil preparation
Soil preparation was carried out mechanically using a disc harrow and complemented manually with a hoe. The total experimental area of 27 × 17 m was divided into three blocks, each containing nine plots (5 × 3 m), resulting in seven ridges per plot spaced 0.8 m apart. The spacing between plants was 0.25 m, corresponding to an estimated plant density of 50,000 plants per hectare. The useful plot area was defined by excluding one ridge at each end and considering two central plants from the remaining five ridges.
2.4. Experimental design and treatment application
The treatments were arranged in a randomized block design in a factorial scheme [(4 × 2) + 1], consisting of four doses of dairy waste (0, 5, 10, and 15 m3 ha−1) in the presence and absence of organic compost, plus an additional mineral-fertilized control, with three replications. The absolute control consisted of the zero-dose dairy waste treatment without organic compost, allowing comparison with the other organic treatments. The organic compost was applied at a rate of 40 t ha−1 (Cavalcanti, 2008). The dairy waste was applied via foliar spraying in three equal portions at 15-day intervals, starting 35 days after sweet potato planting.
Mineral fertilization followed the recommendations of Cavalcanti (2008). The application rates were based on soil analysis results (Table 1), corresponding to 20 kg ha−1 of N for planting and topdressing, 60 kg ha−1 of P as a basal application, and 20 kg ha−1 of K for planting and topdressing. The nutrient sources were urea, single superphosphate, and potassium chloride, respectively.
2.5. Preparation of organic compost and acquisition of dairy waste
The organic compost was prepared in piles consisting of alternating layers of dry plant material (30 cm) and cured cattle manure (5 cm), up to a total height of 1.5 m. The piles were turned every 15 days, and irrigation was carried out according to the internal temperature, measured with a metal rod inserted into the pile. The total composting period was 90 days (Souza, 2006).
The dairy waste, originating from local agro-industries, was obtained in partnership with a dairy processing facility located in Garanhuns, PE. This waste resulted from processes occurring throughout milk processing. A total of 270 liters of dairy waste were used in the experiment, applied manually using backpack sprayers to ensure uniform distribution across the plots. The waste was supplied already processed and ready for experimental use. The prior chemical characterization of this material is presented in Table 2.
Chemical characterization of the organic compost and liquid dairy waste. Garanhuns, PE, 2019.
2.6. Crop management
Crop management practices included weed control performed manually. A total of four weedings were carried out during the experiment, with the last one conducted before full canopy closure. Irrigation was applied through a drip system, following the methodology proposed by FAO 56. During the experimental period, no pest or disease problems were observed, and therefore, no phytosanitary interventions were required.
2.7. Data collection
The determination of starch (ST) and ash (AS) contents was conducted at the Postharvest Biology and Technology Laboratory of the Federal University of Paraíba, in the Center for Agricultural Sciences. Starch content was determined according to the method described by IAL (2008), while ash content was obtained by combustion of the organic matter in a muffle furnace at 550-570 °C, following the AOAC (1995) method.
The number of marketable tubers (NMT) and non-marketable tubers (NNMT) was analyzed at the Plant Production Laboratory of UFRPE. After harvest from the useful plot area, tubers were weighed and classified as marketable (weight ≥ 80 g) or non-marketable (weight < 80 g).
The mean weight of marketable tubers (MWMT) and non-marketable tubers (MWNMT) was calculated as the ratio between the total mass of marketable (≥ 80 g) or non-marketable (< 80 g) tubers and their respective number.
Total tuber yield (TTY) and marketable tuber yield (MTY) were determined based on the average production per plant, multiplied by the estimated stand of 50,000 plants per hectare, according to the spacing described for the crop.
Yield per useful plot (YUP) was determined by weighing all tubers obtained from the useful plot area. The weight of non-marketable tubers per useful plot (WNNMTUP) was determined by weighing all non-marketable tubers obtained from the same area.
Harvesting was carried out manually with hoes, 140 days after planting the sweet potato cuttings, following Embrapa’s recommendations for the crop in Rio Grande do Sul (Embrapa-RS).
2.8. Statistical analysis
The obtained data were subjected to analysis of variance (ANOVA), and treatment means with and without organic compost were compared using Tukey’s test at a 5% probability level (p < 0.05). The effect of dairy waste doses was evaluated through polynomial regression analysis. The means of the additional control (mineral fertilization) and the absolute control (zero dairy waste dose without organic compost) were compared to the other treatments using Dunnett’s test.
Subsequently, principal component analysis (PCA) was performed to examine the interrelationships among the studied variables. Significant principal components (PCs) were selected according to Kaiser’s (1960) criterion, considering only eigenvalues greater than 1 (Lamichhane et al., 2021). All statistical analyses were conducted using RStudio software (RStudio Team, 2021).
