Experimental area

The experiment was conducted from July to November 2021 at the experimental plot of the Horticultural Research Program, located at the Hernando Bertoni Research Center (CIHB-IPTA), in the City of Caacupé, Paraguay (25°23'16.38'' S, 57°11'22.24'' W). Meteorological data for the period were obtained from the CIHB-IPTA weather station, recording minimum and maximum temperatures of 16.7 °C and 29.2 °C, respectively, with a total accumulated precipitation of 534 mm, whose monthly variations are presented in Figure 1. The soil of the experimental area is classified as Arenic Kandiudult, characterized by a dark reddish-brown color (^{López Gorostiaga et al., 1995}), and its initial chemical properties, determined before the experiment installation, are detailed in Table 1.

Figure 1 Monthly climatic behavior (minimum, maximum temperature and precipitation) during the experimental period at the Hernando Bertoni Research Center of the Paraguayan Institute of Agricultural Technology, Caacupé, Paraguay, 2021. 

Obtaining and characterization of organic fertilizer

The organic fertilizer used in this study was obtained from Cooperativa Manduvira Ltda., located in the city of Arroyos y Esteros. This cooperative produces organic fertilizers from solid waste derived from sugarcane processing, through a composting process without the use of chemical inputs. Raw materials include filter cake (58%), sugarcane bagasse (5%), ash (10%), vegetable protein (10%), minerals (1%), cattle manure (6%), poultry manure (1%), and liquid ecological decomposer inoculants (9%).

Composting is carried out in vertical piles with inoculation of native microorganisms and control of temperature (<70 °C), pH and humidity (20-30%), using mechanical turning to ensure adequate aeration until material maturity is reached. In the final stage, beneficial microorganisms are added to improve the biological quality of the compost. The result is a stable and biologically active product, ready for agricultural use. The main nutritional characteristics of the compost indicate that it possesses 7.45 g/100 g of organic carbon, 4.4 g/100 g of nitrogen, 0.4 g/100 g of phosphorus, 0.3 g/100 g of potassium, 1.7 g/100 g of calcium, and 0.47 g/100 g of magnesium. Chemical characterization of the organic fertilizer was performed through routine analysis in the laboratory at the Hernando Bertoni Research Center of the Paraguayan Institute of Agricultural Technology, Caacupé, Paraguay.

Treatments and experimental design

This study evaluated five treatments consisting of four doses of composted organic fertilizer from sugar industry waste and a control treatment without fertilization. The evaluated levels were: F0 (control, without fertilization), F1 (2 t ha⁻¹), F2 (4 t ha⁻¹), F3 (6 t ha⁻¹), and F4 (8 t ha⁻¹).

The experimental design was organized under a randomized complete block scheme with three replications per treatment, resulting in a total of 15 experimental plots. Each experimental plot of 1.2 x 1.2 m contained four rows separated 0.25 m from each other and 0.08 m between plants, corresponding to a total of 48 plants per plot and 720 plants in the entire experiment. The useful area was considered to be the two central rows with a surface area of 0.816 m², discarding the lateral rows and 0.16 m at the ends due to border effect (Figure 2).

Figure 2 Schematic plan of the experimental plot, with dimensions and distances between rows and plants, highlighting the useful area. Caacupé, Paraguay, 2021. 

Crop establishment and evaluation methods

Soil sampling and analysis were performed at the beginning of July 2021. Fifteen days before sowing, the soil was prepared with the help of a rototiller and raised beds of 20 cm height were formed. Following this, the composted organic fertilizer dose, previously weighed according to assigned treatments, was applied and incorporated with a rake. Subsequently, the drip irrigation system was installed.

Carrot sowing, variety Shin Kuroda, was carried out at the end of July. Furrows were opened with a hoe and seeds were deposited manually at 1 cm depth. At 30 days after sowing (DAS), thinning was performed, leaving plant populations as described in the previous section. Throughout the entire crop cycle, periodic monitoring was conducted to ensure optimal irrigation and cleaning conditions. No phytosanitary products were applied at any stage of the crop.

Plant height (cm) was measured at three time points after sowing (30, 60, and 110 DAS, respectively). For this purpose, six plants were randomly selected from the useful area of each plot. Height was measured from soil level to the apex of the aerial part using a measuring tape.

Harvest was performed immediately after the last plant height evaluation (110 DAS). At this stage, six roots were randomly selected from the useful area, and the following variables were measured: root length (cm), determined from the base of the crown to the longest distal extremity; root diameter (cm), measured 2 cm below the crown with a caliper; root weight (g), after washing the roots to remove soil residues and air drying, they were weighed on a precision digital scale; and yield (kg m⁻²), calculated from the average weight of roots obtained in each treatment, extrapolated by multiplying the average weight by the population density established in the experiment.

