Cotton is a detached dicotyledonous plant, belonging to the Malvaceae family Malvaceae. Heat, light, drought tolerance, avoid stains, suitable for planting in loose deep soil. 

Cotton cultivation has a long history, beginning around 800 BC. China is one of the earliest countries in the world to cultivate cotton, the third century BC, that is, the Warring States period, "Shang Shu", "Later Han Shu" there are records about our country's cotton planting and cotton spinning. 

In the history of cotton cultivation in China, four cultivars have been planted: island cotton (long staple cotton), Asian cotton (coarse staple cotton), upland cotton (fine staple cotton) and grass cotton (coarse staple cotton). In different historical periods, the main cultivars in China are also different. Asian cotton has a long history of introduction, a long planting time, and a wider cultivation area. The history of the introduction of upland cotton into China is relatively short, but it developed rapidly, replacing Asian cotton in the 1850s. At present, the cotton grown in the vast cotton area is mostly land cotton (fine cashmere cotton), and a small amount of island cotton (long staple cotton) is also planted in Xinjiang. 

(1) Cotton varieties  Cotton, which is native to tropical and subtropical areas, is a perennial, short-day crop. After long-term artificial selection and cultivation, it gradually moved north to the temperate zone and evolved into an annual crop. Spring (or early summer) sowing, the bud, flowering, fruiting, the completion of the growth cycle, to the winter cold, the end of life. In its growth and development process, as long as there is sufficient temperature, light, water and fertilizer conditions, just like perennial plants, can continue to grow branches, long leaves, buds, flowers, bolls, continuous growth and development, with unlimited growth and strong regeneration ability. In the life of cotton, temperature has a great impact on its growth and development, yield and product quality. In addition to temperature, cotton is very sensitive to light, more resistant to drought, afraid of waterlogging. Cotton grows through the four seasons of spring, summer, autumn and winter. There are 16 solar terms from the spring equinox to start of winter (from mid-late April to mid-November). The life can be divided into five stages: sowing period, seedling period, bud period, flower and bell period and batting period. Compared with other agricultural products, cotton has a longer growing period and is greatly affected by natural factors. 

(2) Quality of cotton fiber  Cotton fiber is a seed fiber made of elongated and thickened epidermal cells of fertilized ovule, which is different from common bast fiber. Cotton fiber is mainly cellulose, accounting for 93%-95% of the dry weight, and the rest is the companion of the fiber. Because cotton fiber has many excellent economic properties, it has become the main raw material of textile industry. 1. Length. At present, the fiber lengths of upland cotton and island cotton varieties produced in the main cotton regions in China are mostly 25-31 mm and 33-39 mm, respectively. The length of the cotton fiber refers to the length between the two ends of the fiber after stretching, generally expressed in millimeters. The length of cotton fiber is very different, the longer fiber can reach 75 mm, the shorter one is only 1 mm, the general fiber length of fine wool cotton is 25-33 mm, and the long wool cotton is more than 33 mm. The length of cotton fiber on different varieties, different cotton plants and different cotton bolls is very different, even the cotton seeds of different petals of the same cotton boll, even the different seed positions of the same cotton seed, the fiber length is also different. Generally speaking, the fiber of the lower boll of cotton plant is shorter, the fiber of the middle boll is longer, and the fiber length of the upper boll is between the two. In the same cotton boll, the middle cotton seed of each seed cotton has longer fiber. Cotton fiber length is one of the most important indexes of fiber quality, which is closely related to spinning quality. When other qualities are the same, the longer the fiber, the higher the spinning count. The calculation of the number of counts is under the condition of the common moisture return rate (8.5%), when the length of each kilogram of cotton yarn is several meters, it is a number of male counts, the finer the yarn, the higher the number of counts. The higher the spinning count, the smaller the spinning number and the greater the strength. 2. Length uniformity. The effect of fiber length on yarn quality is also affected by its uniformity. Generally, the neater the fiber, the lower the short fiber content, the smoother the yarn surface, and the higher the strength of the yarn. 3. Fiber fineness. Fiber fineness is closely related to the strength of yarn. When spinning yarn of the same thickness, the mature fiber with finer fineness is used, the yarn strength is higher because of the number of fiber roots contained in the yarn, the contact surface between the fibers is larger and the holding is tighter. At the same time, fine fibers are also suitable for spinning finer yarn counts. However, the fineness is not the finer the better, too fine fiber, in the processing process is easier to break, but also easy to produce cotton knots. 

4. Fiber strength. It refers to the larger load that a fiber or a bundle can withstand when it is about to break, generally expressed in grams or grams/mg or pounds/mg, the strength of a single fiber varies according to species or varieties, generally between 3.5-5.0 grams of fine cotton, long staple cotton fiber structure is dense, and the strength can reach 4.5-6.0 grams. 