3. Results
In Figure 2B, bars represent the mean ash content (Ash) under the composting condition (WOF) and are compared among dairy residue doses using uppercase letters according to Tukey’s test at the 5% probability level. The line represents the continuous response of ash content (Ash) as a function of dairy residue doses under the non-composting condition (NoOF), fitted by a quadratic regression model and displayed on the secondary axis.
Mean values of starch content (Starch, g 100 g−1) (A) and ash content (Ash, %) (B) in sweet potato tubers subjected to the presence (WOF) and absence (NoOF) of organic compost combined with different doses of dairy waste. Garanhuns, PE, 2019.
3.1. Sweet potato yield characteristics are influenced by the application of dairy waste and organic compost
In Figure 3B, it can be observed that the mean number of marketable tubers (NMT) showed a significant quadratic fit in the absence of organic compost (NoOF), with a coefficient of determination R2 = 0.84, indicating that the dose of maximum efficiency was 8.05 m3 ha−1, providing 25.09 marketable tubers.
Mean weight of marketable tubers (AWMT, kg) (A), number of marketable tubers (NMT) (B), number of non-marketable tubers (NNMT) (C), mean weight of non-marketable tubers (AWNMT, kg) (D), yield of marketable tubers (MTY, kg ha−1) (E), and total tuber yield (TTY, t ha−1) (F) of sweet potato tubers subjected to the presence (WOF) and absence (NoOF) of organic compost combined with different doses of dairy waste. Uppercase letters compare doses within each compost condition, and lowercase letters compare compost conditions within each dairy waste dose, according to Tukey’s test at 5% probability. Garanhuns, PE, 2019..
In treatments with organic compost (WOF), no significant differences were found among the applied doses of dairy waste (p > 0.05), indicating that compost addition stabilized the NMT variable.
Regarding the average weight of non-marketable tubers (AWNMT), the data in Figure 3C show a significant effect of dairy waste doses only in the presence of organic compost, with a decreasing linear fit, indicating a reduction in weight with increasing doses. In the absence of compost, no statistical differences were observed (p > 0.05), suggesting that dairy waste alone did not influence this variable.
The yields of marketable tubers (MTY) and total tubers (TTY) exhibited similar responses (Figures 3E and 3F), with a significant quadratic fit only in treatments with organic compost. Under these conditions, yield (TTY) increased up to doses between 10-15 m3 ha−1, followed by a slight decline. Conversely, in the absence of compost, there was no significant effect of the doses (p > 0.05), indicating low efficiency of liquid dairy waste when applied alone.
The weights of marketable tubers (AWMT) and the number of non-marketable tubers (NNMT), presented in Figures 3A and 3C, were not affected by the treatments, regardless of the presence or absence of organic compost (p > 0.05), demonstrating that these variables are less sensitive to the adopted nutritional management.
3.2. Fertilization with dairy waste and organic compost can replace mineral fertilizers without yield loss in sweet potato
The graphs presented in Figure 4 illustrate the effects of dairy waste and organic compost application, alone or in combination, on ash and starch contents as well as on sweet potato yield components. Statistical comparisons were performed using Dunnett’s test, which compares all treatments directly to the control (mineral fertilization), adopting a 5% significance level (p < 0.05).
Mean values of starch content (Starch, g 100 g−1) (A), mean weight of marketable tubers (AWMT, kg) (B), yield of marketable tubers (MTY, kg ha−1) and total tuber yield (TTY, t ha−1) (C), ash content (Ash, %) (D), number of marketable tubers (NMT) and non-marketable tubers (NNMT) (E), and mean weight of non-marketable tubers (AWNMT, kg) (F) of sweet potato subjected to the presence (WOF) and absence (NoOF) of organic compost combined with different doses of dairy waste. An asterisk (*) indicates a statistically significant difference compared with the control, according to Dunnett’s test at the 5% probability level. Garanhuns, PE, 2019.
The mean starch contents (Starch), shown in Figure 4A, presented significant differences (p < 0.05), with higher values observed for doses of 0 and 5 m3 ha−1 of dairy waste without organic compost. The yield of marketable tubers (MTY) and total tuber yield (TTY), shown in Figure 4C, also differed significantly, where the treatments with mineral fertilization and 5 m3 ha−1 of dairy waste without organic compost produced higher yields for both variables compared to the others.
The mean weight of non-marketable tubers (AWNMT) decreased significantly with increasing doses of dairy waste (p < 0.05), with the highest dose (15 m3 ha−1) resulting in the lowest weight of non-marketable tubers.
The number of marketable tubers (NMT) and non-marketable tubers (NNMT) were also influenced by the treatments (p < 0.05), with the mineral fertilization treatment producing the highest number of marketable and non-marketable tubers, respectively.
