**Equal K Amounts to N Achieved Optimal Biomass and Better Fiber Quality of Late Sown Cotton in Yangtze River Valley**

### **Xiaolei Ma, Saif Ali , Abdul Hafeez, Anda Liu, Jiahao Liu, Zhao Zhang, Dan Luo, Adnan Noor Shah and Guozheng Yang \***

MOA Key Laboratory of Crop Eco-physiology and Farming system in the Middle Reaches of Yangtze River, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan 430000, China; xiaoleima@webmail.hzau.edu.cn (X.M.); drsaifbhatti@webmail.hzau.edu.cn (S.A.); ahafeez1226@webmail.hzau.edu.cn (A.H.); lada199@163.com (A.L.); liujiahao@webmail.hzau.edu.cn (J.L.); zzhang@webmail.hzau.edu.cn (Z.Z.); ld17191091496@126.com (D.L.); ans.786@yahoo.com (A.N.S.)

**\*** Correspondence: ygzh9999@mail.hzau.edu.cn; Tel.: +139-9555-3884

Received: 24 November 2019; Accepted: 8 January 2020; Published: 13 January 2020

**Abstract:** Potassium (K) fertilizer plays a crucial role in the formation of the biological and economic yield of cotton (*Gossypium hirsutum* L.). Here we investigated the effects of the amount of K on biomass accumulation and cotton fiber quality with lowered N amounts (210 kg ha−<sup>1</sup> ) under late sowing, high density and fertilization once at 2 weeks after squaring. A 2-year field experiment was performed with three K fertilizer amounts (168 kg ha−<sup>1</sup> (K1), 210 kg ha−<sup>1</sup> (K2), and 252 kg ha−<sup>1</sup> (K3)) using a randomized complete block design in 2016 and 2017. The results showed correspondingly, K<sup>3</sup> accumulated cotton plant biomass of 7913.0 kg ha−<sup>1</sup> , next to K<sup>2</sup> (7384.9 kg ha−<sup>1</sup> ) but followed by K<sup>1</sup> (6985.1 kg ha−<sup>1</sup> ) averaged across two growing seasons. Higher K amounts (K2, K3) increased biomass primarily due to a higher accumulation rate (32.68%–74.02% higher than K1) during the fast accumulation period (FAP). Cotton fiber length, micronaire, and fiber strength in K<sup>2</sup> were as well as K<sup>3</sup> and significantly better than K1. These results suggest that K fertilizer of 210 kg ha−<sup>1</sup> should be optimal to obtain a promising benefit both in cotton biomass and fiber quality and profit for the new cotton planting model in the Yangtze River Valley, China and similar climate regions.

**Keywords:** cotton; potassium; fertilizer; biomass accumulation; fiber quality

### **1. Introduction**

Cotton is one of the most important fiber crops grown not only for fiber but also for the paper and oil industries [1,2]. China is one of the leading countries for cotton production. The Yangtze River Valley is one of the three cotton-growing regions in China where seedlings are transplanted after wheat or rapeseed is harvested and more than 300 kg ha−<sup>1</sup> N is applied in three splits (30% at pre-plant, 40% at first bloom, and 30% at peak bloom) [3,4]. However, the arduous procedure and excess fertilizer input are depleting cotton production profits [5]. To improve production benefit, a new planting model with late sowing (mid-May) [6], high density (9–10 plants m−<sup>2</sup> ) [6,7], low N amounts (180–225 kg ha−<sup>1</sup> ) [7], and once fertilization [3,8] has been practiced as an effective way to fight the challenge of high cost in cotton production in the region. The new planting model harvested similar yield to the conventional practice [9] but greatly reduced the cost resulted from less manual work, low N fertilizer amount and less application of chemicals, due to the short cotton growing season with high planting density.

