Yield and Quality of Naturally Colored Cotton Fiber Fertilized with Phosphorus Rates in the Brazilian Semiarid

ABSTRACT

Cultivating naturally colored fiber cotton has excellent potential for the Northeast of Brazil. However, its cultivation in the semiarid occurs with low use of technology and inputs in soils with the low phosphorus content. This work evaluated the productivity and fiber quality of naturally colored cotton cultivars under phosphorus fertilization in the Brazilian semiarid region. The experimental design was randomized blocks, with treatments arranged in split plots and four replications. The five P doses were allocated in the main plot, and four cotton cultivars were in the subplots. Variance analyses were performed for the characteristics evaluated in each agricultural harvest. Tukey test was used to compare the averages for cultivars and agricultural harvests. Increased P availability increased productivity and fiber quality of naturally colored cotton cultivars. Maximum fiber yield was obtained at 240; 240; 199.11, and 60 kg ha⁻¹ in the first crop, respectively. The maximum fiber length was reached at 80.86 kg ha⁻¹ in the BRS Verde cultivar. The maximum uniformity was obtained with the 138.49 kg ha⁻¹, and the cultivar BRS Topázio obtained greater uniformity. Fiber strength increased with the application of P₂O₅.

摘要

在巴西东北部种植天然彩色纤维棉具有巨大的潜力. 然而，它在半干旱地区的种植是在低磷含量的土壤中低技术和低投入的情况下进行的. 本工作评估了巴西半干旱地区天然彩色棉花品种在施磷条件下的生产力和纤维质量. 实验设计是随机分组的，以分割区和四次重复的方式安排治疗. 五个磷剂量分配在主地块，四个棉花品种分配在次地块. 对每次农业收获中评估的特征进行方差分析. Tukey检验用于比较品种和农业收成的平均值. 磷有效性的提高提高了天然彩色棉花品种的生产力和纤维质量. 最大纤维产量为240; 240; 199.11和60 kg ha-1。BRS Verde品种的最大纤维长度达到80.86 kg ha-1. 138.49 kg ha-1获得了最大的均匀性，品种BRS Topázio获得了更大的均匀性. 纤维强度随着P2O5的应用而增加.

KEYWORDS:

• Gossypium hirsutum L.
• cultivar
• phosphate fertilization
• micronaire. resistance
• elongation

关键词:

• Gossypium hirsutum. L
• 栽培品种
• 磷肥
• 马克隆. 反对
• 伸长率

Introduction

Gossypium hirsutum, also known as arboreal cotton or upland cotton, is a species of cotton widely grown worldwide due to its high productivity and fiber quality (Cunha et al. 2019). Cotton fibers produced by the species are widely used in the textile industry to make clothing, fabrics and other textile products. In addition, it is an essential source of edible oil and animal feed. It is currently grown in several countries worldwide, including the United States, China, India, Brazil, and others. It is an essential culture for the global economy and is responsible for employing millions of people worldwide. In Brazil, its cultivation occurs mainly in the semiarid region of the Northeast, emphasizing family agriculture, both in conventional and organic management (Carvalho, Andrade, and Silva Filho 2011).

Cotton cultivation in semiarid regions is a challenge for producers since these areas present unfavorable climatic conditions, such as low air humidity, high temperatures and scarcity of rainfall. However, with the development of appropriate agricultural technologies and practices, it is possible to succeed in cotton cultivation in semiarid regions, increasing fiber productivity and quality (Araújo et al. 2022).

Several colored cotton cultivars can be grown under sweaty conditions, i.e. without irrigation. Among the cultivars, BRS Topázio has characteristics such as high productivity, good fiber quality and resistance to pests and diseases common in cotton crops. In addition, BRS Topázio has brown fibers, making it attractive for producing differentiated and value-added fabrics (Vidal Neto et al. 2010). In addition to color, colored cotton fibers have other characteristics than conventional white cotton fibers. Colored cotton fibers tend to be shorter and thinner, making them smoother and more comfortable. However, colored cotton fibers can also be more fragile and less resistant to factors such as abrasion and traction, affecting the durability of fabrics and products made with this fiber. However, it is essential to highlight that the quality of colored cotton fiber can vary significantly due to the management adopted, especially with the use of fertilizers in cultivars available in the market.

