Fiber Quality of Colored Cotton As a Function of Potassium Fertilization in the Semi-Arid Region

ABSTRACT

Naturally colored fiber cotton is an essential alternative green and organic for the textile industry, as it does not require chemical dyeing. However, the low availability of potassium during production can directly affect the quality of the fiber. Based on this, this study aimed to evaluate the fiber quality in naturally colored cotton cultivars submitted to potassium fertilization in a semi-arid region. The experimental design was in randomized blocks, with four replications, and the treatments were arranged in split plots. In the plot, the following were allocated: five K rates (0, 60, 120, 180, and 240 kg ha⁻¹ of K₂O); and in the subplots, the four colored cotton cultivars (BRS Rubi, BRS Safira, BRS Topázio, and BRS Verde). The results showed that K is involved in the length, uniformity, short fiber index, micronaire index, and reliability index of naturally colored cotton fibers. A longer fiber length of 30.45 mm was obtained at a dose of 240 kg ha⁻¹ of K₂O for the cultivar BRS Topázio. In addition, a higher uniformity (84.95%) was achieved with the dose of 240 kg ha⁻¹ of K₂O for BRS Verde. The dose of 240 kg ha⁻¹ of K₂O reduced (8.62% and 11.28%) the short fiber index in the cultivars BRS Rubi and BRS Safira, respectively.

摘要

天然彩色纤维棉是纺织业必不可少的绿色和有机替代品，因为它不需要化学染色. 然而，生产过程中钾的低可用性会直接影响纤维的质量. 基于此，本研究旨在评估半干旱地区施用钾肥的天然彩色棉花品种的纤维质量. 实验设计是随机分组的，有四个重复，治疗安排在分裂区. 在该地块中，分配了以下内容: 五种钾含量（0、60、120、180和240 kg ha-1的K2O; 在子地块中，四个彩色棉花品种（BRS Rubi、BRS Safira、BRS Topázio和BRS Verde）. 结果表明，K与天然彩色棉纤维的长度、均匀性、短纤维指数、马克隆指数和可靠性指数有关. 品种BRS-Topázio在240 kg ha-1的K2O剂量下获得了30.45 mm的较长纤维长度. 此外，BRS Verde在240 kg ha-1的K2O剂量下获得了更高的均匀性（84.95%). 240 kg ha-1的K2O剂量使品种BRS Rubi和BRS Safira的短纤维指数分别降低（8.62%和11.28%).

KEYWORDS:

• Gossypium hirsutum
• potassium
• micronaire
• reliability

关键词:

• 多毛棉
• 钾
• 马克隆
• 可靠性

Introduction

Cotton (Gossypium hirsutum L.) is a fiber of great importance globally, with multiple applications (Soares et al. 2021). However, dyeing the white fiber can consequently cause harm to the environment and even human health (Chen et al. 2021). Given this, the production of naturally colored cultivars should be enhanced yearly in the search for materials that dispense with the dyeing stage.

Although colored cotton has potential, this segment is also subject to changes in the fiber’s quality standard. These changes can occur due to the management adopted from production in the field. According to Santos et al. (2023), the fiber quality of colored cotton can vary significantly depending on management, mainly due to the use of fertilizers.

In addition, the cultivation of colored cotton in semi-arid regions of the Brazilian Northeast is still a challenge for producers, as it occurs at a low technological level. These areas have unfavorable climatic conditions and may show limitations regarding the availability of essential nutrients to the plant. However, developing appropriate agricultural technologies and practices can increase productivity and fiber quality (Araújo et al. 2022).

Cotton is a crop that has a high requirement for potassium (K) (Tariq et al. 2018), which is the most abundant cation (K⁺) in cotton fibers (Almeida, Oliveira, and Saibo 2017). One study showed that K is the most critical osmolyte in fibrous cells and may be associated with others, such as malate and soluble sugars. Thus, increasing these osmolytes can maximize cell turgor, promote elongation, and increase fiber length (Yu et al. 2022).

K plays a critical role in fiber elongation because fiber cells require it abundantly to maintain turgid pressure (Guo et al. 2017). Therefore, low K⁺ content induces premature senescence in cotton and causes defects in fiber properties such as reduced length and strength (Yang et al. 2016). Implementing K via fertilizer should be recommended to meet the crop’s needs and ensure the fiber’s quality.