5. Fiber maturity. The maturity of cotton fiber refers to the degree of thickening of the fiber cell wall. The thicker the cell wall, the higher the maturity, the more fiber twisting, high strength, strong elasticity, good color and relatively high yarn quality. Low maturity of the fiber - all economic properties are poor, but overripe fiber is not ideal, the fiber is too thick, less twist, yarn strength is not high. 

(3) Classification, processing and inspection of cotton  1. Classification. According to the different physical forms of cotton, it is divided into seed cotton and leather cotton. The cotton farmers picked from the cotton tree is called seed cotton, seed cotton after seed processing is called lint, usually referred to as cotton production, generally refers to lint production.  According to the different processing machinery, cotton is divided into saw cotton and roller cotton. The lint processed by sawtooth gin is called sawtooth cotton; The lint processed by the roller gin is called roller cotton. Roller cotton production efficiency is low, processed cotton impurities content is high, but no damage to the cotton fiber, the fiber is relatively long; The lint processed by sawtooth gin has low impurity content and high working efficiency, but it has certain damage to cotton fiber. At present, fine cashmere cotton is basically sawtooth cotton, and long staple cotton is generally roller cotton. 2. Processing. It is generally used to express the proportion of seed cotton processed into lint, and in normal years, the lint is 36-40, that is, 100 pounds of seed cotton can be processed into 36-40 pounds of lint. Lint can not be scattered, must be baled into a cotton bale that meets the national standard. The standard bale weight of cotton in China is (227±10) kg/ bale.  3. Inspection. The quality inspection of cotton in China is carried out in accordance with the national standard GB1103-2007 for fine wool cotton. The standard stipulates that the inspection of cotton is divided into the following indicators:  grade: according to the maturity of cotton, color characteristics, rolling quality of these three conditions the cotton is divided into 1 to 7 grades length: according to the length of the cotton fiber is divided into length grades, with 1 mm as the grade, the cotton fiber is divided into 25 to 32mm eight length grades.  Micronaire is the transliteration of English Micronaire. Micronaire is a comprehensive index reflecting the fineness and maturity of cotton fiber. The larger the value, the coarser the cotton fiber and the higher the maturity. The specific measurement method is to use an air flow meter to measure the air permeability of constant weight cotton fiber after being pressed into a fixed system, and expressed by the scale value. Micronaire value is divided into three levels, namely A, B, C, B is the standard Micronaire value level. Moisture regain rate: cotton public moisture regain rate of 8.5%, moisture regain higher limit of 10.0%. In practice, the moisture regain of raw cotton is generally measured by the method of electric meter.  Impurity content: roller cotton standard impurity content of 3.0%, sawtooth cotton is 2.5%. In practice, the moisture recovery rate of raw cotton is generally measured by the raw cotton impurity analyzer.  Hazardous sundries: Do not mix hazardous sundries with cotton.  cotton inspection includes sensory inspection and instrument inspection. Due to the current cotton inspection equipment in China is mainly to test some physical indicators of cotton, such as the strength of cotton fiber, Maclon value, etc., there is no cotton grading equipment fully in line with China's national conditions, so the national standard stipulates that cotton grading is based on sensory inspection and instrument inspection. In China, the cotton inspection and arbitration institution is the national fiber inspection bureau (institute) at all levels, and the import and export inspection of cotton is responsible for the commodity inspection Bureau of each province (city).  

(4) Cotton grading  Cotton grading is an indispensable means to determine the quality of cotton and measure the use value and market price in the process of cotton acquisition, processing, storage and sales. It can make full and reasonable use of resources to meet the needs of production and consumption.  Cotton grade consists of two parts: one is grade grading, the other is length grading. 

1. Grade classification. In general, cotton grade classification is carried out against the physical standard (standard sample), which is the basis of classification, while assisted by some other measures, such as hand-pulling, feel to experience the maturity and strength of cotton, look at the color characteristics and rolling quality, according to the comprehensive situation of the above indicators for cotton grading, national standard provisions, three for the grade standard level. 

2. Length level. The length classification is carried out by hand pulling the rule, the main length of the cotton is obtained by hand pulling the fiber (the length of a group of fibers with more content in a bundle of fibers), and the length of the cotton fiber is obtained by measuring the cotton bundle with a special ruler. Each length value is a guaranteed length, that is, 25 mm means that the cotton fiber length is 25.0-25.9 mm, 26 mm means that the cotton fiber length is 26.0-26.9 mm, and so on. At the same time, the national standard also stipulates that 28 mm is the standard level of length; The length of grade 6 and 7 cotton is measured by 25 mm. 

3. Macron rating. Micronaire value is divided into three levels, namely A, B, C, B is the standard Micronaire value level.