Phosphorus fertilization (P) is fundamental for the proper development of the cotton crop since this nutrient is essential for the formation and development of the plant’s roots, flowers and fruits (Lambers 2022). Phosphorus is also necessary for transferring energy within the plant and synthesizing proteins, lipids and other essential compounds for plant growth and development (Taiz et al. 2017).

As in conventional white cotton, phosphorus fertilization is also crucial for the quality of colored cotton fiber. Phosphorus is essential for forming the cellular structure of plants and being fundamental for synthesizing compounds such as DNA, RNA and proteins. The proper application of phosphorus can positively influence the quality of colored cotton fibers, resulting in more robust and more uniform fibers with more excellent resistance to breakage and greater absorption capacity of natural dyes (Malhotra, Sharma, and Pandey 2018). In addition, phosphorus fertilization can also increase crop productivity and producer profitability.

Thus, we hypothesize that (i) the greater availability of phosphorus increases the productivity and quality of the colored fiber of cotton cultivars in the semiarid region; and (ii) cotton cultivars respond differently to increased doses of phosphorus. Given the above, the present study aimed to evaluate the productivity and quality of fiber from naturally colored cotton cultivars submitted to phosphate fertilization, under irrigation, in the Brazilian Semiarid.

Materials and methods

The experiments were conducted in the municipality of Mossoró, Rio Grande do Norte, Brazil (5°03’29.27“S, 37° 23’49.44“W, altitude of 81 m), in the period from August to December 2017 (first agricultural harvest) and 2018 (second agricultural harvest). The climate region is of the BSh type (Alvares et al. 2013), characterized as dry and very hot, with two climatic seasons: a rainy one that covers the months from February to May and a dry one that runs from June to January, with an average temperature of 27.2°C and an average annual rainfall of 766 mm (Borges et al. 2015). The meteorological data for the period of the experiments are shown in Figure 1.

Figure 1. Average values of average, maximum and minimum air temperatures (ºC), global solar radiation (MJ m⁻² day⁻¹), relative humidity (%) and accumulated rainfall (mm) in the 2017 and 2018 agricultural seasons. Source: Rafael Fernandes experimental farm station – UFERSA.

The soil of the experimental areas was classified as Typical Red Dystrophic Argisol (Rêgo et al. 2016). The physical and chemical analyses of the soil before the installation of the experiments in the 2017 and 2018 harvests were: sand = 0.91 and 0.90 kg kg⁻¹; silt = 0.02 and 0.03 kg kg⁻¹; clay = 0.07 and 0.07 kg kg⁻¹; pH (H₂O) = 5.30 and 5.00; organic matter = 3.31 and 3.34 g kg⁻¹; P = 3.3 and 5.9 mg dm⁻³; K⁺ = 0.16 and 0.09 cmol_c dm⁻³; Na⁺ = 0.03 and 0.08 cmol_c dm⁻³; Ca²⁺ = 0.80 and 0.90 dm⁻³; Mg⁺² = 0.90 and 0.00 cmol_c dm⁻³; Al⁺³ = 0.05 and 0.25 cmol_c dm⁻³; cation exchange capacity = 3.04 and 5.03 cmol_c dm⁻³.

The experimental design used was randomized blocks with four replications. In the plots, five doses of P (0; 60; 120; 180 and 240 kg ha⁻¹ of P₂O₅) were used, and in the subplots, four cultivars of naturally colored cotton (BRS Rubi, BRS Safira, BRS Verde and BRS Topázio). The total area of the experiment was 896 m², each plot consisting of four rows of plants, totaling an area of 10.64 m² (3.8 m × 2.8 m). The spacing used was 0.70 m × 0.20 m, with one plant per hole, totaling 34 plants in the useful area of the experimental plot (4.76 m²) and a population density of 71,428 plants ha⁻¹.