In a study evaluating the effects of K doses (range from 0 to 300 kg ha⁻¹ of K₂O) on cotton produced under induced drought, the researchers found greater fiber length due to the greater availability of K (Zhao et al. 2019). The same authors justified the result by emphasizing that this cation is associated with osmolytic dynamics during fiber development. Longer and more resistant fibers are among the standards required by the textile industry.

Based on this information, the variables that constitute the fiber’s quality may depend on the availability of K. However, because it is a natural fiber, it may have very varied physical characteristics among cultivars. Thus, the present study investigates the influence of potassium fertilization on the fiber quality of naturally colored cotton cultivars in the semi-arid region.

Materials and methods

The research was carried out at the Rafael Fernandes Experimental Farm (latitude 5°03’31.00” S, longitude 37°23’47.57” W and 80 m altitude), located in Mossoró, RN, Brazil. The climate of this region is characterized as BSh, tropical semi-arid hot, with an average temperature of 27.4°C and annual rainfall of 673.9 mm (Alvares et al. 2013). The region’s soil, in turn, is classified as Typical Dystrophic Red Ultisol (Rêgo et al. 2016).

Two experiments were conducted, corresponding to 2019 and 2021 crops, developed in the second half of each year. The meteorological data (Figure 1) were monitored throughout the experimental conduction through the Automatic Meteorological Station installed at the Experimental Farm.

Figure 1. Rainfall, solar radiation, air temperature and relative humidity during the 2019 (a) and 2021 (b) crops mossoró, RN, Brazil.

Prior to the installation of the experiments, soil samples were collected at a depth of 0–0.20 m for chemical and physical characterization of the experimental areas, whose results were: pH (H₂O): 7.50 and 5.60; P: 8.00 and 4.20 mg dm⁻³ (extractor: Mehlich⁻¹); K⁺: 0.10 and 0.11 cmol_c dm⁻³; Na⁺: 0.04 and 0.01 cmol_c dm⁻³; Al³⁺: 0.00 and 0.00 cmol_c dm⁻³; Ca²⁺: 1.3 and 0.78 cmol_c dm⁻³; Mg²⁺: 0.2 and 0.3 cmol_c dm⁻³, in the first and second crops, respectively. The physical attributes were the same for both crops: sand: 0.9 kg kg⁻¹; silt: 0.03 kg kg⁻¹; and clay: 0.07 kg kg⁻¹. Soil preparation was then carried out conventionally, with plowing and harrowing.

The recommended fertilization for irrigated cotton was carried out in both crops as a function of soil analysis. Nitrogen (N) was supplied by urea (46% of N) and monoammonium phosphate (MAP −12% of N) fertilizers, while phosphorus (P) was provided only with the use of MAP (61% of P₂O₅). Thus, 90 kg ha⁻¹ of N and 80 kg ha⁻¹ of P₂O₅ were applied (Gomes and Coutinho 2008). In addition, a micronutrient compound, corresponding to 1 kg ha⁻¹ of the commercial product Rexolin® BRA, was supplied and applied at the emergence of flower buds (Pedroso Neto et al. 1999). The composition of the compound corresponded to 2.10% of B, 0.36% of Cu, 2.66% of Fe, 2.48% of Mn, 0.036% of Mo, and 3.38% of Zn.

Potassium (K) was made available as potassium chloride (KCl − 61% K₂O). The distribution of all fertilizers was carried out via fertigation with the aid of diversion tanks. P was applied only to foundations, but N and K were provided in installments, at planting (50%) and two other applications (25% each), with 20-day intervals from one to the other (Gomes and Coutinho 2008).

The experiment was arranged in a randomized block design, in a split-plot scheme, with four replications. Five potassium rates (0, 60, 120, 180, 240 kg ha⁻¹ of K₂O) were fixed in the plots, and four colored cotton cultivars (BRS Rubi, BRS Safira, BRS Topázio and BRS Verde) were fixed in the subplots. The rates were determined considering the range of cotton responsiveness found in the literature (ranging from 0 to 300 kg ha⁻¹). The interval was defined according to the reference dose (60 kg ha⁻¹ of K₂O), considering the potassium contents contained in the soil. Regarding the cultivars, the most cultivated by regional producers were chosen.

