What is the impact of the amount of water used for dry sowing and wet emergence of cotton?

In recent years, with the large-scale promotion of conventional drip irrigation, drip irrigation for seedling emergence after cotton sowing has become a routine technique adopted by most cotton-growing regions. Drip irrigation at the appropriate time can not only avoid the final frost in April, but also produce uniform cotton seedling emergence with better growth vigor, making field management easier. However, the amount of water used for drip irrigation for seedling emergence varies from place to place.

Our region is a marginally suitable cotton-growing area, with frequent cold spells returning each spring, final frost mostly around April 20, and relatively low temperatures during the cotton seedling stage. Cotton root rot is more severe in years with frequent returning cold weather. Since cotton root rot worsens under low-temperature, high-humidity conditions, most areas adopt the approach of light drip irrigation for seedling emergence, with 15-20 cubic meters per 667 sq m, which can basically meet cotton seedling emergence needs, helps reduce field humidity, is relatively favorable for soil temperature recovery, and results in lighter root rot incidence. Through years of comparative observation, this drip irrigation for seedling emergence method has many drawbacks, and switching to a single heavy irrigation with sufficient water is more favorable for high cotton yields.

I. Distribution of different drip water amounts in soil

Through years of observation, soil wetting differs with different drip water amounts. With small water amounts, water distribution in the soil forms a "V" shape, with water mostly distributed in the upper plough layer, having little effect on seedling emergence. However, since water is mainly in the upper layer and water at the bottom is insufficient, cotton roots penetrate relatively shallowly and drought resistance is poor, so irrigation basically cannot be stopped after seedling emergence. In fields with a single heavy irrigation of sufficient water, water distribution in the soil forms a "U" shape, with water distributed throughout the entire plough layer, which is very favorable for cotton root penetration.

II. Effect of different drip water amounts on soil temperature

Different water amounts have different effects on soil temperature. Although small water amounts have less effect on soil temperature than large amounts, they have little effect on cotton seedling growth. With large water amounts applied early, there is only 1 adverse stimulus, and this is also when seed coating agent efficacy is at its best. With small water amounts, however, a low-temperature, high-humidity environment is given to cotton seedlings when air temperature has not yet risen, the seed coating agent efficacy has passed, and root rot occurrence worsens, which is unfavorable for cotton seedling growth.

III. Effect of different drip water amounts on planting measures

The current planting model is "short, dense, early, film, uniform." During the periods of seedling hardening and prevention of premature senescence in the mid-to-late stages, different drip water amounts produce different effects. Fields with small drip water amounts can only keep drip irrigating continuously; delaying the first irrigation makes seedling hardening impossible to achieve. Cotton seedling hardening is the best way to promote root penetration, and cotton root penetration can prevent premature senescence in the mid-to-late stages. At the same time, seedling hardening also uses the overcompensation effect formed by appropriate water deficit in cotton to promote earlier transition of the cotton growth period, which is favorable for forming a high-yield structure. In our region, there is a period of water resource shortage every year, and the irrigation cycle is lengthened, which has a relatively large impact on the yield of cotton plants with shallow root penetration.

IV. Effect of different drip water amounts on cotton physiology

Before cotton enters reproductive growth, its nutrient demand is not large, especially the demand for nitrogen fertilizer. During the vegetative growth period, if the nitrogen fertilizer amount is excessive, it will stimulate increased nitrate reductase activity in the cotton plant, which can make vegetative growth overly vigorous and delay reproductive growth. At the same time, because large amounts of soluble sugar are consumed, the soluble sugar content in buds and young bolls becomes insufficient, leading to heavy shedding of buds, flowers, and young bolls. Therefore, applying large amounts of fertilizer before flowering, especially nitrogen fertilizer, is not advocated. However, continuous small-amount drip irrigation affects whole-layer fertilization, has a large impact on cotton plants, and at the same time the utilization of deep-layer fertilizer also differs greatly.

Years of observation show that a single heavy irrigation with sufficient water has more advantages than small water amounts. Some places also use a small drip amount for seedling emergence water, then supplement with one sufficient irrigation after seedling emergence to facilitate later seedling hardening, but the overall effect is not as good as a single sufficient drip irrigation. Author's affiliation: Shihezi No. 14 Regiment, No. 9 Company