All cultivars used were developed by the Genetic Improvement Program of the National Cotton Research Center (CNPA) of the Brazilian Agricultural Research Corporation (EMBRAPA) in Campina Grande-PB. The colors of the cotton fibers are green (BRS Verde), reddish brown (BRS Rubi and BRS Safira), and light brown (BRS Topázio) (Carvalho, Andrade, and Silva Filho 2011).

The cotton sowing in the first agricultural harvest was carried out on August 8, 2017, and the second on August 23, 2018. Direct sowing was carried out at three centimeters in depth, sowing three seeds per hole. Thinning occurred when the plants emitted three final leaves, leaving only one plant per hole.

The irrigation system was drip, with emitters spaced 0.20 m and a flow rate of 1.5 L per hour. The irrigations were carried out daily, based on the daily evapotranspiration of the culture, using the culture coefficient according to each stage of development (Allen et al. 1998). Fertilization was carried out according to the recommendation for irrigated cotton (Gomes and Coutinho 2008), except for phosphate fertilization, which followed the treatments. The source of P used was simple superphosphate (18% P₂O₅), applied all over the foundation (planting) manually in each harvest. In addition to P, 90 kg ha⁻¹ of nitrogen (N) was made available in the form of urea (45% N), 40 kg ha⁻¹ of K₂O in the 2017 harvest, and 60 kg ha⁻¹ of K₂O in 2018 (chloride potassium, 58% K₂O). N and K were applied at planting and cover (15 and 35 days after emergence) through fertigation (Gomes and Coutinho 2008). The micronutrients were supplied at a dose of 1 kg ha⁻¹ of Rexolin® BRA (2.10% B, 0.36% Cu, 2.66% Fe, 2.48% Mn, 0.036% Mo, and 3.38% Zn), in the appearance of flower buds, through irrigation water (Pedroso Neto et al. 1999). The distribution of fertilizers in the irrigation water using a bypass tank (“lung”). The cultural treatments and the phytosanitary control were carried out according to the technical recommendations and needs of the culture in the field (Beltrão and Azevedo 2008).

The cotton harvest started at 109 (first harvest) and 111 (second harvest) days after sowing. The harvest was manual, in three stages, the first being when the bolls of the lower third of the plant opened and twice more, according to the opening of the other bolls. After the final harvest, soil collections were carried out in each experimental plot, followed by chemical analyses for residual P in the plots (Silva 2009), shown in Table 1.

Table 1. Average values of residual P content in the soil of areas cultivated with naturally colored cotton submitted to phosphorus doses in agricultural harvests.

Rates of P2O5	Cultivars	Content of P (mg dm^−3)*
		1^st year	2^nd year
60 kg ha^−1	BRS Rubi	17.00	23.75
	BRS Safira	15.00	19.00
	BRS Topázio	18.75	25.75
	BRS Verde	18.00	23.00
120 kg ha^−1	BRS Rubi	25.00	26.75
	BRS Safira	34.75	28.25
	BRS Topázio	32.00	27.75
	BRS Verde	21.00	24.75
180 kg ha^−1	BRS Rubi	45.00	39.00
	BRS Safira	33.25	31.75
	BRS Topázio	35.75	39.00
	BRS Verde	30.25	34.25
240 kg ha^−1	BRS Rubi	51.00	56.00
	BRS Safira	32.50	37.00
	BRS Topázio	32.75	49.00
	BRS Verde	33.00	38.00

Variables analyzed were: cotton fiber yield (kg ha⁻¹), fiber percentage (%), fiber length (mm), length uniformity (%), short fiber index (%), strength fiber (gf tex⁻¹), fiber elongation (%), micronaire (µg in⁻¹), fiber maturity and count strength product (reliability index). The count strength product was obtained through a multiple regression calculation, making it possible to draw conclusions about the expected maximum resistance. Cotton fiber yield was determined by multiplying seed cotton yields by fiber percentage. The other characteristics were determined at the Fiber and Yarn Laboratory of Embrapa Cotton, Campina Grande, PB, with the HVI system (High Volume Instrument), model HVI 1000 from USTER®.