The cultivars used in this research were developed by the National Center for Cotton Research (CNPA), which belongs to the Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA). They are cultivars with an annual cycle of around 120 to 140 days and have herbaceous size, with an average height of 1.20 m. The colors of the fibers are dark brown (BRS Rubi and BRS Safira), very light brown (BRS Topázio), and green (BRS Verde). In tests, in the semi-arid region, the average data obtained for length and breaking strength, respectively, were the following: 25.4 and 24.5 gf tex⁻¹ for BRS Rubi; 24.0 and 24.2 gf tex⁻¹ for BRS Safira; 30.4 and 31.9 gf tex⁻¹ for BRS Topázio; and 29.56 and 25.86 gf tex⁻¹ for BRS Verde (Carvalho, Andrade, and Silva Filho 2011). The material was obtained directly from Embrapa and multiplied to obtain seeds and carry out the experiments.

Each subplot consisted of four rows of plants, with 19 plants per row. For research purposes, only the two central lines were determined as functional areas, disregarding plants at the ends (border), so 34 plants were determined in the valuable area of the subplot. The planting spacing was 0.20 m between plants and 0.70 m between rows, and the total area of the subplot was 10.64 m² (3.8 m x 2.8 m). The area of each plot corresponded to 42.56 m² (15.2 m x 2.8 m). Thus, the total area in each experiment was 851.2 m², and the population density was 71,428 plants ha⁻¹ (Santos et al. 2023).

Sowing was done manually, with three seeds per hole, at a depth of approximately 5 cm. Thinning was carried out when the plants emitted three definitive leaves, leaving only one plant per hole. Three manual weedings were performed during the crop cycle at 20-day intervals in both crops for weed control.

The irrigation system installed was localized to the drip type, with emitters spaced 0.20 m apart and a flow rate of 1.6 L h⁻¹. Irrigation was carried out daily with an average daily depth of 6.39 mm in the first crop and 6.47 mm in the second crop. The blade was applied based on the crop’s daily evapotranspiration, using the crop coefficient (Kc) (Allen et al. 1998). The reference Kc’s were 1.10 and 1.15 in the 2019 and 2021 crops, respectively. Irrigation was suspended at 115 and 110 days after sowing (DAS) for the first and second crops, respectively.

The harvest was carried out manually in all plants of the useful area, starting at 106 (2019) and 102 (2021) DAS. All the bolls from the lower third of the plant were harvested in the first harvest. The other harvests were carried out weekly, according to the opening of the other bolls. The crop cycle lasted 133 days in the first crop and 123 days in the second crop.

To evaluate the fiber quality variables, samples of 50 grams of lint from each subplot were used. The sample formation was carried out after harvesting all the bolls collected in the useful area of each subplot (32 plants). Separately, in each subplot, the collected plume was homogenized (mixed), and 50 g of the homogenized material was weighed. Subsequently, the samples were in paper bags and sent to the Embrapa laboratory. The analysis was performed using the High Volume Instrument (HVI), model 1000 from USTER, at the Embrapa laboratory in Campina Grande, PB, Brazil. The variables evaluated were: length (UHM) (mm), fiber uniformity (UNF) (%), short fiber index (SFI), strength at rupture (STR) (gf/tex), elongation at break (ELG) (%), micronaire index (MIC), maturity (MAT) and count strength product (CSP) index. The CSP was derived using a multiple regression calculation, enabling us to draw conclusions regarding the anticipated maximum resistance.

Statistical analysis

The data were submitted for analysis of variance, and each crop was evaluated separately. When the homogeneity of the variances between the crops was observed, the joint analysis was applied. Obeying the principle that the quantities of the mean squares of the residue (QMRes) were homogeneous, i.e., the ratio between the highest and the lowest QMRes were not higher than seven times. Thus, interactions between treatments were evaluated. When significant, the means of the treatments were compared by Tukey’s test at 5% probability and regression, using the SISVAR program (Ferreira 2019). In addition, a Pearson linear correlation analysis was performed to investigate the relationship between variables, using the PAST software, version 4.03. The graphs were prepared using Excel version 2209 (Microsoft®).