Statistical analysis

Data were processed using R software (R Core Team, 2020). Normality of residuals was evaluated using the Shapiro-Wilk test. Once this assumption was confirmed, analysis of variance (ANOVA) was performed to determine the effect of treatments on carrot growth and yield variables. When significant differences were identified (p ≤ 0.05), treatment means were compared using Duncan's test at the same significance level. Additionally, for significant variables, regression analysis corresponding to the evaluated doses was performed.

Effect of organic fertilizer on carrot growth parameters

Carrot plant height measured at three stages of the growth cycle is presented in Table 2. Although the highest dose showed heights up to 17% higher at 30 DAS, 16% at 60 DAS, and 13% at 110 DAS compared to other treatments, vegetative growth was statistically similar for all treatments and evaluation stages. These results contrast with those of ^{Hailu et al. (2024}), who reported a significant increase in carrot plant height with vermicompost application, but coincide with those of ^{Chillo Yupanqui, Peñafiel Rodríguez, W. and Aruquipa Condori (2024}), who observed no significant differences with poultry manure-based fertilizers.

The limited growth response observed in this study could be attributed to the low compost doses applied, which were insufficient to completely satisfy the nutritional needs of the crop, especially in terms of nitrogen (N) and potassium (K). Although the compost used contained considerable amounts of these nutrients, the concentrations present in the soil were relatively low, particularly for K (Table 1). This macronutrient is fundamental for various biochemical functions in plants and is known to improve the quality of horticultural products by participating in processes such as enzymatic metabolism and synthesis of defense compounds (^{Sustr, Soukup and Tylova, 2019}).

Furthermore, the slow release of N in the compost may have restricted its immediate availability, limiting the growth of the crop's aerial part, which is crucial for sustaining high levels of photosynthesis and accumulating reserves for root development, directly impacting final yield (^{Guan, Zhang and Chu, 2025}; ^{Mishra, Levengood, Fan and Zhang, 2024}; ^{Zhou et al., 2022}). This suggests that, in addition to increasing applied doses, it might be necessary to enrich the compost with rapid-release sources of N and K or combine it with mineral fertilizers to optimize the availability of these nutrients in degraded soils and satisfy crop demands more effectively.

Influence of organic fertilizer on crop yield parameters

Root length and diameter increased significantly with the applied organic fertilizer doses (p < 0.05). Regarding length, the treatment with the highest dose (F4) showed increases of up to 24.7% compared to the control (F0), reflecting the positive impact of higher concentrations on this variable (Figure 3a). This trend was confirmed by regression analysis, which showed a significant linear association between applied dose and root length (y = 12.418 + 0.313x; R² = 0.73; Figure 3b). When analyzing the equation, it is observed that for each ton of organic fertilizer applied, root length increases on average by 0.313 cm, with dose being responsible for explaining 73% of the observed variation.

Similarly, root diameter also increased with higher doses, with F4 being the treatment with the greatest increases: 34.9% compared to the control and 15.3% compared to F1 (Figure 3c). This positive relationship was supported by regression analysis, which showed a strong linear fit (y = 2.590 + 0.104x; R² = 0.89; Figure 3d). When interpreting the equation, it is observed that carrot diameter increased on average by 0.104 cm for each ton of fertilizer applied, and that 89% of the observed variation in this variable is explained by the dose used.

Both evaluated parameters suggest that higher doses are required to reach the optimum point in the response curve, highlighting the need to conduct local studies on degraded soils to maximize the agronomic potential of the crop. These results are consistent with previous research that has demonstrated positive effects of organic fertilizers on crop root development. For example, ^{Hailu et al. (2024}) reported increases of 63% in length and 146% in carrot root diameter with 3 t ha⁻¹ of vermicompost, while ^{Zerga Heterat & Tsegaye (2019}) observed increases of 118% in diameter with 75 t ha⁻¹ of compost. These findings support the capacity of organic fertilizers derived from solid waste to significantly improve root structure.

Figure 3 Root length (a-b) and root diameter (c-d) under different doses of composted organic fertilizer made from sugar industry waste. F0 (control, without fertilization), F1 (2 t ha⁻¹), F2 (4 t ha⁻¹), F3 (6 t ha⁻¹), and F4 (8 t ha⁻¹). Vertical lines above bars indicate standard error of means (n = 3). Different letters above bars indicate significant differences between treatments by Duncan's test (p ≤ 0.05). 