Statistical analysis

Variance analyses were performed for the characteristics evaluated in each agricultural harvest using the SISVAR 5.6 application (Ferreira 2011). Observing the homogeneity of variances between agricultural harvests, a joint analysis of these same characteristics was applied (Ferreira 2011). Adjusting response curves was performed using the Table Curve 2D program (Systat Software 2002). Tukey test (p < .05) was used to compare the averages for cultivars and agricultural harvests.

Results and discussion

The homogeneity of variances was accepted for all variables, thus enabling the joint analysis of the experiments. In the first harvest, the dose of 240 kg ha⁻¹ of P₂O₅ allowed maximum fiber yield in the cultivars BRS Safira and BRS Topázio, 888.35 and 1,470.38 kg ha⁻¹, respectively (Figure 2a). In the second harvest, the doses that provided higher fiber yields were 187.78 kg ha⁻¹ of P₂O₅ (595.30 kg ha⁻¹); 240 kg ha⁻¹ of P₂O₅ (667.07 kg ha⁻¹); 127.45 kg ha⁻¹ of P₂O₅ (1,067.36 kg ha⁻¹); 240 kg ha⁻¹ of P₂O₅ (488.45 kg ha⁻¹) in cultivars BRS Rubi, BRS Safira, BRS Topázio and BRS Verde, respectively (Figure 2b). Therefore, it was observed that the cultivar BRS Topázio obtained higher fiber yield regardless of doses and crops (Figure 2).

Figure 2. Cotton fiber yield as a function of P rates in the cultivars of naturally colored cotton in the first (a) and second (b) agricultural harvests.

The increase in fiber productivity of naturally colored cotton is due to the availability of P through the application of the nutrient in the soil (Table 1) because the concentration of P in the soil before the installation of the experiment is considered very low in both crop farms (Malavolta 2006). The increase in availability of P in the soil increases with each increment of the nutrient added; however, this increase is not proportional to the amount added to the soil, which was confirmed in the present study (Table 1). The distinct responses between crop yields in fiber yield in naturally colored cotton cultivars may have occurred due to the chemical reaction of P sorption in the soil, not suppressing the ideal amount for the plant (Gou et al. 2020).

The differences in fiber yield between cultivars are related to the genetic potential of each cultivar as a function of phosphate fertilization. This fact is proven by other studies, in which phosphate fertilization had an effect on increasing fiber productivity (Ahmad et al. 2009; Ali and Ahmada 2020; Batista et al. 2010) and others did not have this effect (Aquino et al. 2011; Santos et al. 2012). Thus, it is evident that factors associated with genotypes and cultivation conditions (soil, irrigated or dry land and meteorological variables) can interfere with the cotton productivity response to phosphate fertilization.

The cultivar BRS Safira reached the maximum percentage of fiber at the dose of 103.18 kg ha⁻¹ of P₂O₅ (37.72%) (Figure 3a). In the absence of fertilization (38.92%) in the second agricultural crop (Figure 3b). The cultivar BRS Topázio obtained the highest mean values of fiber percentage (46.02 and 45.82%) and the cultivar BRS Verde the lowest (27.69 and 28.94%) (Figure 3).

Figure 3. Fiber percentage as a function of phosphorus doses and naturally colored cotton cultivars in the first (a) and second (b) agricultural harvest.

The cultivar BRS Topázio, regardless of the p doses applied and the crop yields had a fiber percentage above 40%. A percentage of fiber above 40% is the minimum standard sought by the improvement to obtain a cotton cultivar with great potential for fiber production (Freire et al. 2008). The cultivar BRS Safira had an increase of 3.53% (120 kg ha⁻¹ of P₂O₅) in the percentage of fiber compared to the absence of P, while BRS Verde obtained a 3.58% increase (180 kg ha⁻¹ of P₂O₅), both in the first agricultural harvest.