Numerous studies have demonstrated that K is a fundamental element for plant growth which markedly affects biomass accumulation and biomass partitioning [10–12]. Applying potassium fertilizer improved cotton plant biomass [13], especially the biomass of cotton bolls [14], and it increased the

reproductive parts biomass per unit area [15–17]. On the contrary, K deficiency reduced not only the production but also the transportation of dry matter, leading to poor growth and reduced biomass accumulation in bolls [18]. Excessive K fertilizer has increased not only luxurious consumption and environmental concern [19] but also canopy closure, leading to rotten bolls and delayed maturation [20]. Tsialtas et al. [21] revealed that 80 kg K2O ha−<sup>1</sup> was sufficient for cotton growth to achieve considerable yields in Australia. However, it remains to study how much K has to be applied to ensure enough cotton products for the new planting model. Previous studies have indicated that the cotton plant could produce a considerable yield of 2691 kg ha−<sup>1</sup> seed cotton when the K amount was in line with N amount. It is hypothesized that K could also be reduced in accordance with N because the plant should keep in balance in nutrients accumulation for normal growth and fruits.

Cotton fiber quality is an important standard in cotton production based on high yield. Many studies focused on the effect of K on cotton fiber quality traits but the results had many differences. Some studies showed that the K amount significantly affected the fiber length [21,22], strength, micronaire, uniformity, and elongation of the cotton [23]. However, some studies indicated that fiber properties were not significantly affected by the K amount [16,24,25].

The study aimed to (1) determine the effects of K fertilizer amount (ranging from168–252 kg ha−<sup>1</sup> K2O) on cotton phenology, biomass accumulation (duration and rate of FAP and distribution) and fiber quality; (2) find the optimal K amount to achieve high productivity and fiber quality of cotton in the new planting model.

### **2. Materials and Methods**

### *2.1. Experimental Site and Cultivar*

The field experiment was conducted in 2016 and 2017 with Huamian 3109 (*G. hirsutum* L.) on the experimental farm of Huazhong Agricultural University, Wuhan, China (30◦37' N latitude, 114◦210 E longitude, 23 m elevation). The soil of the experimental field was yellowish-brown and clay loam comprising of 89.3 mg kg−<sup>1</sup> alkaline N, 26.4 mg kg−<sup>1</sup> P2O5, and 177.0 mg kg−<sup>1</sup> K2O.

### *2.2. Climate*

The mean air temperatures from May to October in 2016 were 25.4 ◦C with 0.1 ◦C lower than that in 2017, and from June to September, air temperatures in 2016 were 0.3–1.6 ◦C lower than that in early 2017. The total rainfall from May to October in 2016 was 1311.6 mm with 925 mm more than that in 2017, and rainfall was mainly concentrated on June and July in 2016 with 823 mm more than that in 2017, but 107 mm less from August to September in 2016 than in 2017 [26].

### *2.3. Experiment Design*

A randomized complete block design was employed with four replicates. Three K fertilizer amounts were 168 kg ha−<sup>1</sup> (K1), 210 kg ha−<sup>1</sup> (K2), and 252 kg ha−<sup>1</sup> (K3).

Fertilizers, as provided by urea (46.3% N) for 210 kg N ha−<sup>1</sup> , calcium superphosphate (12% P2O5) for 63 kg P2O<sup>5</sup> ha−<sup>1</sup> , potassium chloride (59% K2O) for three amounts, and borate (10% B) for 1.5 kg B ha−<sup>1</sup> , were mixed evenly and buried in 10 cm deep between cotton rows in bed 2 weeks after squaring.

### *2.4. Field Management*

The plant density was 9 <sup>×</sup> <sup>10</sup><sup>4</sup> plants ha−<sup>1</sup> with a row to row space of 76 cm. The plot size was 36.48 m<sup>2</sup> (12 m × 3.04 m) with four rows in two beds. Cotton seeds were sown directly on 18 May 2016 and 10 May 2017. Seedlings were thinned at the three leave stage to the target planting density. Other field managements were carried out according to conventional practice.

### *2.5. Data Collection*

### 2.5.1. Cotton Phenology

Fifteen successive and uniform plants in one row from each plot were fixed for the investigation of plant growth stages, such as squaring (50% plant bearing squares), first bloom (50% plants showing flowers), peak bloom (normally 15 d after first bloom), boll opening (50% plants showing open boll), and plant senescence. The specific growth period in days were identified as the duration from the day of the first stage to the day of the next stage, such as seedling, from emergence to squaring; squaring, from squaring to first bloom; flowering, from first bloom to peak bloom; boll setting, from peak bloom to boll opening; flowering and boll setting, from first bloom to boll opening; boll opening (or maturation), from boll opening to plant senescence.