Results and discussion

Homogeneity of variances between crops was observed for all variables, so the data were submitted to joint analysis. In the 2019 crop (Figure 2a), the highest values for UHM (29.81 and 26.80 mm) were obtained with the cultivars BRS Topázio and BRS Verde, due to the estimated doses of 112 and 94 kg ha⁻¹ of K₂O, respectively. On the other hand, with BRS Safira, the maximum (21.11 mm) was obtained in the absence of fertilization. For the cultivar BRS Rubi, no variation was observed as a function of the doses, obtaining a mean value of 21.30 mm.

Figure 2. Fiber length (UHM) of colored cotton cultivars under potassium doses, in two cultivation 2019 (a) and 2021 (b) crops.

In the 2021 crop (Figure 2b), the maximum value obtained (30.45 mm) was reached by BRS Topázio with a dose of 240 kg ha⁻¹ of K₂O. Subsequently, 27.84 mm was obtained with BRS Verde due to the same dose. BRS Safira reached its maximum of 22.67 mm with an estimated dose of 156 kg ha⁻¹ of K₂O. Finally, BRS Rubi obtained a maximum value of 22.34 mm without fertilization. From this, it is suggested that for this cultivar, specifically for the UHM variable, the K present in the soil was sufficient. Furthermore, this may be an indication that the cultivar shows a certain tolerance to soils with lower K levels. However, for BRS Rubi, higher doses kept the values of the variable close to the average value found in the 2019 crop. Thus, the cultivars showed a demand for a higher amount of potassium fertilizer and obtained higher UHM values in the second crop.

According to Santos et al. (2023), the UHM considered ideal for the textile industry is one greater than 29.5 mm. Based on this narrative, only the cultivar BRS Topázio would surpass this value under the conditions of the study. However, the values obtained by the same authors for this variable were lower than those observed in the present study. Mainly for the cultivars BRS Topázio and BRS Verde, inferring the importance of K to maximize fiber length.

The length of the fibers is of great importance in evaluating characteristics that will determine the quality of the yarn, as well as its final properties, as a fabric. Thus, the longer the effective length of the cotton fiber, the better its commercial classification will be. According to Guo et al. (2017), K is critical in fiber elongation because elongating fiber cells require abundant potassium to maintain turgidity pressure.

For the UNF variable, it was verified in the 2019 crop (Figure 3a) that the cultivars BRS Verde and BRS Safira obtained higher values (83.22 and 81.29%) without potassium fertilization. The other cultivars did not show variation as a function of the doses, obtaining average values of 85.04% (BRS Topázio) and 81.52% (BRS Rubi). In the 2021 crop (Figure 3b), only the BRS Verde cultivar responded to fertilization, with its maximum obtained of 84.95% due to the dose of 240 kg ha⁻¹ of K₂O. More excellent fiber uniformity allows for less fiber waste during processing, so it is a desirable feature for the industry.

Figure 3. Fiber uniformity (UNF) of colored cotton cultivars under potassium doses, in two cultivation 2019 (a) and 2021 (b) crops.

The UNF was influenced by K only for the BRS Verde cultivar, with an increase of 1.73% in the 2021 crop compared to 2019. The values obtained by this cultivar were higher than those that Santos et al. (2023) observed in the same region. The demand for K may, therefore, differ between crops. Varying non-controllable conditions, such as weather, should be an essential factor. According to Zhang et al. (2023), climatic conditions accounted for more than two-thirds of the variation in fiber quality. Specifically, among the various climatic factors considered, the temperature and solar radiation indices were the most influenced. Additionally, one study showed that temperature considerably affected cotton fiber quality, according to data from a decade ago (Li et al. 2021).

Given this, it is essential to carry out annual evaluations of fiber quality according to the edaphoclimatic variations of a given region, especially in the semi-arid region. Thus, it will be possible to establish what implications these variations may cause. Darawsheh et al. (2022) reported that the effect of the year (season) on fiber quality was two to six times greater than that of the environment. As a result, the year effect was identified as an essential source of variation in all-cotton quality traits. The same authors pointed out that there was an interaction between the environment and the year. This explains the different results between the crops, which occurred in different years, regardless of whether the management was similar.