Cotton fiber characteristics are influenced by genetic and environmental factors, management and harvesting conditions (Kazama et al. 2015). The temperature variation and the nutritional conditions are some of the factors that affect flowering and fruiting (fall), which consequently influences the occurrence of the capsule in the plant. The grasses of the upper and lower thirds of the plant have a lower percentage of fiber (Farias et al. 2008), which was confirmed in the present study with the differences in the percentage of fiber between the doses of P and cultivars.

P doses increased fiber lengths mainly in cultivars BRS Topázio and BRS Verde (Figure 4). The dose of 80.86 kg ha⁻¹ of P₂O₅ carried the maximum length of the fiber, being observed 26.60 mm in the cultivar BRS Verde, while in BRS Safira a fiber length of 22.00 mm was recorded at the dose of 34.68 kg ha⁻¹ of P (Figure 4). BRS Topázio had the most extended fiber lengths about the other cultivars evaluated (Figure 4).

Figure 4. Cotton fiber length as a function of P rates in naturally colored cotton cultivars.

The cultivar BRS Verde with a dose of 80.86 kg ha⁻¹ of P can be classified as a short fiber 26.60 mm (Figure 4). The fiber length considered ideal for serving the textile industry is above 29.5 mm (SL 2.5%) to be presented by a cotton cultivar. The cultivar BRS Topázio had the desirable pattern with increasing the dose of P. Cultivars had an increase in fiber length because of phosphate fertilization, corroborating the responses found by Gutstein (1970) and Ahmad et al. (2009), who observed an increase in fiber length with the application of P.

Fiber length differed among crops (Table 2), with the highest values obtained in the first harvest (24.76 mm) (Table 2). This was probably due to the characteristics of cotton fiber being influenced by genotype and environment interaction (temperature and precipitation) (Baxevanos, Tsialtas, and Goulas 2013; Kazama et al. 2015). The increase in P doses allowed more uniformity of fiber length, in which the dose of 138.49 kg ha⁻¹ of P₂O₅ provided the maximum estimated value of 83.36% (Figure 5). The cultivar BRS Topázio obtained more excellent uniformity in both crop farms (Table 3).

Figure 5. Length uniformity of naturally colored cotton fiber as a function of P rates.

Table 2. Average length values of naturally colored cotton fiber in agricultural harvests.

Agricultural harvests	Fiber length (mm)
1ª year	24.76a^1
2ª year	24.08b

¹Means followed by the same lowercase letter in the column, do not differ, by Tukey’s test at 5% probability.

Table 3. Average values of fiber length uniformity of naturally colored cotton cultivars in agricultural harvests.

Length uniformity (%)
Cultivars	1^st year	2^nd year
BRS Rubi	81.78cB^1	82.81 bA
BRS Safira	82.38bcA	82.57 bA
BRS Topázio	84.68aA	84.28aA
BRS Verde	83.12 bA	82.61 bA

¹Means followed by the same lowercase letter in the column and uppercase in the row do not differ, by Tukey test at 5% probability.

This effect of P is highly desirable because it decreases fiber waste during processing (Gutstein 1970). P application caused a 1.2% increase in fiber length without fertilization. The fiber length was classified as average (EMBRAPA – HVI) and high, according to Fonseca and Santana (2002). Substantial evidence that although the increase was slight, it allowed a better uniformity of the fiber close to the desired fiber length for the textile industry, above 84% (Freire et al. 2008). The cultivar BRS Topázio obtained more excellent uniformity in length than the other cultivars studied in both crop farms, obtaining the desirable standard for the textile industry (Freire et al. 2008).

The lowest index of short fibers was observed in the cultivar BRS Topázio (7.75%) (Table 4). This index is the proportion in the percentage of short fibers about the weight of fibers with lengths less than 12.5 mm. The cultivar BRS Topázio had a low index of short fiber, which showed its potential for using its fiber in the textile industry. In contrast, the other cultivars had fibers with this index classified as average (Fonseca and Santana 2002). Regarding the agricultural harvest, the lowest index of short fibers was observed in the first, with 9.45% (Table 5).