3.3. Interrelationships between treatments and studied variables
Principal component analysis (PCA) (Figure 5) explained 63.6% of the total variation in the data, with 37.1% attributed to the first component (PC1) and 26.5% to the second component (PC2). Treatments with different doses of dairy waste, applied with or without organic compost, were distributed along both axes, highlighting the influence of treatments on variable behavior.
Principal component analysis (PCA) of ash and starch contents and yield traits of sweet potato subjected to fertilization with different doses of dairy residue, with or without organic compost. For the names of the variables, refer to the Methods section. Garanhuns, PE, 2019.
The variables AWNMT, NNMT, NMT, TTY, and MTY contributed most to data variation, with AWNMT and NNMT aligning more closely with PC2, while TTY and MTY were more strongly associated with PC1. Additionally, proximity was observed between the treatments 0+Org, 5+Org, and 10+Org, suggesting similar behavior, particularly in the absence of organic compost. Conversely, the 15+Org treatment appeared distant from the others, showing little influence on response variation.
4. Discussion
4.1. High doses of dairy residue alter carbohydrate partitioning and increase ash concentration in sweet potato tubers
Starch content in sweet potato was higher in the absence of dairy residue and at lower doses. As the doses increased, starch accumulation decreased. This result may be associated with the greater nitrogen input, which initially promotes carbohydrate accumulation in roots but, in excess, tends to stimulate vegetative growth, leading to nutritional imbalances and reduced partitioning of photoassimilates to storage organs (Zhao et al., 2022; Gao et al., 2023). This behavior is reinforced by the lower starch concentration in organically fertilized plants and by the similarity observed in the regression curve pattern.
Conversely, ash concentration increased with higher doses of dairy residue, indicating greater mineral accumulation, particularly of K, Ca, and Mg. Although the literature highlights that supplemental organic sources enhance nutrient availability and incorporation into tubers (Moore et al., 2024; Möller et al., 2008), in this study the organic compost did not result in higher ash contents in the presence of dairy residue. This may be related to the stabilization of mineralization promoted by supplemental fertilization, which reduced oscillations among treatments. Another plausible explanation is the gradual release of nutrients by the organic compost, ensuring a continuous supply of mineral elements to the plants throughout the crop cycle (Serrano et al., 2010).
4.2. Immediate nutrient availability from mineral fertilization enhances commercial yield, while organic fertilization promotes tuber uniformity
Regression analysis showed no significant changes in AWMT and NNMT, indicating that supplemental fertilization did not interfere with tuber uniformity as a commercial product. It also did not differ from the control, revealing the potential for partial replacement of mineral fertilization in this aspect. However, at the highest dose of residue combined with compost, mineral fertilization showed superior performance, suggesting that a nutritional imbalance may have occurred, favoring vegetative growth over root development (Esteves et al., 2025; Moore et al., 2024).
On the other hand, both TTY and MTY were higher under organic supplemental fertilization, regardless of residue application, improving both tuber quality and quantity. This result reinforces the role of organic compost as a stabilizer, regulating residue mineralization and reducing fluctuations in nutrient availability that could compromise tuber formation and filling. Nevertheless, compared to the control, mineral fertilization still outperformed, particularly in yield parameters, since it provides nutrients more rapidly and in greater amounts. This quick nutrient supply enhances photosynthesis and photoassimilate synthesis, unlike organic fertilization, which releases nutrients more slowly and gradually (Francioli et al., 2016; Yuan et al., 2025).
4.3. Principal component analysis reveals that integrating organic fertilization and dairy residue favors the uniformity of commercial tubers
The proximity between yield components (TTY and MTY) and the control treatment suggests that the immediate nutrient availability provided by mineral fertilization was more closely associated with commercial productivity. Conversely, variables related to non-commercial tubers were positioned in the opposite direction, reflecting a greater influence of treatments containing compost and residue. Overall, the principal component analysis demonstrated that the integration of organic fertilization and dairy residue favors tuber uniformity and number, as well as ash accumulation, possibly due to the slow nutrient release and the organic matter input that alters soil dynamics (Yuan et al., 2025). However, this strategy may reduce the immediate expression of yield components compared to mineral fertilization, as observed by other authors (Wei et al., 2020; Yuan et al., 2025; Körschens et al., 2013).
5. Conclusions
The application of dairy residue associated with organic compost positively affected sweet potato productivity.
Higher ash and starch contents were observed in plants fertilized exclusively with dairy residue.
The use of dairy residues, with or without organic compost, produced agronomic results similar to those obtained with mineral fertilization in sweet potato.
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Data Availability Statement
The data generated and analyzed in this study may be made available upon formal request to the first or corresponding author.
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Edited by
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Editor:
Takako Matsumura Tundisi
The data generated and analyzed in this study may be made available upon formal request to the first or corresponding author.