Regarding SFI, it was observed that in the cultivation of the 2019 crop (Figure 4a), the highest index (12.81) was obtained with BRS Rubi in the absence of potassium fertilization. BRS Safira reached a maximum of 12.36 with an estimated dose of 162 kg ha⁻¹ of K₂O, decreasing from there. The same occurred with BRS Verde, which obtained a maximum of 10.50 with an estimated dose of 137 kg ha⁻¹ of K₂O. The cultivar BRS Topázio did not show variation as a function of the doses, with a mean value of 7.78. In the 2021 crop (Figure 4b), BRS Rubi again showed the highest SFI (13.60), but this time with a dose of 240 kg ha⁻¹ of K₂O. In sequence, BRS Safira with 12.16 was assigned to an estimated dose of 80 kg ha⁻¹ of K₂O. BRS Verde and BRS Topázio obtained the average values corresponding to 9.99 and 7.54, showing no variation as a function of K doses. Among the cultivars, BRS Rubi had the highest SFI in both crops, while BRS Topázio had the lowest SFI. This corroborates the results of Soares et al. (2018), who observed a lower rate of short fiber in the BRS Topázio cotton cultivar, regardless of management strategies. This determines that these are characteristics of the cultivars themselves.

Figure 4. Short fiber index (SFI) of colored cotton cultivars under potassium doses, in two cultivation 2019 (a) and 2021 (b) crops.

The dose of 240 kg ha⁻¹ of K₂O in the 2019 crop caused a reduction of 8.62% of the SFI in the BRS Rubi cultivar compared to the dose of 0 kg ha⁻¹ of K₂O (Figure 4a). A similar result was observed in the 2021 crop for BRS Safira, with a reduction in SFI by 11.28% with the dose of 240 kg ha⁻¹ of K₂O about the dose of 0 kg ha⁻¹ of K₂O (Figure 4b). Thus, higher doses of K sometimes reduce the short fiber index. Oscillations in the response of the cultivars due to the crops may have been motivated by edaphoclimatic variations and chemical attributes of the soil. However, these observations, as they have yet to be investigated in depth in this study, deserve further evidence.

A constant consideration of the content of short fibers is related to reducing the quality and dynamometric properties of cotton yarns. However, there is no validation that this is the only one responsible; the reduction in quality may be related to many other factors. Therefore, studying the correlation between the variables is essential for a more precise analysis. In addition, to ensure higher fiber quality, there must also be a positive relationship with factors extrinsic to the fiber, such as wiring adjustments.

Regarding the MIC variable, it was seen that in the 2019 crop (Figure 5a), only the BRS Rubi cultivar responded to K doses, obtaining a maximum value of 4.02 due to the estimated dose of 162 kg ha⁻¹ of K₂O. For the 2021 crop (Figure 5b), the cultivars BRS Safira and BRS Rubi had the highest indices (5.03 and 4.21, respectively) at the dose of 0 kg ha⁻¹ of K₂O. BRS Topázio showed increases up to the dose of 56 kg ha⁻¹ of K₂O, with a maximum of 4.55, and decreases from this. For BRS Verde, no variations were observed in dose functions, obtaining a mean value of 2.66.

Figure 5. Micronaire index (MIC) of colored cotton cultivars under potassium doses, in two cultivation 2019 (a) and 2021 (b) crops.

The MIC is an index through which a fibrous mass’s behavior and air resistance, without unity, are verified (Mitra and Majumdar 2023). When it is less than 3.0, the interpretation is that it is very fine; between 4.0 and 4.9, regular, and from 5.0 to 5.9, it is considered thick. Usually, a low MIC causes irregularity in the thread and tissue, called “neeps.” A micronaire index between 3.5 and 4.2 allows the fibers to be spun by high-speed rotors in modern spinning equipment (Araújo et al. 2022). Galdi et al. (2022) determine that the micronaire premium is within the 4.2–4.9 range. The results for MIC obtained in our study are primarily within or close to this range, except for BRS Verde, an inherent characteristic of the cultivar.