Table 4. Average values of short fiber index of naturally colored cotton cultivars.

Cultivars	Short fiber index (%)
BRS Rubi	11.15a^1
BRS Safira	10.74a
BRS Topázio	7.75b
BRS Verde	10.24a

¹Means followed by the same lowercase letter in the column do not differ, by Tukey test at 5% probability.

Table 5. Average values of short fiber index of naturally colored cotton in agricultural harvests.

Agricultural harvests	Short fiber index (%)
1^st year	9.45b
2^nd year	10.49a

¹Means followed by the same lowercase letter in the column do not differ, by Tukey test at 5% probability.

P doses affected fiber resistance, so that the cultivar BRS Rubi showed a decrease in P doses, with the highest values observed without the use of phosphate fertilization (Figure 6). The cultivar BRS Verde, at a dose of 240 kg ha⁻¹ of P₂O₅ reached the maximum resistance with 22.52 gf tex⁻¹ (Figure 6). The increase of P up to 180 kg ha⁻¹ of P₂O₅ caused an increase in fiber resistance in the cultivars BRS Safira and BRS Topázio (Figure 6). The maximum fiber strength was 26.68 gf tex⁻¹ (131.84 kg ha⁻¹ of P₂O₅) in the first agricultural harvest (Figure 7).

Figure 6. Resistance of naturally colored cotton fiber as a function of P rates in naturally colored cotton cultivars.

Figure 7. Resistance of naturally colored cotton fiber as a function of P rates in agricultural harvests.

The characteristics of cotton fiber, such as fiber resistance, are influenced by genetic and environmental factors (Baxevanos, Tsialtas, and Goulas 2013; Kazama et al. 2015). The edaphoclimatic factors probably affected the response of naturally colored cotton fiber when submitted to phosphate fertilization. The greater amplitude between maximum and minimum temperature in September in the second harvest and precipitation during the harvest period probably affected cotton fiber’s productive performance and quality.

The cultivars BRS Rubi and BRS Safira obtained the highest fiber elongation in both agricultural harvests (Table 6). The lowest fiber elongation observed was in the cultivar BRS Verde with 4.68%. %. Fibers with larger elongations improve the quality of textile products, resulting in better tensile properties, fewer defects in yarn, and less propensity for breakage (Mathangadeera et al. 2020).

Table 6. Average fiber elongation values of naturally colored cotton cultivars in agricultural harvests.

Fiber elongation (%)
Cultivares	1^st harvest	2^nd harvests
BRS Rubi	6.69aA^1	6.55aA
BRS Safira	6.47aA	6.39aA
BRS Topázio	5.44 bA	5.47 bA
BRS Verde	5.45 bA	4.68cB

¹Means followed by the same lowercase letter in the column and uppercase in the row do not differ, by Tukey’s test at 5% probability.

The increase in p availability through fertilization caused the decrease in micronaire in the first agricultural harvest (Figure 8). The absence of fertilization provided the maximum micronaire of 4.09 μg pol⁻¹ in the absence of phosphate fertilization in the first agricultural harvest (Figure 8). The micronaire in the first agricultural harvest decreases about 5% with the dose of 240 kg ha⁻¹ of P₂O₅. Regarding naturally colored cotton cultivars, BRS Safira obtained the highest micronaire while BRS Verde had the lowest, with 4.72 and 2.52 μg pol⁻¹, respectively (Table 7).

Figure 8. Micronaire of naturally colored cotton as a function of P rates in agricultural harvests.

Table 7. Average values of micronaire in the naturally colored cotton cultivars.

Cultivars	Micronaire (µg pol^−1)
BRS Rubi	4.33b^1
BRS Safira	4.72a
BRS Topázio	4.34b
BRS Verde	2.52c

¹Means followed by the same lowercase letter in the column do not differ, by Tukey’s test at 5% probability.

