Cotton is the most important among the natural plant fibers and land for cotton cultivation covers large areas of the planet. Due to climate, the growing areas are limited to a belt of 30° south and north from the equator. The largest cotton producing areas are in the USA, China, Paraguay, Mexico, Pakistan, Australia, Brazil, the C.I.S., Turkey, Sudan, and Egypt. It is not known precisely how old cotton is. In the Indus River Valley in Pakistan, cotton was being grown, spun, and woven into cloth 3,000 years BC. At about the same time, natives of Egypt’s Nile valley were making and wearing cotton clothing. When Columbus discovered America in 1492, he found cotton growing in the Bahama Islands. By 1500, cotton was known generally throughout the world.
Cotton (Gossypium spp.) belongs to the botanical family Malvaceae. It is an erect branched shrub, up to 3 m high. The cotton plant has an indeterminate growth habit, in that its vegetative and reproductive developments occur at the same time. The leaves are spirally arranged on the stems. They are generally palmate with 3 or 5 lobes, pubescent to glabrous. The flowers are solitary, pentamerous, white to pale yellow in color, and rarely tinted with purple in the center. They are borne on 1-4 cm long pedicels and wrapped into three bracteoles that remain around the fruit after pollination. The fruit (called a “boll”) is a capsule, that splits open when mature. Each capsule contains many seeds, each seed is surrounded with downy fiber, white or creamy in color and easily spun. Cotton has a tap root system, and the roots can grow up, making them twice if the plant height.
The plants development occurs through different phenological stages: vegetative and reproductive growth stages. After sowing, seeds germinate in 5 to 10 days and the cotton plant begins its growth with two cotyledons until the plant forms true leaves. As a cotton plant begins to grow, it develops a series of nodes up the main stem. Beginning with the fifth or sixth node, the plant begins to form fruiting branches, which bear the cotton fruit. Typically, a cotton plant will continue to add nodes and fruiting branches for a total of 16 to 22 nodes, with 12 to 16 fruiting branches.
Once pollinated, the flower petals wither and a green capsule or “boll” is formed. The capsule contains a few oil glands as well as many dark brown seeds that are encased in lint and fuzz. The fibers we associate as “cotton” are a matrix of singular, epidermal cells that are attached to the seed coat. The capsule eventually dries, and splits open on four sutures to reveal the seeds and white fibers. The edge of the opened capsule hardens and turns tan, turning into sharp, thorn-like claws surrounding the fibrous core. If not harvested, weather will eventually cause the fibers to scatter in the wind, dispersing seeds for later germination.
Cotton is a perennial plant cultivated as an annual.
The most widely cultivated cotton species are:
Cotton is a semi-xerophyte plant, and it is grown in tropical & sub-tropical conditions. A minimum temperature of 15 °C is required for better germination at field conditions. The optimum temperature for vegetative growth is 21-27 °C and it can tolerate temperature to the extent of 43 °C but temperature below 21°C is detrimental to the crop. Cotton is best grown in soils with an excellent water holding capacity. Aeration and good drainage are equally important as the crop cannot withstand excessive moisture and water logging. The major soil types suitable for cotton cultivation are alluvial, clayey, and red sandy loam.
All nutrients play an important role in the metabolic activities of a plant. A nutrient higher in quantities cannot replace the role of another deficient nutrient. Plants express deficiency of a nutrient through symptoms on their leaves, stems and in their growth.
The crop also benefits from the application of products with a biostimulant action, based on beneficial microorganisms and vegetable protein hydrolysates. These products can stimulate the emergence and root development in the early stages of seedling development, to improve the availability of nutrients in the soil, to increase the yield from a quantitative and qualitative standpoint, to reduce the negative impact of climatic stresses and of herbicide treatments. The application of biostimulants increases the environmental and economic sustainability of the production system.
Nitrogen (N) is probably the most important fertilizer used on cotton, yet it is the most difficult to manage. Low N rates can reduce yield and quality while excessive N rates can cause rank growth, boll rot, delayed maturity, difficult defoliation, and poor quality and yield. Cotton demand for N is low early in the season, picks up through early fruiting, is high at peak bloom, and gradually declines as the crop approaches and proceeds through a cutout.
Phosphorus (P) is important in promoting early rooting. P can be applied in a starter fertilizer with N, which can be beneficial on heavier soils testing low in P or in cool soils. A starter fertilizer can also provide more benefit in wet and cooler conditions than in dry and hotter conditions. The uptake of P by cotton is most critical early in the growing season because P is necessary to stimulate early root development and early fruiting.
Potassium (K) is critical for boll formation. Like N, K is also required in large quantities after first bloom, and the demand for K can exceed N demand during this time. K uptake increases during early boll set, with some 70% of the total uptake occurring after first bloom. Soil application is the best way to supply K.