For STR, only differences were observed between the crops with the cultivars BRS Topázio and BRS Safira, and these obtained higher averages in the 2021 crop (Table 1). Evaluating the differences between cultivars, in the 2019 crop, BRS Topázio was the one with the highest average, statistically diverging from the other cultivars studied. In the 2021 crop, BRS Topázio again stands out. However, it does not differ statistically from BRS Safira; both obtained higher averages than the other cultivars. Thus, in the 2019 crop, all cultivars obtained lower SRT compared to the reference values established by Carvalho et al. (2011), except BRS Safira. According to Gonçalo-Filho et al. (2023), STR values higher than 30 gf tex⁻¹ are classified as very resistant to rupture. Given this statement, the cultivars BRS Topázio and BRS Safira were classified as resistant to rupture in the 2021 crop.

Table 1. Average values for strength at rupture (STR), elongation at rupture (ELG), and maturity (MAT), in colored cotton cultivars, in two cultivation seasons.

STR (gf/tex)
Seasons	Cultivar
	BRS Rubi	BRS Safira	BRS Verde	BRS Topázio
2019	23.74aC	27.13bB	22.90aC	29.63 bA
2021	24.15aB	30.25aA	23.53aB	30.89aA
ELG (%)
Seasons	Cultivars
	BRS Rubi	BRS Safira	BRS Verde	BRS Topázio
2019	7.13 bA	7.05aA	6.22aB	5.90aB
2021	7.60aA	7.31aA	5.76aB	5.56aB
MAT
Seasons	Cultivars
	BRS Rubi	BRS Safira	BRS Verde	BRS Topázio
2019	0.844aC	0.861bB	0.819bD	0.869 bA
2021	0.845aC	0.866aB	0.824aD	0.871aA

^aMeans followed by different lowercase letters in the column and by different capital letters in the row differ from each other, at a 5% probability level, by the Tukey test.

When ELG was evaluated, no difference was found between the crops for the cultivars, except the BRS Rubi cultivar (Table 1). In the 2019 crop, BRS Rubi and BRS Safira were statistically similar to each other but differed from the other cultivars. The same behavior was observed in the 2021 crop. A similar response was found by Santos et al. (2023), proposing that for this variable, the cultivars BRS Rubi and BRS Safira are similar.

Regarding MAT, there were differences between the crops for the cultivars, except BRS Rubi (Table 1). In the 2021 crop, the highest averages were observed for the cultivars BRS Topázio, BRS Safira, and BRS Verde. Evaluating the differences between cultivars, BRS Topázio had the highest average, differing statistically from the other cultivars in the two crops studied. Araújo et al. (2013) stated that the MAT is influenced by the deposition of cellulose layers in the fiber and may vary depending on climatic conditions, harvest time, and pest attacks. This statement may explain the difference between crops, considering variations in weather conditions and harvest times from one crop to another.

In the CSP variable (Figure 6), an interaction was observed between seasons and K doses. In the 2019 crop, the most significant increase was 2200.80 without potassium fertilization. In the 2021 crop, there was an increase until the estimated dose of 168 kg ha⁻¹ of K₂O, where the maximum of 2354.33 was reached, with decreases from this. The CSP index is related to the strength of the yarns, essentially depending on the individual tenacity of the fibers (Veloso et al. 2023). In the case of this study, although K doses influenced the seasons, the values did not exceed those established as standard by the textile industry, with the range being accepted from 2000 to 2500 (Bachelier and Gourlot 2018).

Figure 6. Count strength product (CSP) index of the fiber of colored cotton under potassium doses, in two cultivation 2019 and 2021 crops.

The UHM strongly correlated with the UNF and CSP variables (Table 2). The UNF correlated very strongly with the CSP in both crops and moderately with the SRT in 2019. SFI was very strongly correlated with ELG, just like the MIC with the MAT. MIC also showed a strong correlation with SRT. On the other hand, SRT showed a moderate to strong correlation with MAT between crops and a weak correlation between SRT and CSP, which was identified only in the 2019 crop.

Table 2. Pearson correlation of fiber quality variables for length (UHM), uniformity (UNF), short fiber index (SFI), strength at rupture (STR), elongation at rupture (ELG), micronaire index (MIC), maturity (MAT) and count strength product (CSP) index of naturally cotton colored in two cultivation seasons.