The desirable micronaire pattern for the textile industry is between 3.7 and 4.2 μg pol⁻¹ to be obtained by a cotton cultivar (Freire et al. 2008). Except for the cultivar BRS Verde, which obtained an excellent fiber, the other cultivars obtained values for micronaire close to ideal. Notably, the cultivars BRS Rubi, BRS Safira and BRS Topázio reached higher values than Carvalho, Andrade, and Silva Filho (2011) found. For fiber maturity, there was a difference between cultivars, in which BRS Safira and BRS Topázio stood out (Table 8). In the second crop, it was observed that the maturity of naturally colored cotton fibers was higher than in the first crop (Table 9). The values found in agricultural harvests are ideal for the textile industry (equal to or greater than 85%) (Freire et al. 2008).

Table 8. Average values of fiber maturity in naturally colored cotton cultivars.

Cultivars	Maturity
BRS Rubi	0.8576b^1
BRS Safira	0.8694a
BRS Topázio	0.8683a
BRS Verde	0.8239c

¹Means followed by the same lowercase letter in the column do not differ, by Tukey’s test at 5% probability.

Table 9. Average maturity values of naturally colored cotton fibers in agricultural harvests.

Agricultural harvests	Maturity
1^st year	0.8537b^1
2^nd year	0.8559a

¹Means followed by the same lowercase letter in the column do not differ, by Tukey’s test at 5% probability.

Phosphate fertilization affected the reliability index of the fibers of naturally colored cotton cultivars. The cultivars BRS Safira, BRS Topázio and BRS Verde reached the highest reliability indexes estimated at doses 37.14 (1,915.61); 140.64 (2,946.15) and 158.74 kg ha⁻¹ of P₂O₅ (2,458.93), respectively (Figure 9). The reliability indexes increased 19.8% and 13.35% in the cultivars BRS Topázio and BRS Verde, respectively. In general, the cultivar BRS Topázio obtained higher reliability indexes among cultivars. In the crop harvests, reliability indexes increased with the increase of phosphate fertilization up to the dose of 135.66 kg ha⁻¹ of P₂O₅ in the first agricultural harvest (Figure 10). The reliability index was 2,304.90, which increased by 10.32% about non-fertilization. In the second harvest, there was no adjustment of the equation, and the mean value of 2,114.80 (Figure 10) was observed. About the crop yields, only a difference was observed between the harvests only at the dose of 120 kg ha⁻¹ of P applied.

Figure 9. Fiber count strength product as a function of P rates in naturally colored cotton cultivars.

Figure 10. Count strength product of naturally colored cotton fiber as a function of phosphorus doses in agricultural harvests.

A study conducted in the Brazilian Cerrado, Santos et al. (2012) did not find the effect of P doses on cotton reliability. In the present work, the responses of naturally colored cotton cultivars possibly occurred due to the initial P conditions in soils that differ from the study conducted by these authors, in which the P content in the soil is classified as average. In the present study, the P content considered is shallow.

Conclusions

The increase in the availability of P allowed to increase in the yield and quality of the naturally colored cotton fiber. The cultivar BRS Topázio stood out in the evaluated characteristics of yield and fiber quality. The maximum productivity of the fibers was obtained with the rates of 240; 240; 199.11, and 60 kg ha⁻¹ of P₂O₅ for the cultivars BRS Safira, BRS Topázio, BRS Verde, and BRS Rubi, respectively. The maximum fiber length was achieved with 80.86 kg ha⁻¹ of P₂O₅ in the cultivar BRS Verde. Maximum uniformity was obtained with a rate of 138.49 kg ha⁻¹ of P₂O₅, regardless of cultivars and harvests.

Highlights

• The productivity and fiber quality of the cotton cultivars responded differently to P.

• The highest concentration of P increased the yield and quality of the colored cotton fiber;

• Cultivar BRS Topázio showed better responses for the characteristics analyzed;

• Fiber length, strength and elongation can be increased with rates of P.

Disclosure statement

No potential conflict of interest was reported by the authors.