Both root length and diameter are critical parameters for commercial quality and support of carrot plants (^{Chillo Yupanqui et al., 2024}; ^{Hailu et al., 2024}). The positive effect observed with increasing doses of organic fertilizer may be related to the gradual supply of essential nutrients (^{Oyege and Balaji Bhaskar, 2023}). Additionally, the incorporation of organic matter may have partially improved soil structure (^{Villalba Algarin, 2025}), increasing moisture retention and aeration, conditions that favor root development and facilitate nutrient uptake in deeper layers of the soil profile (^{Salman et al., 2023}; ^{Tavali, 2020}; Villalba Algarin, Ramírez Paniagua, Sanabria Franco, and da Silva, ^2024b). These benefits may have promoted more efficient root growth in treatments with increasing doses of organic fertilizer.

Although increases in root length and diameter were not significantly reflected in root weight and yield, both variables showed a positive linear response with increasing fertilizer doses (Figure 4). Regression analysis showed that, even at the highest doses, yield and root weight continued to increase proportionally (Figure 4b, d), confirming that the applied doses did not saturate the crop's response potential. This suggests that the crop could continue to respond positively to higher applications, possibly because critical thresholds for maximizing productivity have not yet been reached. This trend is consistent with the gradual nutrient release characteristic of organic fertilizers, which may require higher applications to sustain significant increases in biomass and yield, especially in crops with relatively short cycles (^{Bonanomi, Lorito, Vinale and Woo, 2018}).

When analyzing root weight, it was observed that the lowest weight was recorded in F0 (46.6 g), while F4 reached the highest value with 59.3 g (Figure 4a). When interpreting the regression equation, a linear response is observed where for each ton of organic fertilizer applied there is an increase of 1.386 g in root weight (Figure 4b). Although this does not represent a statistically significant difference between means, it could suggest that the applied amounts of organic fertilizer have not been sufficient to completely cover the nutritional demands of the crop and fully activate its productive capacity.

Likewise, when analyzing root yield, it was observed that the lowest value was obtained in the control treatment (F0 = 2.23 kg m⁻²) and reached its maximum point in F4 (2.85 kg m⁻²) (Figure 4c). This linear response was supported by regression analysis, which showed that for each ton of fertilizer applied, the increase in yield was 0.066 kg per square meter (y = 2.366 + 0.066x; R² = 0.79; Figure 4d).

These results partially contrast with those reported in previous studies that found statistically significant increases in root yield with the use of organic fertilizers. For example, ^{Hailu et al. (2024}) and ^{Zerga Heterat and Tsegaye (2019}) documented substantial improvements in these variables with the highest applied doses. Although a gradual increase was observed here with the studied doses, the degraded soil conditions influenced the agronomic responses of the crop.

Figure 4 Root weight (a-b) and yield (c-d) of carrot under different doses of composted organic fertilizer made from sugar industry waste. F0 (control, without fertilization), F1 (2 t ha⁻¹), F2 (4 t ha⁻¹), F3 (6 t ha⁻¹), and F4 (8 t ha⁻¹). Vertical lines above bars indicate standard error of means (n = 4). NS: no significant difference between treatments by Duncan's test (p ≤ 0.05). 

The use of organic fertilizers in vegetable production has shown mixed results. Some studies reported positive effects on crop productivity and soil quality, while others found limited effects. For example, ^{Núñez Sosa, Liriano González, and López Ceballos (2005}) observed that the efficiency of organic amendments can increase when soil organic matter levels are lower. Similarly, ^{Juárez-Rodríguez, Hidalgo-Moreno, Hernández-López, Padilla Cuevas and Etchevers (2024}) found that organic fertilizers improved or maintained soil fertility and increased commercial productivity of vegetables, although these effects appear to depend on initial nutrient availability and specific soil characteristics.

The original soil conditions where this study was developed are challenging. Like all highly weathered soils in the region, they present fertility limitations (Table 1), a characteristic normally observed on Paraguayan farms, especially in family farming systems that frequently face limited resources, inputs and implements, in addition to scarce knowledge about good agricultural practices (^{Derpsch, Lange, Birbaumer and Moriya, 2016}). This represents a significant challenge for restoring soil health and closing production gaps.

In this context, the results of this study revealed promising findings: organic fertilizers derived from solid waste from the sugar industry showed clear potential to improve carrot production, supported by the positive linear response in all evaluated variables with increasing applied doses. This trend suggests that, with more adequate management strategies, these fertilizers can offer substantial economic and environmental benefits for producers and the country.

Based on these results, it is suggested that future research evaluate crop response to higher doses, especially considering soil nutrient availability and specific plant requirements. This is fundamental to ensure that applications are precise and effective, maximizing fertilizer benefits. Additionally, combinations with rapid-release sources or the use of additives that accelerate compost mineralization could be explored, promoting more efficient nutrient uptake and higher yields. These approaches could optimize the use of organic fertilizers derived from solid waste and strengthen the sustainability of agricultural systems.