### 2.5.2. Cotton Biomass Accumulation

Cotton biomass was measured five times (squaring, first bloom, peak bloom, boll opening, and plant senescence) in the fourth replication. Nine (eighteen at squaring stage) successive plants were carefully uprooted and grouped randomly but equally in number into 3 as replicates from each plot at each stage. Plants were separated into vegetative parts (root, stem, and leaves) and reproductive parts (square, flower, and boll). Sub-samples were packed separately and dried in an electric fan-assisted oven at 105 ◦C for 30 min, at 80 ◦C for constant weight, and then weighted. Vegetative part biomass (VPB) is the total biomass of root, stem, and leaves, and reproductive part biomass (RPB) is the total biomass of squares, flowers, and bolls, and cotton plant biomass (CPB) is the sum of VPB and RPB.

Cotton plant biomass accumulation progress was described by a logistic regression model [3],

$$\mathcal{W} = \frac{\mathcal{W}\_{\rm M}}{1 + a\mathbf{e}^{\rm bf}},\tag{1}$$

where *a* and *b* are constants to be found, *t* is the time as the days after emergence (DAE), *W* is the biomass (g) at *t*, and *W<sup>M</sup>* is the maximum biomass (g).

According to Equation (1), the following equations will be calculated:

$$t\_1 = \frac{1}{b} \ln(\frac{2 + \sqrt{3}}{a}),\tag{2}$$

$$t\_2 = \frac{1}{b} \ln(\frac{2 - \sqrt{3}}{a}),\tag{3}$$

$$T = -\frac{\ln a}{b},\tag{4}$$

$$V\_T = \frac{\mathcal{W}\_1 - \mathcal{W}\_2}{t\_1 - t\_2},\tag{5}$$

$$V\_M = -\frac{b\mathcal{W}\_M}{4},\tag{6}$$

where *t*<sup>1</sup> and *t*<sup>2</sup> (DAE) are the initiation and termination of FAP (fast accumulation period), respectively; *T* (d) is the duration of FAP; *V<sup>T</sup>* and *V<sup>M</sup>* (g d−<sup>1</sup> ) are the average and the highest biomass accumulation rate during FAP, respectively; *W*<sup>1</sup> and *W*<sup>2</sup> are the biomass at *t*<sup>1</sup> and *t*2, respectively.

The accumulation rate (AR) of cotton plant biomass during each period was calculated by the following formula:

$$AR\left(\text{kg ha}^{-1}\text{d}^{-1}\right) = \frac{\text{W}\_{\text{T}} - \text{W}\_{\text{I}}}{\text{period length}^{\prime}}\tag{7}$$

where W<sup>I</sup> and W<sup>T</sup> (kg ha−<sup>1</sup> ) are the biomasses on the first day and the last day of the period, respectively, and the period length (d) is the duration in days of this period.

### 2.5.3. Cotton Fiber Quality

One hundred maturated bolls were picked from each plot before harvest to get the fiber samples. High volume instrumentation (HVI) was used to analyze fiber quality parameters for each fiber sample, as described by [15]. The reports of five important quality parameters describing the fiber length, strength, fineness, elongation, uniformity was provided by HVI.

### *2.6. Statistical Analysis*

Data are processed with Microsoft Excel 2010; ANOVA was performed with SPSS 21.0 (IBM Company, Chicago, IL, USA) and figures were drawn with Sigma Plot 12.5 (Systat Software Inc., San Jose, CA, USA). Least Significant Difference (LSD) among the treatments was conducted with Duncan at a 5% probability level (*p* = 0.05).

Higher K fertilizer amounts (K<sup>2</sup> and K3) increased 10.34%–20.03% seed cotton yield over K<sup>1</sup> due to higher boll density in 2016 and boll weight in both years, although differences existed between years in yield and its components [26,27].

### **3. Results**

### *3.1. Cotton Plant Phenology*

Cotton flowering and boll setting period took the longest while squaring the shortest, although differences existed between years in each specific cotton growth period (seedling, squaring and flowering, and boll setting) (Table 1).


**Table 1.** Cotton growth stages and periods influenced by K fertilizer amounts.