2019
	UHM	UNF	SFI	MIC	STR	ELG	MAT	CSP
UHM	1.00
UNF	0,92**	1.00
SFI	−0,93**	−0,88**	1.00
MIC	−0,20^ns	0,02^ns	0,02^ns	1.00
STR	0,37^ns	0,48*	−0,52*	0,76**	1.00
ELG	−0,91**	−0,82**	0,93**	0,21^ns	−0,32^ns	1.00
MAT	0,02^ns	0,22^ns	−0,20^ns	0,95**	0,89**	−0,004^ns	1.00
CSP	0,98**	0,92**	−0,96**	−0,12^ns	0,47*	−0,92**	0,12^ns	1.00
2021
	UHM	UNF	SFI	MIC	STR	ELG	MAT	CSP
UHM	1.00
UNF	0.96**	1.00
SFI	−0.94**	−0.91**	1.00
MIC	−0.26^ns	−0.16^ns	0.02^ns	1.00
STR	0.26^ns	0.35^ns	−0.45*	0.80**	1.00
ELG	−0.93**	−0.84**	0.88**	0.40^ns	−0.11^ns	1.00
MAT	0.09^ns	0.15^ns	−0.31^ns	0.93**	0.90**	0.06^ns	1.00
CSP	0.98**	0.98**	−0.94**	−0.17^ns	0.38^ns	−0.89**	0.16^ns	1.00

ns; *; ** - Not significant, significant at p ≤ .05 and 0.01, respectively. 0 a 0.19 – Very weak; 0.20 to 0.39 – Weak; 0.40 to 0.69 – Moderate; 0.70 to 0.89 – Strong; 0.90 to 1.00 – Very strong.

According to the study by Nawaz et al. (2019), there was a positive relationship between fiber length and strength, where an increase in fiber length corresponded to an increase in fiber strength. However, our study showed that length is positively correlated, not with strength, but with uniformity and reliability of cotton fibers.

Potassium fertilization at 240 kg ha⁻¹ of K₂O increased UHM and UNF by up to 6.30%, specifically for the cultivars BRS Topázio and BRS Verde. The dose of 240 kg ha⁻¹ of K₂O tended to reduce the SFI of BRS Rubi and BRS Safira. The estimated dose of 162 kg ha⁻¹ of K₂O showed increments to the MIC of BRS Rubi. A close dose (168 kg ha⁻¹ of K₂O) exhibited a higher CSP. In addition, the response of colored cotton cultivars to potassium fertilization showed variation among crops. These results indicate that K should be further explored, testing other doses of fertilizer, other sources, different times, and types of soils.

Among the cultivars, BRS Topázio had the highest SRT and MAT; conversely, BRS Rubi and BRS Safira obtained the highest ELG. Cultivars did not influence CSP. Based on these results, the producer must evaluate the market and determine which variables and cultivars are essential to meet it.

Thus, the present research discussed the appropriate level of K, the most influenced variables, the existing correlation, and the potential colored cotton cultivars for the semi-arid region. In this way, we provide a theoretical basis for managing potassium fertilization for the quality of colored cotton fiber. The quality of the fiber has a significant influence on the commercialization and industrialization of cotton.

Conclusions

Potassium is involved in the length, uniformity, short fiber index, micronaire index and reliability index of naturally colored cotton fibers. A longer fiber length was obtained with 240 kg ha⁻¹ of K₂O in the cultivation of BRS Topázio. Better uniformity was achieved with the potassium fertilization of 240 kg ha⁻¹ of K₂O for BRS Verde. BRS Safira showed a reduction in the short fiber index at the dose of 240 kg ha⁻¹ of K₂O.

Highlights

• Potassium fertilization promotes better fiber quality in colored cotton grown in the semi-arid region.

• The BRS Topázio cultivar has high fiber quality and, regardless of the potassium dose, is recommended for cultivation in the semi-arid region.

• There is a very strong correlation between fiber length and the variables uniformity and count strength product index.

Acknowledgments

We to thank for their financial support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) - Financing Code 001. We also thank the Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA) for all the support in carrying out the analyzes and the Universidade Federal Rural do Semi-Árido (UFERSA) for the space and support for the research.

Disclosure statement

No potential conflict of interest was reported by the author(s).